SECTION 6m

Engine-Fuel System

Table of Contents


Figure Index
Rochester "BC" (One Barrel)
Rochester "2GC" (Two Barrel)
Rochester 4GC (Four Barrel)
Carter WCFB (Four Barrel)
Carter WCFB (Dual Four Barrel)
Fuel Pump
Air Cleaner
Troubles and Remedies-Fuel System
Fuel System Specifications

1957 Fuel Injection Shop Manual
Assembly Manual

Figure Index

Six Cylinder Carburetor

The carburetor used on all six cylinder models is a Rochester Model "BC" downdraft carburetor, equipped with an automatic choke. This carburetor presents several distinct features of importance to the car owner and the mechanic. Foremost among these features are:

• (a) Concentric fuel bowl-Regardless of any shift of fuel level in the bowl, the main metering jet is at all times immersed in fuel.
• (b) Centrally located main discharge nozzle-A shift in fuel level has little or no effect on the rate of discharge from the nozzle.
• (c) The main well and support assembly, which contains the main metering jet and the power valve, is attached to the cover and suspended in the float bowl which provides ease in servicing.
• (d) Automatic Choke-Insures proper starting and driving carburetion during cold weather operation.
• (e) A fast idle mechanism, which is in linkage with the choke lever, is an aid to more efficient cold weather warm-up, insuring correct idle speeds during warm-up period.
• (f) Filter built into bowl cover.

Idle and Part Throttle System

The carburetor section in figure 1 shows the passages for carburetion during idle and part throttle operating conditions. At idle speed, the throttle valve is almost closed. Below the valve is manifold vacuum. Above the valve is atmospheric pressure. Atmospheric pressure is also present in the balanced-type float chamber. This differential of pressure causes fuel to pass from the float chamber, through the main metering jet, to the idle mixture port via the carburetor crossbar. As the fuel passes through the crossbar, it is mixed with air from two air bleed holes in the crossbar and the mixture calibrated by a restriction tube in the crossbar. Additional amounts of air are mixed with the idle fuel, entering from the float chamber via the float chamber passage and from the carburetor bore above the throttle valve via the auxiliary idle passage. The correct amount of fuel air mixture is then metered to the intake manifold by the adjustable idle mixture needle.

As the throttle valve is opened slightly, the auxiliary idle passage is gradually exposed to manifold vacuum, delivering additional fuel-air mixture to the manifold instead of adding air to the mixture. This permits operation of the idle system to provide sufficient fuel-air mixture for the engine until engine speed and throttle opening are high enough to bring the part throttle system into operation.

The float chamber passage also serves a dual purpose. When the engine is stopped, the fuel in the carburetor is heated by the warm air rising from the exhaust manifold and tends to form a vapor. The float chamber passage permits the idle system to vent, preventing hard starting due to a vapor build-up in the idle system.

Further opening of the throttle valve causes sufficient flow through the carburetor bore to develop venturi action at the main discharge nozzle. The end result of the venturi action is the development of a reduced pressure or partial vacuum and a high air velocity at the main discharge nozzle in the crossbar. This pressure differential and high air velocity causes a fuel flow from the float chamber to the cross crossbar and out through the main discharge nozzle, where it atomizes into the air flow through the small venturi.

The transition from the fuel delivered from the idle passage to the fuel delivered from the main discharge nozzle is a gradual move. The idle passage fuel is slowly reduced as the main discharge nozzle delivers more fuel. Thus, the two systems interact and produce a smooth fuel-air flow at all engine speeds.

Power System

The power system of the Rochester Carburetor acts as a supplementary fuel source automatically coupled to engine demands. It is, in effect, an auxiliary main metering jet.

The power valve is a double step, spring-loaded ball type valve, controlled by a vacuum piston and metered by a power jet. When engine demands increase, the load is reflected in a manifold vacuum decrease. As the vacuum is reduced, the vacuum piston is held in the up position by manifold vacuum above the piston and atmospheric pressure and power valve spring force below the piston. A piston spring tends to push the piston down to unseat the power valve ball. When manifold vacuum drops below 5 inches of mercury, the piston unseats the power valve ball, adding an additional fuel supply line to the main discharge nozzle. The power valve contains a double step arrangement, allowing more fuel to pass the valve as manifold vacuum decreases further.

During high vacuum operation, the power valve is closed, and the main metering jet controls an economical fuel-air ratio. A relief passage from a piston groove leads to the carburetor throat, preventing fuel from passing into the intake manifold under high manifold vacuum.

Accelerating Pump System

The accelerating pump system functions to supply added amounts of fuel for increasing engine power output. The pump plunger, equipped with a spring loaded leather seal, is linked to the throttle control lever. A depression of the accelerator pedal by the driver, calling for more engine output, opens the throttle valve to allow a greater air-fuel intake and depresses the pump plunger to obtain a temporarily richer fuel-air mixture for the accelerating process. When the plunger is depressed, it unseats the discharge ball check valve and forces the fuel into the carburetor throat, spraying it against the inner or small venturi to break up the droplets of fuel for better atomization. The downward travel of the plunger also seats a bypass ball check in the plunger.

In the case of a rapid depression of the accelerator pedal, a spring-loaded feature in the pump linkage allows the linkage to over-run the pump plunger. This prevents an excessive pressure build-up in the accelerating system and smooths out the delivery of the acceleration fuel to the carburetor bore.

When the throttle is closed, the plunger is moved up, the discharge ball check is seated, and fuel is allowed into the pump cylinder by a bypass ball check in the plunger itself. This ball check also serves to eliminate any build-up of fuel vapors in the accelerating system when the engine is stopped.

Float System

The level of fuel in the float bowl directly affects the fuel-air ratio by determining the distance that the fuel must rise to enter the crossbar for the idle and main discharge systems. A low level will produce a lean mixture, a high fuel level, a rich fuel-air ratio. The Rochester Carburetor has a concentric fuel bowl to minimize surge effects on acceleration and deceleration. Twin floats operate a needle valve to control the fuel level. Automatic Choke System The automatic choke is designed to insure proper starting and driving during cold weather operation. The system is comprised of a thermostatic coil, choke piston, choke valve and fast idle cam and linkage. It is controlled by a combination of intake manifold vacuum, the offset choke valve, atmospheric temperature and exhaust manifold heat.

The thermostatic coil, which is linked to the choke valve shaft, holds the choke valve closed when the engine is below 85F.

As the engine is started, air velocity against the offset choke valve causes the valve to open slightly against the torque of the thermostatic coil. In addition, as the engine starts, intake manifold vacuum is applied to the choke piston, which also tends to pull the choke valve open.

As a consequence, the choke valve assumes a position where the torque of the thermostatic coil is balanced against the vacuum pull upon the choke piston and air velocity against the offset choke valve. This causes a regulated air flow into the carburetor which provides a proper mixture during the warm-up period.

During warm-up, the choke piston serves to modify the choking action to compensate for varying engine loads or acceleration. Any acceleration or increased road load decreases the vacuum exerted on the choke piston. This allows the thermostatic coil torque to momentarily increase choke valve closure to provide the engine with a sufficiently richer mixture for acceleration.

As the engine warms up, hot air from the exhaust manifold "stove" is drawn into the thermostatic coil cover by the vacuum behind the choke piston. This hot air causes a rise in temperature causing the thermostatic coil to slowly relax its tension, Thus, the choke valve is allowed to move gradually to the full open position.

To prevent stalling during the warm-up period, it is necessary to run the engine at an idle speed slightly higher than that for a warm engine. This is accomplished by the fast idle cam (fig. 2) which is linked to the choke valve shaft and holds the throttle valve open sufficiently during the warmup period to give the increased idle RPM until such time as the choke valve moves to the full open position. While the automatic choke is in operation, the driver may wish to advance the throttle to the full wide open position. Since this would decrease vacuum pull on the choke piston, thereby closing the choke valve, it is necessary to provide increased carburetor air flow by opening the choke valve mechanically. To accomplish this, a tang on the throttle lever is made to contact the fast idle cam linkage at wide open throttle position so as to partially open the choke valve. This will also relieve excess choking on starting by allowing more air to enter the carburetor when the engine is cranked with the accelerator held fully depressed.

Care, Maintenance and Adjustments

Because of the simple construction of the "BC" carburetor, it is possible to make all carburetor adjustments without removing the carburetor from the vehicle.

Idle Speed Adjustment

1. With the engine at operating temperature, make sure choke is entirely open and linkage is free. Also make sure throttle linkage is free so that throttle stop screw "A" is against the low step on the fast idle cam (fig. 3).
2. Set parking brake tight, place selector lever in D (Drive) range on Powerglide models, neutral on standard models and then screw in or out to obtain an idling speed of 425 revolutions per minute on Powerglide models. 475 RPM on standard models.

The idle mixture adjustment should be made to give a smooth idle at the specified idle speed. Missing is an indication of too lean an idle mixture, while rolling" or "loping" indicates too rich a mixture.

1. With the engine at operating temperature, screw idle adjusting screw "B" in until it seats lightly (fig. 3). CAUTION: Turning adjusting screw in too tight may cause scoring or grooving of the needle.
2. Back off idle adjusting screw 2 1/2 turns.
3. Start engine and allow to idle (engine at operating temperature).
4. Turn screw either way from this position until best idle point is reached. Readjust idle speed and again check mixture adjustment.

Normal setting of the choke is such that the scribed index mark on. the choke cover is in line with the long cast mark on the choke housing casting. If it is believed that the indexing is wrong, it may be checked as follows:

1. Remove air cleaner.
2. If engine is hot, remove carburetor from engine and allow to cool uniformly. Removal of the choke cover assembly will accelerate cooling. Carburetor must be normalized to room temperature before attempting to check choke operation. If car has stood for a few hours at room temperature, removal of the carburetor is not necessary.
3. Check choke valve and shaft for free operation.
4. Check room temperature.
5. Reinstall choke cover assembly, rotating clockwise so that thermostatic spring hooks behind choke lever. Rotate cover until choke valve barely closes when the air horn is rapped lightly. For each 5 deg that room temperature is below 85 deg F on standard models, 95 deg on Powerglide models, set cover one (1) mark richer as indicated on choke housing. For each 5 above, set cover one (1) mark leaner. NOTE: Carburetor must be upright on flange and throttle adjusting screw must not touch fast idle cam during above check.
6. Tighten three cover clamp screws. If the indexing was correct, the mark on the cover will align with the long mark on the housing.

No adjustment of the fast idle speed is provided since the steps on the fast idle cam are correctly proportioned to give the correct speed steps above normal speed. It is necessary to have the correct relationship between the fast idle cam position and the choke valve position. To check and adjust this setting, proceed as follows:

1. Place end of idle adjusting screw on the next to highest step of the fast idle cam. Using Tool 5120, see if small end of gauge (.073") just slides easily between lower edge of choke valve and bore of carburetor (fig. 4).
2. If necessary, bend choke rod using Tool 4552, until required clearance as measured in step 1, is obtained (fig. 5).

Check and make any necessary correction of the unloader adjustment as follows:

1. Place throttle in wide open position.
2. Using Tool 5120, see if large end of gauge (.166") just slides freely between lower edge of choke valve and bore of carburetor (fig. 6).
3. If necessary, bend tang of throttle lever with Tool 4552 to obtain necessary clearance.

1. Remove carburetor air cleaner.
2. Disconnect fuel line at carburetor cover fitting.
3. Unscrew fitting on choke vacuum tube at cover.
4. Remove line from choke to carburetor "stove" or manifold heat source.
5. Remove 4 cover attaching screws.
6. Disconnect fast idle rod at choke valve lever.
7. Lift cover straight up to prevent damage to floats.
8. With cover fully assembled and cover gasket in position, place assembly up-ended on a flat surface.
9. Place float level gauge, part number 3696192, into position (fig. 7) with tang at center of gauge located in main discharge nozzle of the cover.
10. Bend float arms vertically so that each float just touches top portion of gauge.
11. Carefully bend flat arms horizontally so that each float is centered in the gauge. Tilt assembly 90 each way and check that floats do not touch gauge (fig. 8). This insures that floats will not rub inner sides of float bowl. Recheck level adjustment.

Float Drop Adjustment

To insure sufficient entry of fuel under high speed operation, it is necessary to check and adjust the float drop.

1. With the cover assembly held right side up and floats suspended freely, carefully bend the float tang at rear of float assembly so that the bottom of the float is 1 3/4" below the gasket surface (fig. 9).

Air Horn Installation

1. Carefully place cover assembly on bowl.
2. Install 4 attaching screws and tighten screws securely.
3. Connect fuel line to carburetor cover fitting.
4. Install choke to stove line.
5. Install and tighten choke vacuum tube.
6. Install fast idle rod to choke lever.
7. Replace carburetor air cleaner and check and adjust, if necessary, idle speed and idle mixture.

Choke Overhaul

Disassembly

1. Remove air cleaner and unscrew the 1/2" brass connector nut on the choke heat tube.
2. Remove three choke cover attaching screws and retainers; then, remove choke cover and thermostatic coil assembly from choke housing.
3. Remove choke cover gasket and baffle plate (fig. 10).
4. Remove retainers from each end of choke rod and remove rod.
5. Remove retaining screw at end of choke shaft and carefully pry off choke trip lever, spacing washer and choke lever and collar assembly.
6. Remove two choke valve screws and then remove choke valve.
7. Rotate choke shaft clockwise to free choke piston from housing and then remove piston and choke shaft from carburetor.
8. Remove choke piston pin and piston from choke shaft.
9. Remove two choke housing attaching screws and remove choke housing and gasket from cover.
10. Wash parts in carburetor solvent. Inspect for wear or damage.

Assembly

1. Place new hex fitting packing on choke heat tube. Then place new gasket in position and attach choke housing to cover. Tighten screws securely.
2. Tighten hex fitting on choke housing. Fitting must be tight to prevent loss of vacuum.
3. Inspect choke shaft assembly and piston for evidence of varnish or carbon deposit, clean when necessary. Then assemble choke piston to shaft with pin and install into choke housing. Index piston with piston bore and rotate choke shaft counterclockwise to enter piston into bore.
4. Install choke valve to choke shaft with letters "RP" or "C" facing upward (fig. 11). Center choke valve before tightening screws.
5. Place baffle plate and choke housing gasket into position and install choke cover and thermostatic coil assembly. Rotate cover clockwise until index mark on cover is aligned with long cast mark on the choke housing casting. Install three retainers and screws to choke housing and tighten securely. NOTE: Choke valve should be lightly closed at room temperature (85 deg F) when index mark on cover and housing are aligned.
6. Place choke lever and collar assembly on end of choke shaft with tang facing choke housing. Install spacing washer and choke trip lever so that tang of trip lever rests on top of tang on choke lever and collar assembly with choke valve closed.
7. Install choke rod and retainer to choke lever and collar assembly and to fast idle cam. The hair pin end of rod must connect to fast idle cam.

NOTE: Check choke valve for free movement.

Automatic Choke Stove Installation

When the "BC" carburetor is installed on past model vehicles or should the stove be removed for any reason, the following procedure of installation should be followed to assure proper alignment of stove unit.

1. Assemble carburetor to engine manifold, connect carburetor controls, gasoline and vacuum spark control lines.
2. Mount manifold stove assembly on exhaust manifold between No. 1 and No. 2 exhaust ports so that hot air tube connection is offset slightly toward front of engine. Do not tighten retainer screw securely until tube installation is made.
3. Place tapered end of hot air tube into manifold stove connection. Then, by adjusting position of manifold stove on the exhaust manifold, assemble the tube fitting to the carburetor choke housing connection finger tight.
4. After this assembly has been made, tighten the stove retainer screw and the tube fitting securely.

NOTE: If heater is being installed on a replacement manifold, remove paint from manifold at heater location before installation.

Throttle Rod Adjustment

The length of the throttle control rod is adjustable to insure wide open throttle with full accelerator pedal depression. The adjustment is made at the bell crank on the left side of the engine block. With the pedal fully depressed and the carburetor throttle fully opened, the swivel should be adjusted on the control rod for free entry into the bell crank. (fig. 12). The swivel should then be given two turns to lengthen the throttle control rod. Assemble the rod to the bell crank.

Major Service Operations

The perfect carburetor delivers the proper gasoline and air ratios for all speeds of the particular engine for which it was designed. By completely disassembling at regular intervals, which will allow cleaning of all parts and passages, the carburetor can be returned to its original condition and it will then deliver the proper ratios as it did when new.

Because of the simplicity of design of the Model "BC" carburetor, few parts are used which will require replacement. Accurate calibration of passages and discharge holes, require that extreme care be taken in cleaning. Use only carburetor solvent and compressed air to clean all passages and passage discharge holes. Never use wire or other pointed instrument to clean as calibration of carburetor will be affected.

Removal

1. Remove air cleaner.
2. Disconnect fuel, vacuum and choke heat lines from carburetor.
3. Disconnect throttle rod from throttle shaft lever.
4. Remove the two carburetor flange to manifold stud nuts and lift carburetor off.

1. Disassemble choke as outlined previously.
2. Disconnect choke vacuum tube at cover.
3. Disconnect fast idle rod at choke lever.
4. Remove four cover screws.
5. Lift cover straight up to prevent damage to floats.
6. Place cover up-ended on flat surface and remove float hinge pin, floats and float needle (fig. 13).
7. Using screw driver of proper width, remove float needle seat and red fibre gasket.
8. Remove main metering jet and power valve assembly from main well support. NOTE: Use care when removing power valve not to lose small spring and ball.
9. Remove main well support attaching screw and remove support. Lift straight up to prevent damage to main well support tube.
10. Lift power piston and spring from cover.
11. Lift air horn gasket from cover.
12. Holding accelerating pump plunger all the way down, remove cotter pin or hairpin retainer from pump link and remove pump link from throttle lever and pump plunger. Pump plunger may now be lifted from bowl.
13. Lift pump spring from pump well.
14. With small needlenose pliers, remove pump discharge guide (fig. 14). Pump discharge spring and ball check will fall from bowl when turned upside down.
15. With bowl upside down, remove 2 throttle body attaching screws and remove throttle body assembly.
16. Remove idle adjusting needle and spring from throttle body.
17. Figure 15 illustrates carburetor completely disassembled.

1. Wash all metal parts thoroughly in carburetor solvent and dry with compressed air.
2. Check all parts and passages for carbon deposits.
3. Blow out all drilled passages with compressed air and check to make sure they are clean. NOTE: Do not, under any circumstances, use wire or other pointed instrument to clean drilled passages or calibrated holes in carburetor. Holes and passages are carefully calibrated and use of wire or other cleaning instrument will destroy calibration of carburetor.
4. Inspect pump plunger. If the leather or its expanding spring are damaged, in any way, the, plunger assembly should be replaced. Shake the plunger to determine if by-pass ball check is free.
5. Check floats for dents and wear on lip and hinge pin. Also check cover for wear in hinge pin holes.
6. Check float needle. If wear is noted on float needle, install new float needle assembly consisting of matched and tested needle and seat and new fibre washer.
7. Check power piston for burrs or other damage. Piston must move freely in cover bore.
8. Check throttle arm for looseness on the shaft and for excessive wear at throttle rod connection.
9. Check throttle shaft for excessive looseness in throttle body. NOTE: Any damage or excessive wear in throttle arm or shaft necessitates replacement of the throttle body assembly. This is due to the close tolerance of throttle valve fit required and the fact that the idle discharge and spark advance holes are drilled in relation to a proper fitting valve.
10. Check the choke valve for freedom of operation in cover.

1. Install idle needle and spring finger-tight in throttle body. As a temporary idle adjustment, back needle out 1 1/2 turns.
2. Using a new throttle body gasket, attach bowl to throttle body. Tighten screws evenly and securely.
3. Place pump return spring into pump well and center it by depressing with finger.
4. Install pump plunger and connect pump link to throttle lever and pump rod. Install cotter pin or hairpin retainers at both upper and lower ends.
5. Drop large steel ball into pump discharge cavity of bowl and place bronze color spring on top of ball (fig. 16).
6. Index end of pump discharge guide into bronze color spring and press down until guide is flush with bowl surface.
7. Place new air horn gasket on air horn.
8. Place power piston spring and power piston in air horn cavity and attach main well support to air horn.
9. Install main metering jet and tighten securely.
10. Hold power piston stem down and install power ball, spring and plug and tighten securely.
11. Install float needle seat using new fibre washer and install float needle.
12. Attach float with hinge pin. NOTE: Float tang must face cover.
13. Adjust float setting as outlined under, Minor Service Operations, Float Level Adjustment.
14. Place cover assembly on bowl. Install attaching screws and tighten securely.
15. Install and tighten choke vacuum tube fitting securely.
16. Reconnect fast idle rod at choke lever.

1. Place carburetor in position on manifold studs and install retaining nuts. Tighten evenly and securely.
2. Connect fuel and vacuum lines to carburetor.
3. Connect throttle rod to throttle shaft lever.
4. Connect choke heat tube to automatic choke.
5. Adjust idle speed and mixture as outlined under Care, Maintenance and Adjustments.

Rochester "2GC" (Two Barrel)

The Rochester Model 2GC two barrel carburetor is standard equipment on all eight cylinder models. While not interchangeable, the carburetors used on Powerglide-equipped models and standard shift models are basically the same, and the description and service operations are identical. 'The carburetor incorporates the calibrated cluster design.

The cluster casting is the heart of the carburetor; it embodies the small or secondary venturi (fig. 19) the high speed passages, the main well tubes and nozzles, the idle tubes (fig. 18) and the calibrated air bleeds for both the low and high speed metering systems, as well as the accelerating pump jets (fig. 20).

When the cluster is removed for service purposes, all of these vital parts can be readily seen, cleaned and examined; the main well tubes and idle tubes are permanently installed in the cluster body by means of a precision pressed fit.

The cluster fits on a platform provided in the body casting of the carburetor so that the main well and idle tubes are suspended in the fuel. A gasket is used between the cluster casting and the body platform. This method of design and assembly serves to insulate the main well tubes and idle tubes from engine heat, thus reducing heat expansion and percolation spill-over during hot idle periods of operation and during the time the hot engine is not operating.

An external vent with a protective cover to keep out dirt and other foreign matter is located in the center of the bowl cover to provide adequate venting of the unit under all types of operating conditions.

The Model 2GC carburetor functions conventionally, using the standard six systems of carburetion: Float, Idle, Part Throttle, Power, Pump and Choke. These systems are basically simple and can easily be traced and understood.

The Model 2GC Carburetor is of side bowl construction. It is designed, however, with fuel supply jets and passages submerged enough below the liquid level to provide proper engine operation under all driving conditions.

Operation

Float System

The float system (fig. 17) controls the level of fuel in the carburetor bowl.

Fuel entering the carburetor first travels through the inlet strainer or filter screen to remove particles which might block jets or passages. Then the fuel passes through the needle and seat into the carburetor bowl; flow continues until the rising liquid level raises the float to a position where the valve is closed. Thus the fuel level can be regulated by setting the float to close the valve when the proper level is reached.

The float tang prevents the float from traveling too far downward. A float needle pull clip connecting the float arm to the needle valve keeps the needle from sticking closed in the seat.

Idle System

The idle system (fig. 18) consists of the idle tubes, idle passages, idle air bleeds, idle adjustment needles and idle discharge holes.

In the idle speed position the throttle valve is cracked slightly open, allowing a small amount of air to pass through between the wall of the carburetor bore and the edge of the throttle valve.

The idle needle hole is in the high vacuum area below the throttle valve, while the fuel bowl is vented to atmospheric pressure. Vacuum can be called a lack of pressure, so a high vacuum area can be spoken of as an area of low pressure. Thus it can be said that there is considerable pressure difference between the normal atmospheric pressure on the fuel in the bowl and the low pressure (or high vacuum) at the idle needle hole.

The fuel and fuel-air mixture is drawn from the highest pressure point to the lowest pressure point and flows from the fuel bowl to the manifold in the following manner.

Fuel is drawn from the fuel bowl through the main metering jets into the main well (shown in fig. 19). It is metered by the idle fuel metering orifice at the lower tip of the idle tube and travels up the idle tube. When the fuel reaches the top of the idle tube, it mixes with air drawn through the primary idle air bleed and the mixture moves through the horizontal idle passage.

The fuel-air mixture next moves down the vertical idle passage to the secondary idle discharge holes located just above the closed throttle valve. Through these holes further air is added to the mixture, which then passes through the idle needle hole. In addition to this mixture of fuel and air, there is air entering the bore through the slightly open throttle valve; for smooth operation, the air from the bore and the fuel air mixture from the idle needle hole must combine to form the correct final mixture to curb idle engine speed.

The position of the idle adjustment needle governs the amount of air-fuel mixture admitted to the carburetor bore.

Except for this variable at the idle adjustment needle, the idle system is specifically calibrated for idle engine speed.

Part Throttle System

As the throttle valve is opened, there is a change in pressure differential points.

Initial opening of the valve progressively exposes the secondary idle discharge holes to manifold vacuum and the air stream with the result that they deliver additional air-fuel mixture for fast idle engine requirements.

Further opening of the throttle valve increases the speed of the air stream passing through the venturi, thus lowering the pressure (or raising the vacuum) in the small venturi area of the carburetor bore (fig. 19). At the same time, the edge of the throttle valve is moved away from the wall of the bore, progressively reducing the vacuum and thus the mixture flow at the idle discharge holes.

Since the low pressure point is now in the small venturi area, fuel and fuel air mixture will be drawn from the fuel bowl through the main metering system to the venturi.

The fuel passes through the main metering jet into the main well, where it rises in the main well tube. Air entering through the main well air bleeds in the cluster is mixed with the fuel through the main well tube vents. The mixture continues up the main well tube through the nozzle, where more air is added. The mixture flows through the high speed passage to the small venturi, mixes with additional air and moves on to the bore of the carburetor, through the intake manifold, and into the cylinder as a final mixture for part throttle operation.

Permanent jets and air bleeds calibrate the main metering system for proper part throttle operation.

Power System

The power system provides additional fuel for heavy load and high speed engine requirements.

A spring loaded power piston, controlled by vacuum, regulates the power valve to supply additional fuel according to engine speed and load.

The power piston vacuum channel is open to manifold vacuum beneath the throttle valves; thus the vacuum in the channel rises and falls with the manifold vacuum.

During idle and part throttle operation, the manifold vacuum in the channel is normally high enough to hold the power piston in the fully raised position against the tension of the spring. As the manifold vacuum drops with load, the calibrated spring forces the piston down against the power valve, to open it and allow additional fuel flow through the calibrated power restrictions into the main wells.

A two-step valve allows a partial increase in fuel flow for low speed-low vacuum without over-enrichment, as the power valve is opened; at full throttle position, the power valve is fully opened to permit maximum calibrated fuel flow from the power system.

As the load decreases, manifold vacuum increases. The increasing vacuum pull on the piston gradually overcomes the spring tension and returns the power piston to its original raised position, with the valve fully closed.

Pump System

Extra fuel for smooth, quick acceleration is supplied by a double spring pump plunger (fig. 20). The combination of the top and bottom springs is calibrated to move the plunger in such a manner that a smooth, sustained charge of fuel is delivered for acceleration.

The fuel passes from the bowl past the pump plunger ball check into the pump well on the intake stroke of the plunger. When the plunger is pushed downward for acceleration, the fuel is forced up the pump discharge passage.

The pressure of the fuel lifts the pump outlet ball check from its seat and the fuel passes on through the pump jets in the cluster, where it is sprayed into the venturi and delivered to the engine.

The pump plunger head embodies a unique ball check and seat, designed to eliminate fuel percolation in the pump system. When the engine is idling or not operating, excessive fuel vapors in the pump well rise through the plunger head and by-pass the ball, then circulate into the fuel bowl, which is vented to the atmosphere.

Without this feature, vapor pressure in the pump system might force fuel from the pump well into the engine, causing hard hot starting because of excess fuel in the manifold or poor initial acceleration due to vapor in the pump system.

Choke System

For cold engine operation, a rich mixture at the carburetor is required, so that a combustible mixture remains to be drawn into the cylinder after condensation of much of the fuel vapor on the cold engine parts. The function of the choke system is to subject all fuel outlets in the bore of the carburetor to high vacuum while restricting the intake of air, thus drawing into the engine the required rich mixture.

The Model 2GC Carburetor employs the newly designed Rochester Multistage Balanced Piston automatic choke system, to insure proper starting and driving during cold weather operation (fig. 21). The choke system includes a thermostatic coil, housing, choke piston, choke valve and fast idle cam and linkage. It is controlled by a combination of intake manifold vacuum, air velocity against the offset choke valve, atmospheric temperature and exhaust manifold heat.

The thermostatic coil, which is attached to the choke housing cover acts to hold the choke valve closed when the engine is cold.

The offset choke valve is first slightly opened by a combination of manifold vacuum on the choke piston and air velocity against the offset choke valve.

In the first choking stage, manifold vacuum is applied to the choke housing through a vacuum slot in the housing. This vacuum pulls the piston to help open the choke valve against the thermostatic coil torque. The piston moves in the housing to a point where the piston blocks the vacuum slot; at this point there is very small amount of vacuum pull on the piston and further choke valve opening is primarily the function of the thermostatic coil.

In the second stage, the coil is gradually heated by air drawn from the exhaust manifold "stove" through the center of the thermostatic coil cover, then through a calibrated vacuum by-pass into the main vacuum passage. The calibrated restriction of the vacuum by-pass slows the flow of air through the coil, so that the heating is gradual. A baffle plate serves to distribute the heat from its entering point at the center of the coil throughout the choke housing, to prevent a "hot spot" in the coil center.

The coil unwinds as it is heated and the piston moves in the housing until the land on the piston uncovers the vacuum slot. A flat on one side of the piston forms a passage so that vacuum from the slot can draw air directly from the choke housing, thus allowing a greater flow of heat through the coils for the rest of the opening operation.

The thermostatic coil controls the majority of the choking action and coils with high thermal activity can be chosen to best fit engine characteristics. Also, low manifold vacuum (during acceleration or load) cannot allow the choke to partially close as much as often happens in the conventional vacuum controlled choke system.

The choke system uses a conventional fast idle cam and linkage to maintain sufficient engine speed for cold operation.

The carburetor is also equipped with a mechanical "unloader" to eliminate the possibility of the choke closing during heavy load or acceleration.

At wide open throttle, the throttle lever lifts a tang on the fast idle cam and restricts the choke from closing past a certain point. This feature is particularly useful in preventing excess choking when the engine is cranked with the throttle wide open.

In addition, the unloader also eliminates the possibility of the choke sticking closed, or closing during heavy load or acceleration due to low vacuum on the piston.

Care, Maintenance and Adjustments

The carburetors are carefully designed and calibrated to meet engine requirements. They are tested and adjusted at the factory to meet specifications. It is a common practice to blame the carburetor for many engine problems, and before making carburetor adjustments, there are several items to be checked.

Preliminary Checks

1. Thoroughly warm-up engine. If the engine is cold, allow to run for at least 15 minutes.
2. Inspect torque of carburetor to intake manifold bolts and intake manifold to cylinder head bolts to exclude the possibility of air leaks.
3. Inspect manifold heat control valve for freedom of action and correct spring tension. NOTE: For float level and pump lever operations, warm-up is not required.

Adjustment

1. Connect tachometer and vacuum gauge to engine. Set parking brake and place transmission in neutral.
2. With a thoroughly warmed-up engine, check to see that choke is fully off and carburetor is on slow idle.
3. Start engine and allow to idle. Adjust idle speed (fig. 22) to give 475 R.P.M.
4. On Powerglide models, move selector lever to Drive Range. Adjust each idle mixture screw (fig. 23) separately to give peak vacuum and R.P.M. indications on tachometer and vacuum gauge. CAUTION: Do not turn idle mixture screws tightly against stop or damage to needle seat will result.
5. If necessary, readjust engine idle to 475 R.P.M. on standard shift models, 425 R.P.M. on Powerglide models with selector lever in Drive Range. Readjust idle mixture as outlined in Step 4.
6. Shut off engine and remove tachometer and vacuum gauge.
7. Road test vehicle to check performance.

The length of the throttle linkage is adjustable to assure wide-open throttle with full accelerator pedal depression. To check, depress accelerator pedal fully and check to see if throttle is wide open. If not, adjust threaded swivel at throttle lever to suit. With the accelerator pedal fully depressed and the carburetor throttle valve fully open (through detent on Powerglide models) the threaded swivel should be adjusted for free entry into the throttle ever (fig. 24). The swivel should then be turned two full turns to lengthen the control rod.

Float Level and Drop Adjustment

1. Remove bowl cover with gasket from carburetor as described under Major Service Operations.
2. Invert the cover on a flat service. Place float Tool 6326 over float as shown in fig. 25.
3. Bend float hinge arm as required until the highest part of the float just clears the gauge step. The scale dimension, bottom of float to gasket, is 1 1/4".
4. Hold bowl in an upright position and measure the distance from the gasket to the bottom of the float, as shown in fig. 26. This dimension should be 1 29/32 ". Bend the tang at the end of the float hinge arm to obtain the correct drop.
5. Install the bowl cover as outlined under Major Service Operations.
6. Readjust idle mixture.

To provide accurate accelerator pump action, the position of the plunger, measured at the accelerator pump lever (fig. 22 and fig. 27), is adjustable. The accelerator pump rod may be carefully bent to obtain the correct adjustment. Automatic Choke Adjustment Normal setting of the choke is such that the scribed index mark on the cover is in line with the long cast mark on the choke housing casting. If it is believed that the indexing is wrong, it may be checked as follows:

1. Remove air cleaner.
2. If engine is hot, remove carburetor from engine and allow to cool uniformly. Removal of the choke cover assembly will accelerate cooling. Carburetor must be normalized to room temperature before attempting to check choke operation. If car has stood for a few hours at room temperature, removal of the carburetor is not necessary.
3. Check choke valve and shaft for free operation.
4. Check room temperature.
5. Reinstall choke cover assembly rotating clockwise so the thermostat spring hooks behind choke lever. Set cover index mark to long index mark of choke housing if room temperature is 75 deg; set one (1) mark leaner for each 5 that temperature is below 75 deg or one (1) mark richer for each 5 that temperature is above 75 deg. Choke valve should close when air horn is vibrated or rapped lightly. NOTE: Carburetor must be upright on flange and throttle adjusting screw must not touch fast idle cam during above check.
6. If original setting was correct, reset cover index mark to long index mark on choke housing and tighten three cover screws. If above check indicates need for changing original cover setting, make necessary correction and tighten three cover screws.

No adjustment of the fast idle speed is provided since the steps on the fast idle cam are correctly proportioned to give the correct speed steps above normal idle speed. It is necessary, however, to have the correct relationship between the fast idle cam position and the choke valve position. To check and adjust this setting, proceed as follows:

1. Place end of idle adjusting screw on the next to highest step of the fast idle cam. Using Tool 5921, see if small end of tool just slides easily between upper edge of choke valve and bore of carburetor (fig. 28). NOTE: Tool must be held vertically.
2. If necessary, bend choke rod until required clearance as measured in step 1 is obtained.

Check and make any necessary correction of the unloader adjustment as follows:

1. Place throttle in wide open position.
2. Using Tool 5921, see if large end of gauge just slides freely between upper edge of choke valve and bore of carburetor (fig. 29). NOTE: Gauge must be held vertically.
3. If necessary, bend tang of throttle lever to obtain necessary clearance.

Removal

1. Remove air cleaner wing nut, air cleaner, gasket and stud.
2. Disconnect vacuum line, spark control line, fuel line and choke heat tube.
3. Disconnect throttle rod at carburetor and remove throttle return spring. (On Powerglide models, disconnect transmission control rod from throttle lever.)
4. Remove four nuts and washers retaining carburetor to manifold. (On overdrive models, lay kick-down switch aside without disconnecting wires.)
5. Lift carburetor off manifold.
6. Remove and discard carburetor to manifold gasket.

Flooding, stumble on acceleration and other performance complaints are, in many instances, caused by the presence of dirt, water or other foreign matter in the carburetor. To aid in diagnosing the cause of the complaint, the carburetor should be carefully removed from the engine without draining the fuel from the bowl. The contents of the fuel bowl may then be examined for contamination as the carburetor is disassembled.

The following is a step-by-step sequence by which the Rochester Model 2 GC Carburetor may be completely disassembled and reassembled. Adjustments may be made and various parts of the carburetor may be serviced without completely disassembling the entire unit.

Choke

1. Remove the three choke cover attaching screws and retainers then remove choke cover and coil assembly from carburetor.
2. Remove choke cover gasket and baffle plate.
3. Remove retaining screw at end of choke shaft (fig. 30) and carefully pry off choke trip lever, and fast idle choke rod link and lever. Lever can be removed from rod by turning until slot in lever will pass over tang on link rod.
4. Remove two choke valve screws, then remove choke valve.
5. Rotate choke shaft counterclockwise to free choke piston from housing, then pull piston and choke shaft from carburetor.
6. Remove two choke housing attaching screws. Choke housing and gasket may now be removed from air horn assembly.

Bowl Cover

1. Disconnect pump rod from throttle lever by removing retainer. Rod can be removed completely by rotating until it clears pump lever.
2. Remove eight cover screws and lift cover from bowl.
3. Place upended cover on flat surface. Remove float hinge pin and lift float assembly from cover. Float needle may now be removed from float.
4. Remove float needle seat and gasket with wide blade screw driver and remove filter screen.
5. Remove power piston by depressing shaft and allowing spring to snap, thus forcing piston from casting.
6. Remove retainer on pump plunger shaft, remove plunger assembly from pump arm. The pump lever and shaft may be removed by loosening set screw on inner arm and removing outer lever and shaft.
7. Remove the cover gasket. Figure 31 shows the choke and bowl cover exploded. NOTE: The pump plunger may be further disassembled by compressing the spring and removing the retainer.

Bowl and Throttle Body

1. Remove pump plunger return spring from pump well.
2. Remove main metering jets and power valve.
3. Remove three screws on top of cluster, after which cluster and gasket may be removed.
4. Using a pair of long nose pliers, remove the pump discharge spring retainer. Then the spring and check ball may also be removed.
5. Upend carburetor and remove three large bowl to throttle body attaching screws. Throttle body and gasket may now be removed.
6. Remove fast idle cam.
7. Remove idle adjusting needles and springs from throttle body. Figure 32 shows the bowl and throttle body exploded.

Dirt, gum, water, or carbon contamination in or on the exterior moving parts of a carburetor are often responsible for unsatisfactory performance. For this reason, efficient carburetion depends upon careful cleaning and inspection while servicing.

1. Thoroughly clean carburetor castings and metal parts in clean cleaning solvent. CAUTION: Choke coil and housing and pump plunger should not be immersed in solvent. Clean pump plunger in clean gasoline only. DO NOT SOAK AIR HORN ASSEMBLY IN CLEANER OR SOLVENT FOR MORE THAN ONE HALF HOUR IF CHOKE PISTON HOUSING HAS NOT BEEN REMOVED.
2. Blow all passages in castings dry with compressed air and blow off all parts until they are dry. CAUTION: Do not pass drills or wires through calibrated jets or passages as this may enlarge orifice and seriously affect carburetor calibration.
3. Check all parts for wear. If wear is noted, defective parts must be replaced. Note especially the following:
   • Check float needle and seat for wear. If wear is noted the assembly must be replaced.
   • Check float lip for wear and float for dents. Check floats for leaks by shaking.
   • Check throttle and choke shaft bores in throttle body and cover castings for wear or out of round.
   • Inspect idle adjusting needles for burrs or ridges. Such a condition requires replacement.
   • If wear is noted on steps of fast idle cam, it should be replaced as it may upset engine idle speed during the choking period.
   • Inspect pump plunger leather. Replace plunger if leather is damaged.
4. Inspect gaskets to see if they appear hard or brittle or if the edges are torn or distorted. If any such condition is noted they must be replaced.
5. Check filter screen for dirt or lint. Clean and if it is distorted or remains plugged, replace.
6. If for any reason parts have become loose or damaged in the cluster casting, it must be replaced.

Throttle Body

1. Screw idle adjusting needles and springs into throttle body until finger tight. Back out screw 1 1/2 turns as a temporary idle adjustment. CAUTION: Do not force needle against seat or damage will result.
2. Upend bowl, place new throttle body gasket in position and attach throttle body to bowl. Tighten screws evenly and securely.

Bowl

1. Install fast idle cam.
2. Drop pump discharge check ball into discharge hole. Ball is 3/16" diameter steel. Replace spring and retainer.
3. Replace cluster and gasket, tighten screws evenly and securely. Make certain center screw is fitted with red fibre gasket.
4. Replace main metering jets and power valve gasket and power valve. Figure 33 shows the bowl and throttle body at this stage of assembly.
5. Install pump return spring, pressing with finger to center it in pump well.

Bowl Cover

1. Replace pump lever assembly.
2. Install filter screen, float needle seat and gasket, using wide blade screw driver.
3. Replace power piston in vacuum cavity; piston should travel freely in cavity. Lightly stake retainer in place.
4. Attach plunger shaft with retainer, with shaft end pointing inward.
5. Install cover gasket.
6. Attach needle to float, carefully position float and insert float hinge pin. Figure 34 shows the bowl cover assembled.
7. Adjust float as previously outlined.
8. Place cover on bowl, making certain that accelerator pump plunger is positioned and will move freely. Raise cover and lower straight down to insure proper installation.
9. Install and tighten cover screws evenly and securely.
10. Install pump rod and retainer.

Choke

1. Place new gasket into position and attach choke housing to cover. Tighten screws securely.
2. Place shaft in choke housing and rotate clockwise so that the piston rides in bore in housing.
3. Install choke valve on choke shaft with letters "RP" facing upward. Center choke valve before tightening screws.
4. Place baffle plate and choke housing gasket into position and install choke coil and cover. Rotate cover counterclockwise until index marks on cover and housing are aligned. Attach three retainers and screws to choke housing and tighten securely. NOTE: Choke valve should be slightly closed at room temperature (75 degrees F) when index mark on cover and housing are aligned.
5. Replace link in fast idle cam and choke lever, place choke lever on cover with the tang facing outward and toward the pump lever. Install trip lever so that tang of trip lever is under tang of choke lever.

It is good shop practice to fill the carburetor bowl before installing the carburetor. This reduces the strain on the starting motor and battery and reduces the possibility of backfiring while attempting to start the engine. A fuel pump clamped to the bench, a small supply of fuel and the necessary fittings enable the carburetor to be filled and the operation of the float and intake needle and seat to be checked. Operate the throttle several times and check the discharge from the pump jets before installing the carburetor.

Installation

1. Be certain throttle body and intake manifold gasket surfaces are clean.
2. Install new carburetor to manifold gasket over manifold carburetor studs.
3. Start spark control pipe fitting into carburetor fitting.
4. Lower carburetor onto manifold studs, choke housing to right side of vehicle.
5. Install and tighten securely four washers and nuts, using a short, open end wrench. (On overdrive models, install and adjust kickdown switch, see Transmission Section 7.) NOTE: Copper washers are used on two front studs.
6. Tighten spark control pipe fitting and connect and tighten choke heat tube, vacuum line and fuel line.
7. Install throttle rod and throttle return spring. (On Powerglide models, install transmission control rod.)
8. Install air cleaner gasket, stud, air cleaner and wing nut. Tighten securely.
9. Adjust idle speed and mixture.

Rochester 4GC (Four Barrel)

Fig. 1

The Rochester Model 4GC four barrel carburetor is optional equipment and is used in mixed production for 1956 Chevrolet V-8 Power Package models equipped with Powerglide. This four barrel carburetor is basically two 2 jet Rochester carburetors combined in a single casting. The . carburetor incorporates the Rochester Calibrated Cluster designed for fuel economy and simplification of maintenance.

The cluster casting is the heart of the carburetor; in the primary side of the carburetor it embodies the small or secondary venturi, the high speed passages, the main well tubes and nozzles, the idle tubes and the calibrated air bleeds for both the low and high speed metering systems, as well as the accelerating pump discharge jets. The secondary cluster is different in as much as it does not contain pump discharge jets or idle tubes.

When the cluster is removed for service purposes all of, these vital metering parts can be readily seen, cleaned and examined; the main well tubes and idle tubes are permanently installed in the cluster by means of a precision pressed fit. The clusters also contain the mixture passages, air bleed and on the primary side, pump jets.

The clusters fit on a platform of the carburetor bowl provided on the body casting of the carburetor so the main well tubes and idle tubes are suspended in fuel at all times. A gasket is used between the cluster casting and the body platform. This method of design and assembly serves to insulate the main well tubes and idle tubes from engine heat, thus reducing heat expansion and percolation spill-over during hot idle periods of operation and during the time the hot engine is not operating,

The Model 4GC functions conventionally, using the six basis systems-Float, Idle Part Throttle, Power, Accelerating Pump and Choke. The "Primary Side" contains all six of the carburetion systems; the "Secondary Side" supplements the "Primary Side" with separate Float and Power systems. The following explanation and illustrations show how each system operates to provide efficient carburetion through all operating conditions.

Operation

Float System (fig. 2)

The float system controls the level of fuel in the carburetor bowl. Fuel enters the carburetor on the primary side and is fed to the secondary side through a connecting passage in the bowl cover casting. Entering fuel passes first through the inlet screens (1) then through the fuel valves (2) into the float bowls. Flow continues until the fuel lever raises the floats (3) to a position where they close the valves (2). The fuel inlet needles are provided with a pull clip (5) to eliminate the possibility of sticking in the closed position. The float setting and thus the fuel level is the same in both sides of the carburetor. On one side of the float bowl there is a cored passage which connects the two float chambers just above the fuel level so that a temporary rise in fuel level in one float chamber will spill over into the other float chamber, rather than into the bore of the carburetor. Thus engine operation is not disrupted. Both sides of the carburetor are individually and internally vented (4) to transmit the pressures from beneath the air cleaner to the fuel in the float bowl. External vents (6) are also provided for vapor release at idle and to improve hot starting.

Idle System (fig. 3)

At low idle the throttle valves (1) are nearly closed. Since there is not sufficient air flow to the venturi to draw any fuel up the main nozzles, fuel must be supplied through the idle system by application of vacuum directly to the fuel. With the throttle valve in this nearly closed position, there is a very low pressure point at the idle needle hole (2) which causes the fuel to flow through the idle system in the following manner: Fuel passes through the main metering jet (3) and is drawn through the idle tube restriction (4) up the idle tube to meet with air being drawn in the top bleed(s). This mixture is metered by the passage restriction (6) and mixes again with air drawn through the side bleed (7). As the mixture travels downward towards the idle needle, more air enters through the lower idle bleed (8) and the secondary discharge holes (9). The mixture is delivered through the idle needle hole and is regulated by the position of the idle needle to combine properly with the slight amount of air which is passing the throttle valve, so that the proper mixture is furnished for low idle speeds.

As the throttle valves are opened from low idle position, and the engine demands are increased, the secondary discharge holes (9) are exposed to manifold vacuum and become additional suppliers of fuel-air mixture for increased engine needs. With further throttle valve opening, as the transition is made to part throttle operation, the lower idle air bleeds (8) also come under the influence of vacuum or low pressure and commence to feed fuel, continuing through the part throttle and power ranges.

Part Throttle System (fig. 4)

During part throttle when the throttle valves are opened to the point where air velocity through the venturi system will draw fuel through the main metering system, operation is as follows:

Fuel is drawn from the fuel bowl through the calibrated main metering jets (2) into the main well, where it is drawn up the main well tube (3) and mixed with air entering through the main well bleed (4) and the small drilled holes in the main well tube. The mixtures rise to the upper end of the main well tube, commonly called the nozzle, at which point it is delivered into the mixture passage (5) and passes on to the small venturi (6) and on into the manifold.

Power System (fig. 5)

The power system provides additional fuel for heavy load and high speed engine requirements.

A spring loaded power piston (1) located in the primary side of the carburetor controls the power valve (2) through a connecting vacuum passage (3) from the base of the carburetor to the chamber above the power piston, exposing the chamber to manifold vacuum at all times.

During idle and part throttle, the vacuum in the channel (3) is normally high enough to hold the piston (1) in the fully raised position overcoming the tension on the power valve spring. As the manifold vacuum drops with heavy load, the spring overcomes the vacuum pull on the piston and pushes the power valve open, allowing an additional supply of fuel to flow through the power restriction (4) and mix with fuel from the main metering jet.

As the load decreases, manifold vacuum increases. The increasing vacuum pull on the piston (1) gradually overcomes the spring tension of the power valve spring and the power piston returns to its original raised position; then the valve (2) is fully closed. Thus fuel for power operation is used only when required.

In the secondary side of the carburetor whenever the secondary valves (6) are open, fuel will be fed through a metering system similar to that in the primary side.

To improve low speed wide open throttle operation a pair of spring loaded, air velocity operated, auxiliary throttle valves (5) are located in the secondary bores above the regular throttle valves. When the throttle valves are moved to their wide open position and engine speed is low there is insufficient air flow through the secondary bores to force the spring loaded auxiliary valves to open. This will concentrate all air flow through the primary throttle bores with better metering of fuel and air. In this condition the carburetor is functioning as a 2-barrel carburetor. As the engine speed increases, the force of the air acting on the auxiliary valves increases to the point where the auxiliary valves are forced to open. The calibration of the auxiliary valve spring tension is such that valve opening occurs when greatest metering efficiency is possible. With the addition of the auxiliary throttle valves, low speed power operation is improved with a smoother transition from low to high speed occurring. In this way the power is maintained at low speed by providing good metering and at high speed by allowing complete opening of all four bores.

Pump System (fig. 6)

More fuel is supplied for smooth quick acceleration without hesitation by a double spring loaded, pump plunger (1). When the throttle valves are quickly opened the motion is transmitted through the pump lever to the pump shaft, which quickly moves down against the tension of the duration spring, which is a part of the plunger. The combination of the top and bottom springs is calibrated to move the plunger in such a manner that a smooth sustained charge of fuel is delivered for acceleration.

The fuel is drawn into the pump chamber on the upward stroke of the pump plunger through the inlet screen and inlet ball check (2). When the plunger is pushed down for acceleration, the force of the stroke seats the ball check to prevent flow to the fuel bowl and the fuel is forced up the discharge passage (3), unseats the discharge check ball (4) and passes on through the passage to the pump discharge jets (5) in the cluster, where it sprays into the venturi and is delivered to the engine. A third ball check (6) inside the pump plunger head serves as an additional fuel inlet and also as a vapor vent for the pump chamber during hot operation. When the engine is idling or not operating, excessive fuel vapors in the pump will rise through the plunger head and by-pass the ball, then circulate into the fuel bowl which is vented to the atmosphere.

Without this feature, vapor pressure in the pump system might force fuel through the pump passage and into the engine causing hard (hot) starting because of excess fuel in the manifold, or vapor pressures might cause poor initial acceleration due to insufficient fuel in the pump system.

Choke System (fig. 7)

For cold engine operation, a rich mixture at the carburetor is required, so that a combustible mixture remains to be drawn into the cylinder after condensation of much of the fuel vapor on the cold engine parts. The function of the choke system is to subject all fuel outlets in the bore of the carburetor to high vacuum while restricting the intake of air, thus drawing into the engine the required rich mixture. The model 4GC carburetor employs the Rochester Multistage Balanced Piston automatic choke system, to insure proper starting and driving during cold weather operation.

The choke system includes a thermostatic coil, baffle plate, choke piston, choke valve and fast idle cam and linkage. It is controlled by a combination of intake manifold vacuum, air velocity against the offset choke valve, atmospheric temperature and exhaust manifold heat. The thermostatic coil, which is attached to the choke housing cover acts to hold the choke valve closed when the engine is cold.

The offset choke valve is first slightly opened by a combination of manifold vacuum on the choke piston and air velocity against the offset choke valve.

In the first choking stage, manifold vacuum is applied to the choke piston through a vacuum slot in the housing. This vacuum pulls the piston to help open the choke valve against the thermostatic coil torque. The piston moves in the housing to a point where the piston blocks the vacuum slot; at this point there is a very small amount of vacuum pull on the piston and further choke valve opening is primarily the function of the thermostatic coil.

In Figure 7, view A, the choke valve is in fully closed position, held there by the tension of the thermostatic coil (1). At this time there is no manifold vacuum applied to the choke piston (2) because the engine is not yet started.

In view B, the engine has been started and it immediately becomes necessary to partially open the choke valve to prevent over choking. Air velocity past the choke valve (3) and vacuum on the piston (2) combine to open the choke valve against tension of the coil (1). In view A, application of vacuum to the choke piston will immediately provide positive movement of the piston. In view B, the effect is lessened because the piston covers the vacuum hole and vacuum is applied to the end of the piston only through a small bleed hole (4). During this portion of choke operation heated air is drawn through the choke heat tube and past the baffle plate on the choke cover to warm up the coil and cause it to relax its tension gradually.

In view C, the coil tension has relaxed sufficiently that the piston has moved further and exposes the cutaway portion (6) on the side of the piston, which allows an immediate increase in air flow through the coil (1), which heats the coil until it relaxes enough to allow the choke valve to stay wide open.

The thermostatic coil controls the majority of choking action and coils with high thermal activity can be chosen to best fit engine characteristics. Also, low manifold vacuum (during acceleration or load) cannot allow the choke to partially close as much as often happens in the conventional vacuum controlled choke system.

The choke system uses a conventional fast idle cam and linkage to maintain sufficient engine speed for cold operation.

The carburetor is also equipped with a mechanical "unloader" to eliminate the possibility of the choke closing during heavy load or acceleration.

At wide open throttle, the throttle lever lifts a tang on the fast idle cam and restricts the choke from closing past a certain point.

This feature is particularly useful in preventing excess choking when the engine is cranked with the throttle wide open.

In addition, the unloader also eliminates the possibility of the choke sticking closed, or closing during heavy load or acceleration due to low vacuum on the piston.

Care, Maintenance and Adjustments

Adjustments on the Car

All adjustments listed below can be done without removal of the carburetor from the car. The adjustments described below should be made in the following sequence:

1. Accelerator Linkage Adjustment.
2. Idle Speed and Mixture Adjustment.

If trouble is not corrected with the previous adjustments, continue with the following:

1. Float Level and Drop Adjustment.
2. Pump Rod Adjustment.
3. Automatic Choke Adjustment.
4. Choke Rod Adjustment.
5. Choke Unloader Adjustment
6. Secondary Throttle Lockout Adjustment.
7. Secondary Throttle Contour-Clearance Adjustment
8. Idle Speed and Mixture Adjustments
9. Accelerator Linkage Adjustment

NOTE: No adjustment of the fast idle speed is provided since the steps on the fast idle cam are correctly proportioned to give the correct speed steps above normal idle speed. It is necessary, however, to have the correct relationship between the fast idle cam position and the choke valve position. This adjustment is outlined under "Choke Rod Adjustment" in this section.

Idle Speed and Mixture Adjustment

With the engine at operating temperature (choke entirely off) adjust idle speed screw (fig. 12) to 425 RPM for Powerglide equipped engines (selector lever in DR (Drive) position) and 450 RPM for standard and overdrive transmission.

The idle mixture screws should be adjusted separately to give a smooth idle at the above mentioned idle speed. Missing or tendency to stall is a sign of too lean an idle mixture, while "rolling" or "loping" indicates too rich a mixture. Turning the idle mixture adjustment screws in will lean out the idle mixture. As a preliminary adjustment, idle mixture screws should be set one to one and one half turns from seat before making final adjustments (fig. 16).

Float Level and Drop Adjustment

1. Remove bowl cover with gasket from carburetor as outlined under "Major Service Operations."
2. Invert the cover on a flat surface. Place Tool 5399A centered over floats on bowl cover gasket as shown in Figure 8, so that it rests against the curvature of the bore of the air horn.
3. Bend float arms (fig. 9) at rear of assembly until floats just touch the tool at the center notch. The scale dimension, bottom of float to gasket is 1 5/8".
4. Bend float arms (fig. 9) horizontally until each float is centered between tool legs (fig. 8). Repeat operation 3 and 4 on other set of floats.
5. Hold the, bowl cover in an upright position and measure the distance from the bowl cover to the bottom of the float, with the float hanging free. This dimension should be 2 1/4". Bend float tang (fig. 9) at end of the float hinge arm to obtain the correct drop (fig. 10).
6. Install the bowl cover as outlined under "Major Service Operations."

1. Back off the idle speed screw until the throttle valves are completely closed.
2. Measure from top of bowl cover to bottom of pump plunger rod. This measurement should be 1 1/16", if necessary bend arm with Tool 4552 at upper angle of pump rod (fig. 11).

Automatic Choke Adjustment

Normal setting of the choke is such that the scribed index mark on the cover is one notch lean with the cast mark on the housing. If it is believed that the indexing or spring tension is wrong, it may be checked as follows:

1. If engine is hot, remove carburetor from engine and allow to cool uniformly. Removal of the choke cover assembly will accelerate cooling. Carburetor must be normalized to room temperature before attempting to check choke operation. If engine is inoperative for period of four hours, carburetor temperature will be normalized and removal is unnecessary.
2. Check choke valve and shaft for free operation.
3. If choke cover was removed, reinstall choke cover assembly, rotating until the thermostat spring hooks behind choke lever. Set cover index mark in line with the correct mark of choke housing if room temperature is 75 deg; set one (1) mark richer for each 12 deg that temperature is below 75 deg or one (1) mark leaner for each 12 deg that temperature is above 75 deg . At room temperature of 75 deg, the choke valve should be lightly closed. NOTE: If carburetor is removed, carburetor must be upright on flange and throttle adjusting tang must not touch fast idle cam during above check.
4. If original setting was correct, reset cover index mark one notch lean with cast mark on the housing. if above check indicates need for changing original cover setting, make necessary correction and tighten cover screws.

1. Turn idle speed screw in until it just contacts the second step of the fast idle cam.
2. Be certain that choke trip lever is in contact with choke counterweight lever.
3. Holding the screw on the second step and against the shoulder of the high step, there should be .043" clearance (Tool 6492) between top edge of choke valve and dividing wall in the air horn (fig. 12). Bend choke rod at lower angle if necessary to adjust.

Choke Unloader Adjustment

1. With choke trip lever contacting choke counterweight lever, hold throttle valve in the wide open position.
2. There should now be a clearance of .235" (Tool 6492) between top of choke valve and the dividing wall of the air horn (fig. 13). Bend tang on the fast idle cam with Tool 5197 to correct adjustment.

1. With the choke valve closed so that secondary lockout tang is in the fast idle cam slot, check clearance between fast idle cam and tang (fig. 14). The clearance should be .015".
2. Bend tang horizontally, using Tool 4552.

Secondary Throttle Contour Clearance

Adjustment

1. With choke valve held wide open and the fast idle cam and secondary lockout lever positioned as shown in Figure 15 there should be a clearance of .030" between the cam and the tang.
2. Bend tang vertically to adjust, using Tool 6058-A.

Removal

1. Remove air cleaner wing nut, air cleaner, gasket and stud.
2. Disconnect vacuum line, spark control line, fuel line and choke heat tube.
3. Disconnect throttle rod at carburetor and remove throttle return spring. (On Powerglide models, disconnect transmission control rod from throttle lever).
4. Remove four nuts and washers retaining carburetor to manifold. (On overdrive models, lay kick-down switch with bracket aside without disconnecting wires).
5. Lift carburetor off manifold.
6. Remove and discard carburetor to manifold gasket.

Choke Assembly

1. Mount carburetor on suitable holding fixture such as Tool 5923.
2. Remove the three choke cover attaching screws and retainers, then remove choke cover, gasket, and coil assembly from carburetor (fig. 18).
3. Carefully lift the baffle plate from the choke housing.
4. Unhook the spring clip from the upper end of the pump rod, and remove rod and clip from pump lever.
5. Remove the small retainer screw holding the trip lever to the choke shaft, then remove the trip lever.
6. Remove the two choke valve retaining screws and slide the choke valve from the shaft. Discard retaining screws.
7. Rotate choke shaft counter-clockwise to free choke piston from housing, and remove piston and choke shaft assembly by sliding from bowl cover.
8. Remove the choke housing attaching screws and remove choke housing and gasket from bowl cover.

Bowl Cover

1. Remove gasoline inlet fitting and inlet screen.
2. Perform procedure for "Disassembly of Choke Assembly".
3. Remove the 13 bowl cover to carburetor body attaching screws.
4. Carefully lift the bowl cover straight up until the float is clear of the carburetor body fig. 17.
5. Invert the bowl cover and remove the hinge pin from the primary float, then lift the float and needle from the air horn. Mark float assembly "primary" to prevent unnecessary bending during assembly.
6. Remove the primary float needle seat using a wide blade screwdriver. Remove seat gasket and strainer. NOTE: The float needle and seat are factory matched and must be installed in mated pairs. Never mix float needle and seats during cleaning or replacement.
7. Remove the hinge pin, float, needle seat, gasket and strainer from secondary side of bowl cover. Remove bowl cover gasket.
8. Remove the power piston by depressing the stem and allowing it to snap into position.
9. Remove the horseshoe type retainer from the pump plunger shaft.
10. Remove pump plunger by sliding the shaft through the rubber seal. Remove the rubber seal from the top side of the bowl cover casting.

Fig. 22

1. Remove the three attaching screws and lock washers from venturi cluster on the primary side, and carefully remove cluster and gasket. NOTE: The primary venturi cluster (fig. 19) contains the pump discharge nozzles and idle tube in addition to main well tubes, and must always be installed on primary side. The venturi clusters are serviced as an assembly.
2. Remove both main metering jets from primary (pump) side of carburetor body.
3. Remove power valve and gasket. NOTE: Do not mix metering jets. Primary main metering jets are different from secondary side.
4. Remove the three attaching screws and lock washers from venturi cluster on secondary side, and carefully remove cluster and gasket.
5. Remove both main metering jets from secondary side of carburetor body. Keep separate from primary jets.
6. Remove the pump return spring from the pump well, and turn the carburetor over to remove the pump inlet aluminum ball.
7. Using needle nose pliers remove "T" shaped pump discharge spring guide, then remove the small spring and steel ball (fig. 20).
8. If required, pump inlet strainer (fig. 21) may be removed by prying up with pointed tool.
9. Invert carburetor body (fig. 27) and remove four throttle flange to carburetor body attaching screws. Then remove throttle flange, and gasket from carburetor body. NOTE: Do not immerse auxiliary throttle valves in cleaning solvent. Clean with WARM WATER and brush only fig. 26.
10. Remove the secondary auxiliary throttle valve assembly from the carburetor body as shown in Figure 25. NOTE: Do not attempt any further disassembly of the auxiliary throttle valve assembly (Fig. 23). The spring tension is exactly calibrated and any change will completely upset the secondary side of the carburetion operation, primarily during speed wide open throttle.
11. The idle mixture needle screws may be removed for cleaning or replacement. The idle speed screw and spring can be replaced if necessary. NOTE: The throttle flange and levers are serviced as a unit. No further disassembly of throttle flange should be attempted as it may be impossible to again reassemble throttle valves correctly in relation to vacuum and idle discharge orifices.

Other than removal of the idle mixture screws, idle speed screw, and their springs (fig. 24), no disassembly of the throttle flange is necessary or should be attempted. Only these screws and their springs are serviced as replacement parts; the remainder of the throttle flange is serviced as a unit.

Cleaning and Inspection

Dirt, gum, water or carbon contamination in the carburetor or on the exterior moving parts of a carburetor are often responsible for unsatisfactory performance. For this reason, efficient carburetion depends upon careful cleaning and inspection while servicing.

1. Thoroughly clean carburetor castings and all metal parts in clean carburetor cleaning solution. Caution: Composition and plastic parts such as pump plunger, auxiliary valves and gaskets should not be immersed in cleaner.
2. Blow out all passages in castings with compressed air and blow off all parts so they are free of cleaner (be sure to follow instructions furnished with cleaning solution). CAUTION: Do not use drills or wire to clean out jets or passages as this may enlarge orifices and seriously affect carburetor calibration.
3. Carefully inspect parts for wear and replace those which are worn. Check the following specific points:
   • Inspect choke piston and choke piston housing for carbon. If necessary to clean choke piston housing, remove Welch plug in the top of housing. Plug can be removed by piercing center with a small instrument and prying outward. Care should be exercised so that damage will not result to the casting when removing this plug. Before installing new plug, carbon present in piston cylinder slots should be removed and the Welch plug seat should be carefully cleaned.
   • Remove carbon from bores of throttle flange with sandpaper; never use emery cloth.
   • Inspect float needles and seats for wear; if worn, both needle and seat must be replaced.
   • Inspect float pins for excessive wear.
   • Inspect floats for dents and excessive wear on lip or at hinge pin holes. Check for fluid inside floats by shaking. Replace float if any of above are present.
   • Inspect throttle shafts for excessive wear (looseness or rattle in body flange casting) replace flange assembly if worn excessively.
   • Inspect idle mixture adjusting screws for burrs. Replace if burred.
   • Inspect pump plunger assembly. If leather is not in good condition, replace plunger as a complete assembly.
   • Inspect the upper and lower surfaces of the carburetor body to see that the small sealing beads are not damaged. Damaged beading may result in loss of air or fuel.
   • Inspect holes in the pump rocker arm, fast idle cam and throttle shaft lever. If holes are worn excessively or out of round to the extent of improper operation of the carburetor, the worn parts should be replaced.
   • Inspect the steps on the fast idle cam for excessive wear. If excessive wear is noted, it should be replaced to assure proper warm-up and choking periods.
   • Check all filter screens for lint or dirt. Clean or replace as necessary.
   • Check venturi clusters for loose or worn parts. If damage or looseness exists, replace cluster assembly.

Carburetor Body

1. Place the carburetor body on the holding stand in an inverted position.
2. Install the auxiliary throttle valve assembly (with the calibrated spring operating pin) down into the base of the carburetor body.
3. Position the throttle flange gasket on the carburetor body so that all holes are properly aligned.
4. Place the throttle flange on the carburetor body and install four attaching screws, then tighten the 3/8" x 24 center screw 9 to 10 ft. lbs. and 12 x 28 outer screws 3 to 4 ft. lbs. torque.
5. Install the idle mixture screws and springs as they were removed from the throttle flange. Back off screws 1 - 2 turns for initial adjustment.
6. Place the carburetor upright on Tool 5923 and install steel pump outlet ball, spring and "T" shaped guide (fig. 28). NOTE: The aluminum inlet ball is 5/32" in diameter. The steel outlet ball is 3/16" in diameter.
7. Install the pump outlet aluminum ball, then the pump return spring in the pump plunger cylinder (fig. 29). NOTE: The spring must be all the way down and seated over the ball.
8. Replace the pump inlet screen if it was removed in disassembly.
9. Install the two primary main metering jets and the power valve.
10. Install the two secondary main metering jets.
11. Install the secondary venturi cluster and gasket with three attaching screws and lock washers. (The secondary venturi does not carry the idle tubes or pump discharge nozzles (fig. 19).
12. Install the primary venturi cluster and gasket using three attaching screws and lock washers.

1. Install the power piston into bowl cover and stake very lightly to hold piston in place.
2. Assemble rubber seal on pump plunger shaft.
3. Index pump plunger with rubber seal in the bowl cover by inserting the small end through from the bottom. The lips of the seal must be sealed on both sides of the bowl cover.
4. Position the gasket on the bowl cover.
5. Install needle seat gasket, then install inlet screen on lower shank of needle seat.
6. Install inlet screen and needle seat as assembly with wide bladed screwdriver. Repeat operation for other needle and seat. CAUTION: Needle seats must be installed on the same sides from which they were removed to match their respective Float needle.
7. Install both float and needle assemblies on the bowl cover, retaining in place with the hinge pin.

NOTE: The float level and float drop adjustments are made independent of each other and must be made at this stage of the assembly procedure. All other adjustments are made after the assembly of the carburetor has been completed. The float adjustments are outlined previously in this section under "Adjustments on the Car" of "Care, Maintenance and Adjustments".

Completion of Carburetor Assembly

1. Carefully guide bowl cover assembly on carburetor body, indexing pump plunger into well and closely observing alignment of floats and power piston shaft.
2. Align bowl cover and gasket to holes in body and START the 13 bowl cover attaching screws (fig. 30).
3. Tighten evenly and securely the two inner attaching screws, then tighten the remaining outside screws in the same manner.
4. Assemble the choke housing and gasket to the bowl cover and tighten with two countersunk head screws (fig. 31).
5. Install the choke shaft and piston assembly by carefully sliding the shaft through its bores. Then carefully fit the piston in the bore of the housing.
6. Rotate the choke shaft clockwise to check for free movement of the shaft and the piston (fig. 31).
7. Slide the choke valve through shaft so the letters "R.P." on the valve are facing up with the valve closed.
8. Start the two small brass choke valve to shaft attaching screws. DO NOT TIGHTEN.
9. Position the choke counterweight over the choke shaft.
10. Position the choke trip lever over the end of choke shaft. Be sure the small tab of the trip lever is above the choke counterweight tang and keyed to the choke shaft before tightening the retaining screw (fig. 32).
11. To provide correct fit of the choke valve in bowl cover, push lightly on the end of the choke shaft to obtain a minimum clearance of .020" between the trip lever and choke counterweight as shown in Figure 32.
12. While holding the choke shaft in this position, tighten choke valve retaining screws. Check for free operation of choke valve in air horn.
13. Place baffle plate in position in choke housing.
14. Install thermostat cover and coil assembly, gasket and retaining screws (with retainers). Leave housing cover retaining screws loose enough to permit rotating of cover.
15. Rotate cover counter-clockwise until coil engages tang.
16. Set the cover at the one notch "LEAN" mark. Tighten coil cover retaining screws.
17. Install the upper end of pump rod (with spring retaining clip) on the pump lever.
18. Perform the following operations in their sequence, as outlined previously under "Adjustments on the Car" of "Care, Maintenance and Adjustments".
    1. Pump Rod Adjustment
    2. Choke Rod Adjustment
    3. Choke Unloader Adjustment
    4. Secondary Throttle Lockout Adjustment
    5. Secondary Throttle Contour Clearance Adjustment

Test Before Installation On Engine

1. Be certain throttle flange body and intake manifold gasket surfaces are clean.
2. Install new carburetor to manifold gasket over manifold carburetor studs.
3. Start spark control pipe fitting into carburetor fitting.
4. Lower carburetor onto manifold studs, choke housing to right of vehicle. NOTE: If vehicle is overdrive equipped, install kickdown bracket on left front manifold stud and adjust for proper kickdown action.
5. Install and tighten four washers and nuts, using a short open end wrench.
6. Tighten spark control pipe fitting and connect and tighten choke heat tube, vacuum line and fuel line.
7. Install throttle rod and throttle return spring. (On Powerglide models, install transmission control rod).
8. Install air cleaner gasket, stud, air cleaner and wing nut. Tighten securely.
9. Perform idle speed and mixture adjustment as outlined in "Adjustments on the Car".

Carburetor - Rochester
Part No - 7008737
Throttle Bore
Primary - 1 5/16"
Secondary - 1 5/16"
Main Venturi
Primary - 1"
Secondary - 1 1/16"
Small Venturi
Primary - 1/4"
Secondary - 1/4"
Low Speed Jets
Idle Needle Orifice - .046"
Idle Tube Restrictions - .026"
Main Metering Jets
Primary - .048"
Secondary - .046"
Nozzle Clearance
Primary - 0
Secondary - 0
Main Well Vents - .032"
Power Valve Restrictions - .035"
Cluster Top Bleed - .034"
Cluster Side Bleed - .034"
Cluster Channel Restriction - .044"
Lower Idle Bleed - .033"
Second Idle Drilling - .038"
Third Idle Drilling - .027"
Fourth Idle Drilling - .028"
Spark Drillings - .052"
Choke Restriction - .098"
Choke Piston Restriction - .028"
Pump Jets - .028"
Pump Capacity per 10 Strokes - 18.5 to 21.5 cc's
Float Setting - Gasket to Float Lip
Drop Adjustment - 2 1/4"
Level Adjustment - 1 5/8"
Choke Setting - 1 Notch Lean
Pump Rod Adjustment - 1 1/16"
Choke Rod Adjustment - .043"
Unloader Adjustment - .0235"
Secondary Lockout Adjustment - .015"
Secondary Contour Clearance Adjustment - .030"

Carter WCFB (Four Barrel)

Carter WCFB (Four Barrel)

A Carter WCFB Climatic Control carburetor (fig. 1) is available as optional equipment on 8-cylinder engines. This carburetor is used in conjunction with a dual exhaust system and higher compression heads when higher engine performance is desired. Functionally, the Carter Model WCFB carburetor is two dual carburetors contained in one assembly and is built from four basic castings: choke housing, bowl cover, carburetor body, and throttle flange. The section containing the metering rods, accelerating pump and choke is termed the secondary side of the carburetor, and the other section is termed the secondary side. It has five conventional systems as have been used in previous carburetors: float system, low speed system, high speed system, pump system, and Climatic Control system (choke).

FLOAT SYSTEM

The float systems (fig. 2) maintain supply of fuel at the proper level in the fuel bowls for the low speed, high speed, pump, and choke systems. Primary and secondary bowls are separated by a partition. The fuel line connection is on the primary side. Fuel is supplied to the primary and secondary intake needles and seats through a passage in the bowl cover. A strainer located at the fuel inlet in the bowl cover prevents the entry of dirt and oxides.

Fuel level in the carburetor is maintained by needle valves and floats. As fuel is consumed, the floats descend with the fuel level allowing the float needles to unseat. The resulting opening allows fuel to flow into the bowls until the floats are lifted by the increased level and again seat the needles. Because of this critical function, the floats must be carefully adjusted both vertically and laterally. These adjustments are fully covered under "MAJOR SERVICE OPERATIONS."

Intake needles and seats are carefully matched during manufacture. Do not use the primary needle in the secondary seat or vice-versa. To avoid unnecessary bending, floats should be reinstalled in their original positions and then adjusted.

The bowls are vented to the inside of the bow cover. Bowl vents are calibrated to provide proper air pressure above the fuel at all times. To assure a positive seal, always use a new bowl cover gasket and a new dust cover gasket when reassembling as an air leak at either place can result in a mileage complaint.

LOW SPEED SYSTEM

Fuel for idle and early part throttle operation is metered through the low speed system (fig. 3). Fuel enters the idle wells through the metering rod jets on the primary side of the carburetor. No idle system is used in the secondary side of the carburetor.

The low speed jets measure the amount of fuel for idle and early part throttle operation. The air by-pass and idle air bleeds are carefully calibrated and serve to break up the liquid fuel and mix it with air as it moves through the passages to the idle ports and idle adjustment needle ports. Turning the idle adjusting needles toward their seats reduces the quantity of fuel mixture supplied by the idle system.

The idle ports are slot shaped. As the throttle valves are opened, more area of the idle ports is uncovered allowing a greater quantity of the fuel air mixture to enter the carburetor bores. The secondary throttle valves remain seated at idle.

The vapor vent, operated by the vapor vent arm on the countershaft, provides a vent for fuel vapors to escape from the carburetor bowls to the outside at idle and when the engine is not in operation.

All by-passes, idle ports, idle adjustment needle ports, as well as the bores of the throttle body must be clean and free of carbon. Obstructions will cause poor low speed engine operation.

HIGH SPEED SYSTEM

Fuel for part and full throttle operation is supplied through the high speed system (fig. 4).

Primary Side

The position of the metering rods (fig. 4) in the metering rod jet controls the amount of fuel flowing into the high speed system on the primary side of the carburetor. The position of the metering rods is dually controlled; mechanically by the movement of the throttle and by manifold vacuum applied to the vacumeter piston which is suspended on the metering rod hanger (fig. 6).

MECHANICAL OPERATION OF METERING RODS

During part throttle (high vacuum) operation, the position of the metering rods is controlled by the metering rod arm attached to the pump countershaft (fig. 5). Mechanical positioning of the rods is required to prevent fuel starvation as under high vacuum conditions the vacuum pull completely overcomes the vacumeter piston spring tension and the metering rods are pulled down into the metering rod jets.

Vacuum Operation of Metering Rods

When vacuum pull is less than the tension of the vacumeter piston spring, the metering rods (fig. 5) are moved toward their "wide-open" position. Low vacuum occurs during acceleration, hill climbing, and other engine load conditions. As the engine load decreases, manifold vacuum increases until once again mechanical metering rod action takes place.

Secondary Side

Fuel for the high speed system of the secondary side is metered by the secondary jets (fig. 4); no metering rods are used.

Throttle valves in the secondary side remain closed until the primary throttle valves have been opened a predetermined amount. This is accomplished by linkage between the throttle levers. Although the secondary throttle valve opening is delayed, both the primary and secondary throttle valves reach the "wide-open" position simultaneously. Mounted directly above the secondary throttle valves are the air velocity operated auxiliary throttle valves. These valves are normally held in the closed position by a counterweight and open only when air velocity through the secondary side is sufficient to permit efficient operation of both sides of the carburetor. This directs air through the primary side only during low speed wide open throttle operation permitting more efficient metering of air and fuel under these conditions. During choke operation, the secondary throttle valves are locked closed to insure faster cold engine starting and warm-up.

Anti-Percolator

To prevent vapor bubbles in the nozzle passages and low speed wells from forcing fuel out of the nozzles, anti-percolator passages (fig. 3) and calibrated plugs and bushings are used. Their purpose is to vent the vapors and relieve the pressure before it is sufficient to push the fuel out of the nozzles and into the intake manifold. Anti-percolator plugs and bushings are permanently installed and must not be remo