2.2.7 Holley Four-Barrel Carburetor

Fig. 57: Holley Four-Barrel Carburetor

Fig. 57: Holley Four-Barrel Carburetor

The Holley Four-Barrel Carburetor (Figs. 57 and 58) is a downdraft, two-stage carburetor. It can be considered as two, dual carburetors; one supplying a fuel-air mixture throughout the entire range of engine operation (primary stage) and the other functioning only when a greater quantity of fuel-air mixture is required (secondary stage).

Fig. 58: Holley Four-Barrel Carburetor - Bottom View

Fig. 58: Holley Four-Barrel Carburetor – Bottom View

The primary stage (front section) of the carburetor contains a fuel bowl, metering assembly, an accelerating pump assembly, and two barrels. The barrels each contain a primary and booster venturi, main fuel discharge nozzle, throttle plate, and idle system fuel passage. The choke plate, mounted in the air horn above the primary barrels, is controlled by an automatic choke mechanism mounted on the main body.

The secondary stage (rear section) of the carburetor contains a fuel bowl, metering assembly, secondary throttle operating diaphragm assembly, and two barrels. The barrels each contain a primary and booster venturi, main secondary fuel discharge nozzle, throttle plate, and a transfer system fuel passage.

Operation

To effectively provide the correct fuel-air mixture during all phases of engine operation, the carburetor is equipped with four primary stage metering systems and three secondary stage metering systems.

The primary stage metering systems are the idle fuel system, the accelerating system, the main fuel system, and the power fuel system. In addition, there is an automatic choke system which provides a means of temporarily enriching the mixture to aid in starting and operating a cold engine.

The secondary stage metering systems are the idle transfer system, secondary main fuel system, and the power fuel system.

A fuel inlet system for both the primary and secondary barrels provides the various fuel metering systems with a constant supply of fuel.

The difference in air pressures within the carburetor causes the proper discharge of fuel for various engine speed and load conditions.

FUEL INLET
Fig. 59: Primary Fuel Inlet System

Fig. 59: Primary Fuel Inlet System

A separate fuel inlet system consisting of a fuel bowl, a float assembly, and an inlet needle and seat assembly is provided for the primary and secondary barrels (Figs. 59 and 60). The fuel for both bowls first enters the primary fuel bowl. A fuel tube at the primary fuel inlet connects both fuel bowls. The operation of each fuel inlet system is the same.

Fig. 60: Secondary Fuel Inlet System

Fig. 60: Secondary Fuel Inlet System

The fuel enters the fuel bowl through the fuel inlet needle valve and seat assembly. The amount of fuel entering the fuel bowl is determined by the distance the needle valve is moved off of its seat and by fuel pump pressure. Movement of the needle valve in relation to the seat is controlled by the float and lever assembly which rises and falls with the fuel level. As the fuel level drops, the float lowers, opening the needle valve to allow fuel to enter the float chamber. When the fuel in the float chamber reaches a preset level, the float moves the needle valve to a position where it restricts the flow of fuel, admitting only enough fuel to replace that being used. Thus, any change in the fuel level causes a corresponding movement of the float, opening or closing the needle valve to maintain the preset level of fuel. The fuel inlet system must maintain this preset level, because the basic metering systems are calibrated to deliver the proper mixture only when the fuel is at the proper level.

A spring and pin, inside the hollow fuel inlet needle valve, cushions the needle valve against road shocks and vibrations. A float spring located under the float assists in keeping the float stable.

The float chamber is internally vented by the vent tube at all times. In addition, when the engine is stopped and at curb idle, the primary fuel bowl is vented externally by a vent on top of the primary fuel bowl. This vent provides an external release of excess fuel vapors from the bowl.

AUTOMATIC CHOKE

When a cold engine is being started, much of the fuel discharged by the carburetor is unable to vaporize during its travel to the combustion chamber until sufficient heat is developed in the intake manifold to maintain a homogeneous mixture for efficient combustion. Therefore, a much larger quantity of fuel must be supplied to compensate for this lack of vaporization when starting and running a cold engine.

The choke plate, located in the air horn above the primary stage venturis, when closed, provides a high vacuum above as well as below the throttle plates. With a vacuum above the throttle plates, fuel will flow from the main system as well as from the idle system, thus bringing about the extremely rich mixture necessary for cold engine operation.

Fig. 61: Automatic Choke System

Fig. 61: Automatic Choke System

The carburetor choke shaft is linked to a choke control housing, mounted directly on the main body (Fig. 61).

The thermostatic spring in the choke control housing will unwind when cold and wind up when warm. When the engine is cold, the thermostatic spring, through attaching linkage, holds the choke.plate in a closed position. Manifold vacuum channeled through a passage in the choke control housing, draws the choke vacuum piston downward, exerting an opening force on the choke plate. When the engine is started, manifold vacuum, acting directly on the piston located in the choke housing, and the flow of air acting on the offset choke plate, immediately moves the plate against the tension of the thermostatic spring to a partially open position to prevent stalling. As the engine continues to run, manifold vacuum, acting on the choke vacuum piston, draws air from the heat tube in the intake manifold where the air is warmed by the engine heat. The warmed air enters the thermostat housing and heats the thermostat spring, causing it to wind up. The air then flows through a passage and is exhausted into the air stream in the throttle body. The tension of the spring gradually decreases as manifold temperature rises, allowing the applied vacuum on the choke piston to further open the choke plates.

When the engine reaches its normal operating temperature, the spring no longer exerts an opposing tension on the choke piston, allowing the piston to pull the choke plate to the full open position. In this position, the choke piston is at its lowest point in the cylinder. Slots in the piston will allow sufficient air to bleed past the piston and into the intake manifold, causing a continual flow of warm air to pass through the thermostat housing. The thermostat spring thus remains heated and the choke plate remains fully open until the engine is stopped and allowed to cool.

The choke rod at the carburetor actuates a fast idle cam during choking. The fast idle cam has a series of steps on one edge designed to increase the idle rpm for smoother running when the engine is cold. As the choke rod is moved through its range of travel from the closed position to the open position, the fast idle cam rotates, presenting successive steps to a throttle stop screw. Each step permits a slower idle rpm as engine temperature rises and choking is reduced.

If the engine should reach the stall point, due to a lean mixture, during the warm-up period, manifold vacuum will drop considerably. The tension of the thermostat spring then overcomes the lowered vacuum acting on the choke piston, and the choke plate will be moved toward the closed position, providing a richer mixture to help prevent engine stalling.

IDLE SYSTEM
Fig. 62: Idle Fuel System

Fig. 62: Idle Fuel System

At idle and low speed operation, the engine does not draw sufficient air through the primary booster venturi to create a vacuum great enough to operate the main metering system. Therefore, an idle fuel system is provided, which is not dependent upon venturi vacuum, to discharge fuel (Fig. 62). At idle and low engine speeds, intake manifold vacuum is high. This high manifold vacuum provides a pressure differential great enough to operate the idle fuel system.

The carburetor has two identical idle systems, one for each primary barrel. Since the two systems function identically, only one side will be discussed.

At idle speed, the normal air pressure in the float chamber causes fuel to flow through the idle fuel system passages to the greatly reduced pressure area (vacuum) below the throttle plate. Fuel flows from the float chamber through the main jet and into a small angular passage that leads across to the idle well. The fuel flows up the idle well where it is mixed with air from the idle air bleed. The fuel-air mixture flows through a short horizontal passage and then down another vertical passage. At the bottom of this vertical passage the fuel-air mixture branches in two directions, one through the idle discharge passage and the other to the idle transfer passage.

The fuel in the idle discharge passage flows past the idle adjusting needle which controls the mixture delivered at idle. From the idle adjusting needle chamber, the fuel goes through a short passage, past an asperating restriction in the main body. The fuel is discharged into the throttle bore below the throttle plate.

Fig. 63: Transfer System

Fig. 63: Transfer System

During off-idle operation when the throttle plate is moved slightly, the fuel flows through the idle transfer passage from the main metering assembly into the main body passage through a restriction and then into the throttle body passage (Fig. 63). As the idle transfer slot is exposed to manifold vacuum, fuel is discharged into the throttle bore.

As the throttle plate is opened still wider and engine speed increases, the air flow through the carburetor is also increased. This creates a vacuum in the booster venturi great enough to bring the main metering system into operation. The flow from the idle system begins tapering off as the main metering system begins discharging fuel. The two systems provide a smooth gradual transition from idle to cruising speeds.

ACCELERATING SYSTEM
Fig. 64: Accelerating System

Fig. 64: Accelerating System

Upon acceleration, the air flow through the carburetor responds almost immediately to the increased throttle opening. There is, however, a brief interval before the fuel, which is heavier than air, can gain speed and maintain the desired balance of fuel and air. During this interval, the accelerating pump supplies fuel until the other systems can once again provide the proper mixture (Fig. 64).

The accelerating pump is located in the bottom of the primary fuel bowl. The pump begins to function when the pump operating lever is actuated by throttle movement. When the throttle is opened, the pump linkage, actuated by a cam on the primary throttle shaft, forces the pump diaphragm up. As the diaphragm moves up, the pump inlet ball check is forced on its seat preventing fuel from flowing back into the float chamber. The fuel flows from the short passage in the fuel bowl into the long diagonal passage in the primary metering assembly. The fuel passes into the main body and then into the pump discharge chamber. The pressure of the fuel causes the discharge ball check and weight to raise and fuel is discharged into the venturi.

As the throttle is moved toward the closed position, the linkage returns to its original position and the diaphragm spring forces the diaphragm down. As the diaphragm returns to its original position the pump inlet ball check is moved off its seat and the diaphragm chamber is filled with fuel from the float bowl.

MAIN METERING SYSTEM
Fig. 65: Main Metering System

Fig. 65: Main Metering System

As engine speed increases, the air passing through the booster venturi creates a vacuum. The amount of vacuum is determined primarily by the air flow through the venturi, which in turn is regulated by the speed of the engine. The pressure differential in the venturi and fuel bowl causes fuel to flow through the main metering system (Fig. 65).

At a predetermined throttle opening, fuel flows from the fuel bowl, through the main jets, and into the bottom of the main well. The fuel moves up the main well past the main well air bleeds in the side of the well. Filtered air enters through the main metering air bleeds in the main body and then into the main metering assembly by interconnecting passages. This mixture of fuel and air, being lighter than raw fuel, responds faster to any change in venturi vacuum and vaporizes more readily when discharged into the air stream of the venturi. The mixture of fuel and air moves up the main metering passage and passes into the short horizontal passage leading to the main body, then through the horizontal channel of the discharge nozzle. The fuel is discharged into the booster venturi and then in the air stream of the carburetor venturi.

POWER FUEL SYSTEM
Fig. 66: Power Fuel System

Fig. 66: Power Fuel System

During periods of high power operation, the carburetor has a tendency to lean out as the air flow is increased. To supplement the main and secondary fuel systems, additional fuel is required to maintain the proper fuel-air ratio. The added fuel required during this period is supplied by the power fuel system (Fig. 66).

The power fuel system is controlled by manifold vacuum, which gives an accurate indication of the power demands placed on the engine. Manifold vacuum is highest at idle speed and decreases as the load on the engine is increased.

Manifold vacuum is transmitted from an opening in the base of the throttle body, through a passage in the throttle body, and main body to the power valve chamber in the main metering assembly. The manifold vacuum, acting on the diaphragm at idle or normal load conditions, is strong enough to overcome the tension of the power valve spring and holds the diaphragm closed. When high power operation places a greater load on the engine and manifold vacuum drops below a predetermined value, the power valve spring overcomes the reduced vacuum and opens the power valve. Fuel flows from the float chamber, through the valve and out the small holes in the side of the valve through the diagonal restrictions in the main metering body and then into the main well. In the main well, the fuel joins the fuel flow from the main metering system, enriching the mixture.

As engine power demands are reduced, manifold vacuum increases. The increased vacuum overcomes the tension of the power valve spring and draws the valve diaphragm closed. This closes the power valve and shuts off the added fuel supply.

SECONDARY STAGE

To provide sufficient fuel-air mixture to operate the engine at maximum power, the mixture supplied by the primary stage of the carburetor is supplemented by an additional quantity of fuel-air mixture from the secondary stage of the carburetor.

Fig. 67: Secondary Throttle Operation

Fig. 67: Secondary Throttle Operation

This additional supply of fuel-air mixture is delivered through the two secondary (rear) barrels of the carburetor. The secondary stage throttle plates are operated by a spring-loaded vacuum diaphragm assembly attached to the side of the main body and linked to the secondary throttle shaft (Fig. 67).

Opening of the secondary throttle plates is controlled by vacuum from the right primary and secondary Venturis. At high speeds when engine requirements approach the capacity of the two primary barrels, the increased vacuum at the right primary venturi moves the secondary diaphragm, compressing the diaphragm spring. The diaphragm, acting through the diaphragm link and lever, starts to open the secondary throttle plates. The position of the throttle plates depends upon the strength of the vacuum. This in turn, is determined by the air-flow through the barrels. As the secondary throttle plates begin to open, the vacuum in the secondary barrels increases. The increased vacuum in the right secondary venturi assists the primary venturi vacuum in further opening the secondary throttle plates. As top speed is reached, the secondary throttle plates will approach wide open.

The bleed past the ball check valve in the vacuum passage of the carburetor limits the rate at which the secondary throttle plates will open. Any rapid increase in vacuum which would tend to open the secondary throttle plates too suddenly merely holds the ball check valve securely against its seat. The opening of the throttle plate is slowed to a rate governed by the amount of air passing through an air bleed in the check valve seat. This allows the vacuum to build up slowly at the diaphragm which results in a controlled rate of opening for the secondary throttle plates.

When engine speed is reduced, venturi vacuum in the barrels becomes weaker. The momentarily stronger vacuum at the secondary throttle operating diaphragm moves the ball check valve off its seat in the vacuum passage, permitting an immediate flow of air into the diaphragm chamber. As the vacuum acting on the diaphragm is lessened, the load on the diaphragm spring will start closing the secondary plates. The diaphragm spring is assisted by the design of the secondary plates. Each secondary plate is slightly offset. When the plates are closing, the combined force of manifold vacuum and the air stream has greater effect on the larger, upstream area of the plates forcing them to a closed position. The secondary plates are retained in the closed position when the primary plates are fully closed by the secondary throttle connecting rod. This rod, which is fastened to the primary throttle lever, rides in a slot in the secondary throttle lever.

The secondary system is supplied with fuel from the secondary fuel bowl.

The three secondary stage fuel systems are the transfer system, main metering system, and the fuel enrichment system.

The transfer system begins to function when the secondary throttle plates begin to open. As the platesbegin to open, the fuel flows through the secondary main jets into the transfer passages which are similar to those in the primary metering assembly.

When the secondary throttle plates are opened further the pressure differential causes the secondary main metering system to begin discharging fuel. The passages in this system are identical to those in the primary main metering system (Fig. 65).

When the secondary throttle plates are wide open the secondary fuel enrichment system starts to operate. This system is identical to the primary power fuel system (Fig. 66). The fuel flows into the power valve through the metering assembly into the main body and is discharged in the secondary booster venturi.

These three secondary stage fuel systems provide a smooth transition of power instead of a sudden surge.

Carburetor Removal

  1. Remove the air cleaner. Remove the throttle rod from the throttle lever. Disconnect the choke control heat tube, the fuel line, and the distributor vacuum line from the carburetor.
  2. Remove the four nuts and lock washers which secure the carburetor to the manifold, then remove the carburetor. Remove the spacer and two gaskets from the manifold.
  3. Install bolts about 2¼ inches of the correct diameter through the carburetor retaining bolt holes with a nut above and below the flange (or install carburetor legs) to facilitate working on the carburetor and prevent damage to the throttle plates.

Carburetor Disassembly

Use a separate container for the component parts of the various assemblies to facilitate cleaning, inspection, and assembly.

SEPARATE MAIN BODY AND THROTTLE BODY
  1. Remove the air cleaner anchor screw and lock-washer, and remove the secondary diaphragm link retainer.
  2. Invert the carburetor and remove the throttle body to main body screws and lockwashers. Lift off the main body and remove the main body gasket.
PRIMARY FUEL BOWL AND METERING ASSEMBLY
  1. Remove the fuel inlet fitting, gasket, and screen, then remove the primary fuel bowl retaining screws. The fuel bowl, fuel bowl gasket, metering assembly, and metering assembly gasket will slide off the main body. Remove the fuel line tubing and discard the “O” ring seals.
  2. Separate the main metering assembly from the fuel bowl and discard the gasket, then remove and discard the main metering assembly gasket.
  3. Remove the vent valve, the idle adjusting needles and gaskets, the power valve and gasket, and the jets from the metering assembly.
  4. Remove the external vent linkage retainer and spring, then remove the vent and linkage from the fuel bowl. Remove the accelerating pump cover assembly, then remove the diaphragm and spring. Remove the float retainer, then lift the float and spring out of the fuel bowl. Remove the “O” ring seal from the fuel valve seat assembly, then remove the baffle plate. Remove the fuel valve seat and let the valve, spring, and pin fall into the hand.
SECONDARY FUEL BOWL AND METERING ASSEMBLY
  1. Remove the secondary fuel bowl retaining screws, then remove the fuel bowl from the main body. Separate the secondary main metering assembly and gasket from the fuel bowl, then remove the secondary main metering assembly gasket.
  2. Remove the vent valve, the jets, and the power valve and gasket from the metering assembly.
  3. Remove the float retainer, then lift the float and spring out of the fuel bowl. Remove the “O” ring seal from the fuel valve seat assembly, then remove the baffle plate. Remove the fuel valve seat assembly and let the valve, spring, and pin fall into the hand.
CHOKE PLATE AND CHOKE HOUSING
  1. Remove the choke rod retainer from the choke housing lever assembly. Remove the thermostat housing and gasket, then remove the choke housing and gasket from the main body.
  2. Remove the choke housing shaft nut, lock washer, and spacer, then remove the choke housing lever and shaft assembly and fast idle cam assembly. Remove the choke piston and lever assembly.
  3. Remove the choke rod retainer from the choke lever. Remove the choke rod, washers, and seal from the main body. Remove the choke plate from the choke shaft, then slide the shaft and lever out of the choke housing.
SECONDARY DIAPHRAGM HOUSING

Remove the secondary diaphragm housing and gasket from the main body, The housing must be removed before the cover can be removed. Remove the diaphragm housing cover, then remove the spring and diaphragm from the housing.

ACCELERATING PUMP DISCHARGE

Remove the pump discharge nozzle screw, then lift the pump discharge nozzle and gasket out of the main body. Invert the main body and let the pump check weight and ball fall into the hand.

THROTTLE BODY
  1. Remove the dashpot assembly, then remove the pump operating lever retainer and remove the pump operating lever. Remove the secondary throttle connecting rod and washer,
  2. Remove the secondary diaphragm lever assembly. Remove the fast idle pick-up lever and the fast idle cam lever and spring.
  3. If it is necessary to remove the primary throttle plates, lightly scribe both plates along the throttle shaft and mark each plate and its corresponding bore with a number or letter for proper installation. Remove the throttle plates. Remove the two throttle connecting rod retainers and the washer. Slide the primary throttle shaft out of the throttle body, then remove the throttle connecting rod and throttle return spring.
  4. If it is necessary to remove the secondary throttle plates, mark the plates as described for the primary throttle plates, then remove the plates and slide the secondary throttle shaft out of the main body.

Carburetor Cleaning and Inspection

Wash all the carburetor parts (except the accelerating pump diaphragm, the power valves, the secondary throttle operating diaphragm, and the dashpot) in clean solvent. Rinse the parts in hot water to remove all traces of the cleaning solvent, then dry them with compressed air.

Make sure all the carbon is removed from the choke piston and piston bore. Force compressed air through all passages of the throttle body, main body, choke housing, secondary diaphragm housing, and metering assemblies. Wipe all parts that cannot be immersed in solvent with a clean, soft, dry cloth. Do not use a wire brush to clean any parts or wire to clean out any openings or passages in the carburetor. A drill or wire may enlarge the hole or passage, thus changing the calibration of the carburetor. Discard all gaskets, compression-type washers, and “O” ring seals.

Check the choke shaft for wear, and excessive looseness or binding in the air horn. Inspect the choke plate for nicked edges. Replace the choke shaft and/or the choke plate if necessary.

If the primary or the secondary throttle shafts are excessively loose or bind in the throttle body, or if the plates are burred preventing proper closure, replace them.

Inspect the throttle body, main body, metering assemblies, fuel bowl, choke housing, choke thermostat housing and the secondary diaphragm housing for cracks.

Check the floats for leaks by holding them under water that has been heated to just below the boiling point. Bubbles will appear if there is a leak. If a float leaks, replace it. Polish the needle contact surface of the float arm. Replace the float if the arm contact surface is grooved.

Inspect the fuel inlet needle valves and seats. Replace both parts if either part is defective as they are matched assemblies.

Inspect all diaphragms for cuts, tears, etc., and replace any that are defective.

Inspect all screws and nuts for stripped threads, replace any that are defective.

Replace all distorted or broken springs.

Inspect the discharge nozzles, metering jets, and idle restrictions. If any of the openings are blocked, open them up with compressed air.

If the screwdriver slots or threads of the metering jets are found to be damaged or burred, replace them.

Check the accelerator system discharge nozzles. If the holes are blocked, open them with compressed air. Blow out the pump discharge passages in the main body. Check the accelerator discharge ball weight. If the weight is nicked or grooved, replace it. Check the accelerator pump ball check valve. If the ball is nicked or corroded, replace the ball. Check the condition of the ball seat. Check the accelerating pump fuel inlet ball check in the primary fuel bowl for freeness in its seat, nicks, or corrosion. If it binds or is defective replace the fuel bowl because the inlet ball is not removable.

Clean the idle adjusting needle threads, and inspect the threads and needle faces. Replace the needles if they are grooved or bent.

Inspect the dashpot rubber boot for proper installation in the groove of the stem bushing. Check the stem movement for smooth operation. Do not lubricate the stem. Replace the dashpot if it is defective.

Inspect all gasket surfaces. Repair or replace any parts that are damaged.

Carburetor Assembly

Fig. 68: Holley 4-Barrel Carburetor

Fig. 68: Holley 4-Barrel Carburetor

Install all the new gaskets and parts furnished in the carburetor overhaul kit. Make sure all holes in the new gaskets have been properly punched and that no foreign material has adhered to the gaskets. Make sure the accelerating pump and secondary operating diaphragm are not cut or torn.

A disassembled view of the carburetor is shown in Fig. 68.

PRIMARY FUEL BOWL AND METERING ASSEMBLY
Fig. 69: Primary Fuel Bowl Assembly

Fig. 69: Primary Fuel Bowl Assembly

Fig. 71: Secondary Fuel Bowl and Metering Assemblies

Fig. 71: Secondary Fuel Bowl and Metering Assemblies

Refer to Fig. 69 for the correct location of the fuel bowl parts. The primary and secondary fuel bowls and metering assemblies are installed on the main body in the same manner (Fig. 71).

  1. Place the fuel inlet valve spring over the valve pin and insert these parts, spring first, into the hollow valve. Place the assembly in the fuel seat, then install the seat and gasket in the fuel bowl. Position the baffle plate over the fuel inlet valve seat, then install the baffle retainer. Position the float spring on the boss on the float, then install the float on the shaft. Install the float retainer. Refer to Carburetor Bench Adjustments and check the float setting.
  2. Place the accelerating pump diaphragm spring and diaphragm in the accelerating pump chamber. Install the cover with the screws finger tight. Make sure the diaphragm is centered, then compress the diaphragm with the pump operating lever and tighten the cover screws. After the carburetor is assembled, refer to “Carburetor Bench Adjustments” for the correct adjustment of the accelerating pump.
  3. Install the power valve and gasket, jets, and the idle fuel adjusting needles and springs in the metering assembly. The primary power valve can be identified by the number “85” stamped on it. Turn the idle adjusting needles in gently, until they just touch the seat, then back them off 1 turn for preliminary idle adjustment.
  4. Place the metering assembly gasket on the dowels on the back of the metering assembly and insert the bowl vent valve in position on top of the metering assembly, then lay the assembly in place on the main body. Position the bowl to metering assembly gasket on the dowel on the fuel bowl. Be sure the small hole in the bottom of the gasket is aligned with the hole in the fuelbowl. Lay the bowl in place. Install the compression gaskets and retaining screws.
  5. Lubricate the fuel line “O” ring seal and place the seel against the flange on one end of the fuel line and install this end of the line in the fuel bowl.
  6. Position the external vent and linkage on the fuel bowl, then install the spring and retainer.
SECONDARY FUEL BOWL AND METERING ASSEMBLY
Fig. 70: Secondary Fuel Bowl Assembly

Fig. 70: Secondary Fuel Bowl Assembly

Refer to Fig. 70 for the correct location of the fuel bowl parts, and refer to Fig. 71 for the correct order of installing the fuel bowl and metering assembly on the main body.

  1. Place the fuel inlet valve spring over the valve pin and insert these parts, spring first, into the hollow valve. Place the assembly in the fuel seat, then install the seat and gasket in the fuel bowl. Position the baffle plate over the fuel inlet valve seat, then install the baffle retainer. Position the float spring on the boss on the float, then install the float on the shaft. Refer to Carburetor Bench Adjustments and check the float setting.
  2. Install the power valve and gasket and the jets in the metering assembly. The secondary power valve can be identified by the number “105” stamped on it.
  3. Position the metering assembly gasket on the dowels on the back of the metering assembly and insert the bowl vent valve in position on top of the metering assembly, then lay the metering assembly in place on the main body. Lubricate the fuel line “O” ring seal and place it against the flange on the fuel line. Position the bowl to metering assembly gasket on the fuel bowl. Lay the bowl in place on the metering assembly, guiding the fuel line into the recess on the bowl. Install the compression gaskets and retainer screws.
ACCELERATING PUMP DISCHARGE
Fig. 72: Accelerating Pump Discharge Assembly

Fig. 72: Accelerating Pump Discharge Assembly

Refer to Fig. 72 for the correct location of parts.

Drop the accelerator pump discharge ball check into its well. Seat the ball with a brass drift and a light hammer. Make sure the ball is free, then drop the ball check weight into the well. Position the nozzle gasket and nozzle on the discharge well, then install the nozzle retaining screw.

SECONDARY DIAPHRAGM HOUSING
Fig. 73: Secondary Diaphragm Assembly

Fig. 73: Secondary Diaphragm Assembly

Refer to Fig. 73 for the correct position of the parts. The secondary diaphragm housing must be installed before the choke housing.

  1. Position the secondary diaphragm in the housing and place the spring on the diaphragm disc, then install the cover with the screws finger tight. Pull the diaphragm rod as far as it will go and tighten the cover screws. The diaphragm housing must be removed from the main body to install the cover.
  2. Place the gasket on the secondary vacuum passage opening on the main body. Lay the diaphragm housing in position on the main body and install the lockwashers and retaining screws.
CHOKE PLATE AND CHOKE HOUSING
Fig. 74: Choke Plate and Housing Assembly

Fig. 74: Choke Plate and Housing Assembly

Refer to Fig. 74 for the correct location of parts.

  1. Position the choke plate shaft in the air horn, then install the choke plate on the shaft. Install the rod seal between the two brass washers on the choke rod. Slide the choke plate rod through the opening in the main body and secure it to the choke lever with a cotter pin. Push the rod seal and washers against the underside of the air cleaner flange between the retaining flanges on the main body.
  2. Install the fast idle cam assembly on the brass bushing on the back of the choke housing with the bushing on the fast idle cam facing outward. Position the piston assembly in the choke housing, then install the choke housing shaft and lever assembly and secure the lever and piston assembly to it with the spacer, lock washer, and nut.
  3. Lay the main body assembly on its side and position the gasket on the vacuum passage opening on the main body. Secure the choke rod to the choke housing shaft lever with a cotter pin. Place the choke housing in position on the main body, then install the lock washers and screws. Place the thermostat housing gasket in position on the choke housing, engage the thermostat spring on the spring lever, then install the thermostat housing, clamp and screws. Adjust the thermostat housing to the mid position mark.
THROTTLE BODY
  1. Slide the secondary throttle shaft assembly into the throttle body.
  2. Referring to the line scribed on the throttle plates, install the plates in their proper location with the screws snug, but not tight. Little or no light should show between the throttle plates and the throttle bores. If the throttle plates are properly installed and there is no binding when the throttle shaft is rotated, tighten the throttle plate screws.
  3. Place the primary throttle return spring on the throttle shaft. Place the smallest bend of the throttle connecting rod in position in the throttle lever. Slide the throttle shaft into the throttle body. Guide the connecting rod so that the largest bend will be in place in the secondary throttle shaft lever. Position the return spring so that the small hook fits into the slot alongside the throttle lever adjusting screw, while the other end rests against the stud which houses the throttle stop screw.
  4. Install a washer on the secondary end of the connecting rod, then secure both ends with a pin retainer.
  5. Install the primary throttle plates using the same procedure as for the secondary throttle plates.
  6. Place the fast idle cam spring inside the fast idle cam lever and position the lever on its stud. Install the fast idle pick-up lever on the stud, with the longest end of the spring resting on the longest arm of the pick-up lever.
  7. Install the secondary diaphragm lever, the dashpot assembly, and the accelerating pump actuating lever and retainer.
ASSEMBLE MAIN BODY TO THROTTLE BODY

Invert the main body and position the main body to throttle body gasket on the main body. Place the throttle body on the main body so that the fuel inlet fitting is on the same side as the dashpot. Slide the secondary diaphragm rod into the operating lever and secure it with the retainer as the throttle body is placed in position. Be sure the accelerating pump diaphragm lever rests on the adjusting screw. Install the throttle body to main body screws and lockwashers. Install the air cleaner anchor screw.

Carburetor Installation

  1. Be sure all old gasket material is removed from the manifold heat riser flange, then place the spacer between two new gaskets and position them on the manifold. Position the carburetor on the manifold and install the lock washers and nuts. Tighten the nuts alternately to 12-15 foot-pounds torque.
  2. Connect the throttle rod, the choke heat tube, the fuel line, and the distributor vacuum line. Place the air cleaner on the carburetor and tighten the wing nut. Refer to “Carburetor In-Chassis Adjustments” and adjust the idle speed, idle fuel mixture, and the dashpot.

Carburetor Bench Adjustments

FLOAT ADJUSTMENT
Fig. 75: Float Setting

Fig. 75: Float Setting

If the floats are checked with the carburetor installed on the engine, remove one lower bowl retaining bolt and drain the fuel in a suitable container before removing the bowl.

Invert the fuel bowl and check the float setting (Fig. 75) with the cardboard gauge (Tool-9590-10) provided in the carburetor repair kit.

One side of the gauge is used to check the primary float setting and the other side is used to check the secondary float setting. Bend the gauge tabs as shown. If a gauge is not available, the float should be set so that there is 13/16 inch clearance (primary float) or 3/4 inch clearance (secondary float) between the bottom of the float and floor of the float bowl measured at a point on the float 11/16 inch from the inside of the float bowl (at the end farthest from the fuel inlet needle) and ½ inch in from the gasket mounting surface. If necessary bend the tab on the float arm to bring the float setting within limits. This should provide the proper fuel level.

PRIMARY FUEL BOWL EXTERNAL VENT ADJUSTMENT

With the throttle plates in the closed position, the clearance between the vent button and the top of the fuel bowl should be 1/16 inch. To adjust the clearance, bend the end of the horizontal arm on the accelerating pump lever.

ACCELERATING PUMP ADJUSTMENTS

With the primary throttle plates in the wide open position there should be a clearance of 0.015 inch between the pump lever adjusting screw head and the pump arm when the pump arm is fully depressed manually. Turn the adjusting screw “in” to increase the clearance and “out” to decrease the clearance. One-half turn of the adjusting screw is equal to 0.015 inch.

To satisfy acceleration requirements in various climates the accelerating pump cam can be placed in one of two positions. Aligning the top hole in the cam with the top hole in the throttle lever will give the longest stroke which is recommended for cold weather operation. Aligning the bottom hole in the cam with the bottom hole in the throttle lever gives the shortest stroke which is recommended for warm weather or average conditions.

Carburetor In-Chassis Adjustments

IDLE SPEED ADJUSTMENT

The engine idle speed must be adjusted to the proper hot and cold setting.

Hot Engine Idle Speed
Fig. 76: Idle and Dashpot Adjustments

Fig. 76: Idle and Dashpot Adjustments

Adjustment of the left side stop screw controls the hot engine idle speed (Fig. 76). Clockwise rotation increases the engine idle speed and counterclockwise rotation decreases it.

Set the hand brake. Place the transmission selector lever in the neutral position. Operate the engine until the temperature has stabilized, and the choke fast idle cam is in the slow position.

Back off the choke fast idle adjustment screw from the fast idle cam, then turn the hot engine idle adjustment screw in a direction to obtain 475-500 rpm. Open the throttle by hand and allow it to close normally. Recheck the hot engine idle speed.

On vehicles equipped with Fordomatic, set the hand brake and place the selector lever in Dr (drive) position. Check the engine idle speed. Adjust the idle speed to 425-450 rpm in Dr (drive) position if necessary.

Adjust the cold engine idle speed setting.

The adjustment screw, on the right side of the carburetor contacts steps on the fast idle cam during the engine warm-up period and controls the cold engine idle speed (Fig. 76).

Adjust the hot engine idle speed to the recommended rpm before attempting to set the cold engine idle speed. Make this adjustment with the engine at normal operating temperature.

With the fast idle cam in the slow position, turn the cold engine idle speed adjustment screw in until it just touches the lowest step on the fast idle cam, then back it off ¼ turn. In localities where normal setting of the cold engine idle speed may be considered unnecessarily high, the cold engine idle speed may be reduced by backing off the adjusting screw not in excess of one full turn.

IDLE MIXTURE ADJUSTMENT

The idle fuel mixture is controlled by the idle mixture adjustment needles (Fig. 76). Turn the needles “in” to lean the mixture, and “out” to enrich the mixture. Make the initial mixture adjustment by turning the screws in until they lightly touch the seat, then back them off 1 turn. Do not turn a needle against the seat tight enough togroove the point. If a needle is damaged, it must be replaced before a proper mixture adjustment can be obtained.

Run the engine at fast idle speed for 20 minutes to bring it to normal operating temperature.

Turn the mixture needles in until the engine begins to run rough from the lean mixture. Turn the needles out until the engine begins to “roll” from the rich mixture. Then, turn the needles in until the engine runs smoothly. Always favor a slightly rich mixture rather than a lean setting.

It may be necessary to reset the idle speed after the correct idle mixture is obtained.

ANTI-STALL DASHPOT ADJUSTMENT

Adjust the engine idle speed, then loosen the dashpot locknut. Hold the throttle in the closed position and depress the dash-pot plunger with a screwdriver blade, then turn the dashpot adjustment nut in a direction to provide the specified clearance of 0.045-0.064 inches (Fig. 76). Tighten the locknut to secure the adjustment.

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One Response to 2.2.7 Holley Four-Barrel Carburetor

  1. Jim says:

    Excellent! I needed to see the exploded diagram to refresh my memory. I’m 79 yrs. old and fifty plus years ago were my hotrod days. Now working on a 1987 Ford RV with a Holly carburetor problem. Thanks for the info.

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