2.2.3 Holley Single-Barrel Carburetor

Fig. 2: Holley Single-Barrel Carburator

Fig. 2: Holley Single-Barrel Carburator

The Holley single-barrel carburetor (Fig. 2), is used on all 6-cylinder engines. It consists of two main assemblies, the main body and the throttle body.

The main body contains the float, fuel-inlet valve, fuel bowl, the carburetor air inlet, the main and booster venturi, the choke plate, the main well body, the power fuel assembly, and the accelerating pump assembly. The main well body contains the majority of the fuel passages and the fuel metering parts.

The throttle body houses the throttle plate, the idle discharge ports, the idle speed screw, and the idle fuel mixture adjustment needle.

Operation

The carburetor has four fuel metering systems to provide the correct fuel-air mixture for all phases of engine operation. The four systems are: the idle fuel system, the main fuel system, the accelerating system, and the power fuel system. In addition, a fuel inlet system regulates the fuel supply to the various systems, and a manual choke provides an enriched mixture to aid in starting and running a cold engine.

FUEL INLET SYSTEM
Fig. 3: Fuel Inlet System

Fig. 3: Fuel Inlet System

Fuel under pressure from the fuel pump enters the float chamber through the fuel inlet needle valve and seat assembly (Fig. 3).

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 admit fuel.

When the fuel in the float chamber reaches a pre-set level, the float moves the needle valve to a position where it restricts the flow of fuel into the float bowl. Changes in the fuel level cause a corresponding movement of the float which opens or closes the needle valve to maintain the pre-set fuel level. This level must be maintained because the carburetor is calibrated to deliver the proper mixture only when the fuel is at this level.

A spring and pin are located in the hollow needle valve to cushion the valve against vibrations. A clip, to assure reaction of the valve to any float movement, is attached to the valve and float.

IDLE FUEL SYSTEM
Fig. 4: Idle Fuel System

Fig. 4: Idle Fuel System

During idle, fuel passes through the main jet into the bottom of the main well (Fig. 4). High manifold vacuum acting through the idle passages draws fuel from the main well through a short horizontal passage into the idle well.

The fuel is metered through a calibrated restriction, at the top of the idle well, on its way into the idle channel. Air is introduced through an idle air bleed at the top of the idle channel. The air bleed also acts as a vent to prevent siphoning at high speeds or when the engine is stopped. The fuel-air mixture travels down the idle channel past two idle transfer holes in the throttle body and is discharged through the idle discharge hole below the closed throttle plate. As the throttle plate is moved past the two transfer holes, during off idle, each hole begins to discharge fuel as it is exposed to manifold vacuum. The transfer holes act as additional air bleeds at idle.

Fuel discharge at idle is controlled by an idle adjusting needle which seats in the discharge hole.

MAIN FUEL SYSTEM
Fig. 5: Main Fuel System

Fig. 5: Main Fuel System

The velocity of air flow through the carburetor increases as the throttle plate opens. When this causes a great enough pressure drop in the venturi, the main metering system starts flowing. Fuel from the idle system tapers off as the main system begins discharging fuel. At this time, there is a definite blend of the idle and main systems.

Fuel passes through the main jet into the bottom of the main well and flows up the main well (Fig. 5). Filtered air from the carburetor air inlet passes through the high speed air bleed into the air bleed well and enters the fuel in the main well through three short horizontal air passages. This mixture of fuel and air, being lighter than raw fuel, responds faster to any change in venturi pressure and also vaporizes more readily when it is discharged. The fuel continues up the main well and flows into the main discharge nozzle where it is sprayed onto the open choke plate and the walls of the booster venturi. Here, the mixture is vaporized and mixed with the air stream passing through the booster venturi. It then passes the throttle plate into the intake manifold.

ACCELERATING SYSTEM
Fig. 6: Accelerating System

Fig. 6: Accelerating System

During periods of sudden acceleration, the air flow through the carburetor responds very quickly to a sudden throttle opening. However, there is a brief interval before the heavier fuel-air mixture in the narrow passages can gain speed and maintain the desired balance of fuel and air. The accelerating system (Fig. 6) operates during this interval to supply fuel until the other systems can provide the proper mixture.

When the throttle is suddenly opened, the diaphragm, which is connected by linkage to the throttle, forces fuel from the pump chamber into the pump discharge passage. The fuel under pressure forces the pump discharge ball check valve and weight up. The fuel then passes into the pump discharge nozzle where it is sprayed into the air stream of the venturi. The discharge nozzle is vented to prevent siphoning at high engine speeds.

When the throttle is closed, the pump return spring forces the pump diaphragm toward the back of the pump chamber, drawing fuel into the chamber through the pump inlet. A ball check valve in the pump inlet opens to admit fuel from the float chamber and closes when the pump is operated to prevent a reverse flow of fuel. The outlet ball check valve prevents air from entering when the diaphragm draws fuel into the pump chamber.

POWER FUEL SYSTEM
Fig. 7: Power Fuel System

Fig. 7: Power Fuel System

The power fuel system (Fig. 7) operates when additional fuel is required for a richer mixture during high speeds, heavy loads, and for low speeds at full throttle.
Manifold vacuum is transmitted from below the throttle plate through the vacuum passage to the vacuum chamber on top of the diaphragm. At idle and normal speeds the manifold vacuum is great enough to hold the diaphragm up against the tension of the diaphragm spring. This raises the diaphragm stem clear of the power valve. The power valve is held closed by the tension of its spring.
When high power places a greater load on the engine, manifold vacuum is reduced. When the vacuum drops below 7-6 inches of mercury, the diaphragm can no longer overcome the tension of the diaphragm spring and the diaphragm stem is forced down on the power valve. This depresses the pin in the center of the power valve, opening the valve. Fuel from the float chamber flows into the valve and passes through a restriction into a horizontal passage which leads to the main well where it is added to the fuel from the main fuel system.

Carburetor Removal

Remove the air cleaner. Disconnect the accelerator rod, choke wire, fuel line, and the distributor vacuum line. Remove the carburetor hold-down nuts, then remove the carburetor and gasket from the manifold.

Carburetor Disassembly
Use a separate container for the component parts of the sub-assemblies to facilitate cleaning, inspection, and assembly.

  1. Remove the accelerator pump link cotter pin and slide the upper end of the link out of the pump operating lever. Remove the two throttle body screws and
    lock washers. Separate the throttle body and main body, and remove the gasket.
  2. Remove the fuel inlet fitting with a box wrench, and remove the gasket. Remove the dashpot if so equipped. Remove the four float bowl retainer screws, lock washers, and clamps. Remove the retainer, retainer gasket, float bowl, and bowl gasket.
  3. Remove the fuel inlet seat screw and gasket located in the fuel inlet opening. Remove the fuel inlet needle valve and float assembly, and the gasket from inside the main body. Remove the float shaft, releasing the float. Slide the fuel inlet needle assembly off the float lever tab. Remove the wire clip, spring, and plunger from the fuel inlet needle. Remove the three power valve diaphragm screws and lock washers, then lift the power valve diaphragm and stem assembly out of the main body. Separate the cover from the diaphragm and stem assembly.
  4. Fig. 8: Choke Plate and Main Discharge Nozzle Removal

    Fig. 8: Choke Plate and Main Discharge Nozzle Removal

    Remove the five main well screws and lock washers, then remove the main well. Remove the pump return spring from the metal disc on the accelerating pump piston, then remove the spacer gasket. Pull the accelerating pump diaphragm out of the main body. Remove the pump operating lever retainer, then slide the lever off the stud. Remove the choke bracket screw and lock-washer, the choke plate screw and lockwasher, and the choke shaft locating screw and lockwasher. Slide the choke shaft and lever assembly out of the main body, then remove the choke bracket. Slide the main discharge nozzle out of the main body, then remove the choke plate and valve assembly (Fig. 8). Invert the main body and remove the distributor passage ball retainer and the distributor passage ball.

  5. Remove the main jet, the pump inlet check valve retainer, and the pump discharge valve retainer from the main well body. Invert the body, allowing the pump inlet check ball, pump outlet check ball weight and check ball to fall out into the hand.
  6. Press the pump rod sleeve toward the diaphragm until the pump rod sleeve retainer ball drops out. Remove the pump rod sleeve and spring.
  7. Remove the idle adjusting needle and spring. Remove the pump link cotter pin and link.

On carburetors equipped with a dashpot, remove the dashpot lever.

At times it may be necessary to remove the throttle plate and shaft to accomplish a thorough cleaning job-If this is done, be sure to mark the throttle plate before removal so it can be installed in exactly the same position. Throttle plates and shafts cannot be interchanged between carburetors, nor are they serviced as separate parts.

Carburetor Cleaning and Inspection

Many carburetor troubles are the result of deposits accumulating in the carburetor. A thorough cleaning must be performed to assure satisfactory carburetor performance.

CLEANING

Soak all castings and metal parts (except the dashpot) in a cleaning solution to soften and loosen all foreign deposits. If a commercial carburetor cleaning solvent is not available, lacquer thinner or denatured alcohol may be used.

INSPECTION

Replace the float if it leaks or if the assembly is damaged in any way. Replace the main body if the protective plating is damaged exposing bare metal to corrosion. Check the action of the poppet valve in the choke plate, and free it up if necessary. Replace the choke lever and shaft assembly if the threads in the shaft are stripped or if it is not securely riveted to the lever.

Carburetor Assembly

Always install new gaskets when rebuilding the carburetor. A carburetor overhaul kit is available for service. A disassembled view of the carburetor is shown in Fig. 9.

  1. Fig. 9: Holley Single-Barrel Carburetor

    Fig. 9: Holley Single-Barrel Carburetor

    Install the pump link in the throttle lever, then
secure it with a cotter pin. Install the idle adjusting
needle and spring. Turn the needle in gently with the
fingers until it seats, then back it off 1 1/2 turns for a
preliminary idle adjustment. To avoid grooving the
 tip of the needle, do not force the needle against its
seat.
    On carburetors equipped with a dashpot, install the dashpot lever.

  2. Install the distributor passage ball and ball retainer. Place the choke bracket in position on the main body and install the choke bracket screw and lockwasher. Insert the choke plate and valve assembly into the main body. The poppet valve stem should be pointing down. Install the main discharge nozzle. Slide the choke shaft in position and install, but do not tighten, the choke plate screw. Close the choke plate and hold the main body up to the light. Little or no light should show between the choke plate and the walls of the bore. Make sure the choke plate does not bind, then tighten the choke plate screw. Install the choke shaft locating screw and lockwasher.
  3. Place the pump operating lever on the stud in the main body, and fit the pump operating lever retainer on the stud. Place the spring on the pump diaphragm rod, and press the pump rod sleeve into the rod to compress the spring. Drop the pump rod sleeve retainer ball into the hole in the sleeve. Be sure the main discharge nozzle gasket is in place in the main body, then position the pump assembly in the main body. Place the main well spacer gasket over the pump assembly.
  4. Insert the pump inlet check ball, and the pump outlet check ball in the main well body. The pump inlet check ball is slightly larger than the pump outlet check ball. Be sure they are installed in their proper chamber. Seat the check balls with one gentle tap of a light hammer and a soft brass drift. Be sure the check balls move freely in their chambers, then install the pump inlet check ball retainer, and the outlet check ball weight and retainer. Install the main jet in the main well body.
  5. Seat the large end of the pump return spring in the metal disc on the accelerating pump diaphragm. Position the main well body screws and lock washers in the body. The two long screws are placed in the center top and center bottom holes; the short screws are used in the three remaining holes. Insert the power valve end of the main well body into the main body, then press the main well body into position against the spacer gasket as follows:
    Apply pressure with the index finger against the protruding end of the pump rod sleeve, to fully compress the pump return spring, as the thumb presses the main well body into position. This will prevent the pump return spring pressure from disturbing the alignment of the holes in the diaphragm, spacer gasket, and main body. Before releasing the pump rod sleeve, tighten the five main well body screws.
  6. Position the power valve gasket, power valve diaphragm stem assembly, and the power valve body cover in the main body. Install the three power valve body cover screws and lock washers.
  7. Place the inlet needle spring over the fuel inlet needle plunger, and insert these parts, spring first, into the hollow fuel inlet needle. Install the wire fuel valve clip on the fuel inlet needle. Clip the needle on the float lever tab. Guide the needle into the inlet needle seat, and position the float lever between the two float hinge bracket arms. Install the float lever shaft. Do not attempt to interchange fuel inlet needles or seats; they are matched assemblies.
  8. Fig. 10: Float Hinge Bracket Alignment

    Fig. 10: Float Hinge Bracket Alignment

    Install the fuel inlet seat screw gasket on the screw, and insert the screw through the fuel inlet fitting boss in the main body. Place the seat gasket on the threaded end of the inlet seat screw which protrudes into the fuel bowl, Set the float and fuel inlet valve assembly into position, and install the carburetor float gauge under the float hinge bracket (Fig. 10) to prevent the assembly from tilting when the seat screw is tightened. Tighten the screw securely, then remove the gauge. Install the fuel inlet fitting and gasket.

  9. Fig. 11: Float Setting - Bench Check Only

    Fig. 11: Float Setting – Bench Check Only

    Invert the main body assembly, and check the setting of the float with the cardboard gauge provided in the repair kit (Fig. 11). If necessary, bend the tab on the float arm to bring the float setting within limits. This should provide the proper fuel level.

  10. Install a new float bowl gasket into the recess in
the main body. Install the retainer gasket over the
bowl. Place the retainer on the bowl, and set the bowl
into position. Install the four clamps, screws, and lock
washers. Tighten the two center screws, then the two
end screws, alternately, to evenly compress the gasket
(approximately 8-10 inch pounds torque.) Do not overtighten these screws as the bowl may crack. Install the dashpot if so equipped.
  11. Place a new throttle body to main body gasket on the throttle body, and check the alignment of all holes in the gasket with the corresponding holes in the throttle body. Insert the two throttle body screws and lock washers through the throttle body and gasket to maintain gasket alignment, then set the main body on the throttle body. Invert the carburetor, and tighten the two throttle body screws evenly. Insert the upper end of the pump link through the hole at the end of the pump operating lever, and install the cotter pin.

Carburetor Installation

If the carburetor to intake manifold gasket is not serviceable, install a new one. Place the carburetor on the manifold, and secure it with the lock washers and nuts. Tighten the nuts evenly. Connect the choke and throttle linkage to the carburetor, and adjust if necessary.

Connect the fuel line and the distributor vacuum line. Install the air cleaner and tighten the clamp. Be sure the air cleaner gasket is in place. Adjust idle fuel mixture, engine idle speed, and the dashpot (Fordomatic). Check the fuel level, and adjust it if necessary.

Adjustments

IDLE FUEL MIXTURE ADJUSTMENT
Fig. 12: Idle Fuel Mixture Adjustment

Fig. 12: Idle Fuel Mixture Adjustment

The idle fuel mixture is controlled by the idle mixture adjustment needle (Fig. 12). Turn the screw “in” to lean the mixture, and “out” to enrich the mixture. Make the initial mixture adjustment by turning the needle “in” until it lightly touches the seat. Then back off the screw 1 turn. Do not turn the needle against the seat tight enough to groove the point. If the needle is damaged, it must be replaced before proper mixture adjustment can be obtained.

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

Turn the mixture adjusting needle in until the engine begins to run rough from the lean mixture. Slowly turn the needle out until the engine begins to “roll” from the rich mixture. Then slowly turn the needle 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 stop screw after the correct idle mixture is obtained.

IDLE SPEED ADJUSTMENT
Fig. 13: Idle Speed Adjustment

Fig. 13: Idle Speed Adjustment

A stop screw controls the engine idle speed (Fig. 13). Run the engine until normal operating temperature has been reached. Turn the idle stop screw “in” to increase the engine speed and “out” to decrease the engine speed. Idle speed should be 475-500 rpm on all cars with a conventional drive or overdrive transmission.

On Fordomatic equipped cars set the hand brake and place the selector lever in the N (neutral) position, then run the engine until normal operating temperature is reached. Idle speed should be 475-500 rpm. Check the idle speed in Dr (drive) position; it should be 425-450 rpm. If it is not 425-450 rpm in Dr (drive) range, adjust the idle speed so it falls in the Dr (drive) range specification. Adjust the anti-stall dashpot.

ANTI-STALL DASHPOT ADJUSTMENT—FORDOMATIC EQUIPPED CARS

Adjust the engine idle speed, then loosen the dashpot adjusting screw locknut (Fig. 13).
Hold the throttle in the closed position, and depress the dashpot plunger with a screwdriver blade. Turn the adjusting screw in a direction to provide the specified dashpot clearance of 0.045-0.064 inches. Tighten the adjusting screw locknut to secure the adjustment.

ADJUSTING THE ACCELERATING PUMP STROKE AND CHECKING THE PUMP

The quantity of fuel discharged by the accelerating pump is controlled by changing the position of the pump link in the throttle lever holes. The inner hole is for average or hot weather operation, and the outer hole is for cold weather operation.

To check the accelerating pump, remove the air cleaner, then operate the throttle and observe the fuel flow from the discharge outlet. If the system is in good condition, a quick steady stream will flow from the outlet when the throttle is opened.

CHECKING AND ADJUSTING FUEL LEVEL
Fig. 14: Checking Fuel Level

Fig. 14: Checking Fuel Level

To check the fuel level, remove the power valve diaphragm cover and valve assembly. Place the fuel gauge in this opening and crank the engine. The fuel should touch the tip of the “low” gauge pin and should not touch the tip of the “high” gauge pin (Fig. 14).

Fig. 15: Fuel Level Adjustment

Fig. 15: Fuel Level Adjustment

If the fuel level is too high or too low, remove the carburetor fuel bowl, and install the dummy bowl using the fuel bowl gasket and three of the retaining screws (Fig. 15). Position a suitable container under the carburetor to prevent fuel from splashing on the engine and manifolds. To adjust the fuel level, bend the float arm tab. Start the engine and recheck the fuel level.

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