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See Figures 1 and 2

The carburetor is the most complex part of the fuel system. Carburetors vary greatly in construction, but they all operate basically the same way; their job is to supply the correct mixture of fuel and air to the engine in response to varying conditions.

Despite their complexity in operation, carburetors function because of a simple physical principle; the venturi principle. Air is drawn into the engine by the pumping action of the pistons. As the air enters the top of the carburetor, it passes through a venturi, which is nothing more than a restriction in the throttle bore. The air speeds up as it passes through the venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowl through a nozzle into the throttle bore, where it mixes with the air and forms a fine mist, which is distributed to the cylinders through the intake manifold.

Fig. Fig. 1: Exploded view of the 4A-F carburetor air horn assembly

Fig. Fig. 2: Exploded view of the 4A-F carburetor float bowl and throttle body assembly

There are multiple systems (air/fuel circuits) in a carburetor that make it work:

The way these systems are arranged in the carburetor determines the carburetor's size and shape.

Carburetors all function in the same fashion; larger engines have larger carburetors to move more air and fuel, but the principle is still the same. Older units don't have as many external linkages and controls to manage emissions and driveability, but the principle is still the same.

It's important to remember that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speed is OK at every tune-up, there's not much maintenance you can perform on the average carburetor. Quality of fuel and presence of water in the system will affect the carburetor; dirt particles in the fuel can clog the jets and water causes rust and corrosion. If the vehicle is to be parked or stored for a long period of time, drain the carburetor to prevent the evaporating fuel from gumming up the system.

The carburetors used on 4A-F engines are conventional 2 bbl, downdraft types similar to domestic carburetors. The main circuits are: primary, for normal operational requirements; secondary, to supply high speed/high load fuel needs; float, to supply fuel to the primary and secondary circuits; accelerator, to supply fuel for quick and safe acceleration; choke, for reliable starting in cold weather; power valve, for fuel economy.

It is important to know that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speeds are correct at every tune-up, there's not much maintenance you can perform on the average carburetor.

PRELIMINARY ADJUSTMENTS

The following adjustments are to be made with the carburetor removed from the engine. Adjustments on carburetors interrelate; if you change one setting you may affect other adjustments. Therefore, these procedures must be performed in the order presented. Read these procedures thoroughly before continuing.

See Figures 3, 4, 5, 6, 7 and 8

It will be necessary to remove the air horn from the carburetor to gain access to the float. Follow the procedures outlined in the carburetor overhaul.

Fig. Fig. 3: Installing the float and pivot pin to the side of the carburetor

Fig. Fig. 4: With the float hanging, check the clearance between the float tip and air horn

Fig. Fig. 5: Bend the portion of the float lip marked (A) to adjust the float

Fig. Fig. 6: Lift up the float and check the clearance between the needle valve plunger and the float lip

Fig. Fig. 7: Adjust the float lip by bending the portion marked (B)

Fig. Fig. 8: Assemble the pin clip onto the needle valve

See Figures 9, 10 and 11

You will need the use of SST 09240-00014 and 09240-00020 or equivalent angle gauge to make any adjustments.

Fig. Fig. 9: To adjust the full operating angle of the throttle valve, bend the first throttle lever stopper

Fig. Fig. 10: Check the full opening of the secondary throttle valve from horizontal plane

Fig. Fig. 11: Adjusting the secondary throttle lever stopper

See Figures 12 and 13

You will need the use of SST 09240-00014 and 09240-00020 or equivalent angle gauge to make any adjustments.

Fig. Fig. 12: Check the clearance between the secondary throttle valve and body for the kick-up setting

Fig. Fig. 13: Adjust the kick-up setting by bending the secondary throttle lever

See Figures 14 and 15

You will need the use of SST 09240-00014 and 09240-00020 or equivalent angle gauge to make any adjustments.

Fig. Fig. 14: You will need the use of an angle gauge to make any adjustments for the secondary touch angle

Fig. Fig. 15: To adjust the secondary touch angle, bend the first kick lever

See Figures 16 and 17

You will need the use of SST 09240-00014 and 09240-00020 or equivalent angle gauge to make any adjustments.

Fig. Fig. 16: Set the throttle shaft lever to the first step of the fast idle cam

Fig. Fig. 17: Turn the fast idle screw to adjust

You will need the use of SST 09240-00014 and 09240-00020 or equivalent angle gauge to make any adjustments.

See Figures 18, 19, 20 and 21

Fig. Fig. 18: While holding the throttle slightly open, push the choke valve closed, and hold it closed as you release the throttle valve

Fig. Fig. 19: Apply vacuum to the diaphragm and check the choke valve angle

Fig. Fig. 20: Apply vacuum to the choke breaker diaphragms (A) and (B)

Fig. Fig. 21: Adjust the choke breaker by turning the diaphragm screw

See Figure 22

Fig. Fig. 22: Test the pump stroke with the choke valve fully opened, check the length of the stroke

FINAL ADJUSTMENTS

See Figures 23 and 24

To perform the final carburetor adjustments, the following conditions must be met:

Fig. Fig. 23: The float level should be even with the correct level in the sight glass

Fig. Fig. 24: Connect the tachometer to the positive terminal of the service connector at IIA distributor

See Figure 25

Adjust the idle speed by turning the idle speed adjusting screw. It should be 650 rpm on M/T and 750 rpm on A/T. Leave the tachometer attached for further adjustments.

Fig. Fig. 25: Adjust the idle speed by turning the idle speed adjusting screw

See Figures 26, 27 and 28

Fig. Fig. 26: Make sure to plug these hoses prior to adjusting the fast idle

Fig. Fig. 27: Disconnect the hose from the TVSV (M) port and plug the port

Check the fast idle cam is set at the first step.

Fig. Fig. 28: Check that the fast idle cam is set at the first step

See Figures 29 and 30

As stated in Engine Electrical , the Mixture Adjusting Screw (MAS) should be the very last item you try to adjust during tune-up or troubleshooting. The MAS is concealed behind a plug; the plug cannot be removed with the carburetor on the car. If adjustment is to be done, great care must be taken during removal of the plug; clearances are very tight and damage to the carburetor can occur.

Fig. Fig. 29: The head of the screw is only 0.04 in. (1mm) below the plug. Drill carefully and slowly to avoid damage

Fig. Fig. 30: Once the screw has bottomed, turn it out 31/4 turns

The method used to set the idle mixture and speed is also known as the Lean Drop Method.

REMOVAL & INSTALLATION

See Figures 31, 32, 33, 34 and 35

Fig. Fig. 31: Remove the air cleaner to access the carburetor

Fig. Fig. 32: Tag and disconnect all fuel, vacuum, coolant and electrical lines or hoses

Fig. Fig. 33: Disconnect the accelerator and throttle linkages from the carburetor

Fig. Fig. 34: Remove the four carburetor mounting nuts

Fig. Fig. 35: Discard the base gasket and remove the insulator; do not discard the insulator

OVERHAUL

See Figures 36 through 58

Efficient carburetion depends on careful cleaning and inspection during overhaul, since dirt, gum, water, or varnish in or on the carburetor parts are often responsible for poor performance.

Overhaul your carburetor in a clean, dust-free area. Carefully disassemble the carburetor, referring often to the exploded views. Keep all similar and look-alike parts segregated during disassembly and cleaning to avoid accidental interchange during assembly.

Carburetor overhaul kits are recommended for each overhaul. These kits contain all gaskets and new parts to replace those that deteriorate most rapidly. Failure to replace all parts supplied with the kit (especially gaskets) can result in poor performance and a leaks.

The following procedure is organized so that only one group of components is worked on at a time. This will help eliminate confusion of parts or sub-assemblies on the bench. Always keep parts in order; take great care not to lose small parts or clips.

Fig. Fig. 36: Separate the fuel pipe clamp from the side of the carburetor

Fig. Fig. 37: Unscrew the pump arm pivot and pump arm, remove them with the connecting linkage

Fig. Fig. 38: Disconnect the choke linkage

Fig. Fig. 39: Remove the eight air horn retaining screws

Fig. Fig. 40: Carefully disengage the solenoid harness

Fig. Fig. 41: Remove the float pivot pin, float and needle valve subassembly from the body

Fig. Fig. 42: Remove the power piston retainer (1), power piston (2), spring and pump plunger (3)

Fig. Fig. 43: Unscrew the solenoid valve from the side of the carburetor

Fig. Fig. 44: Remove the (a) stopper gasket, pump discharge weight, long spring, discharge large ball, (b) pump damping spring, (c) plunger retainer and small ball

Fig. Fig. 45: Remove the bolts retaining the throttle positioner

Fig. Fig. 46: Remove the jets and power valve

Fig. Fig. 47: Carefully remove the slow jet and set aside

Fig. Fig. 48: Unscrew the primary and secondary passage plugs

Fig. Fig. 49: Remove the primary and secondary main jets

Fig. Fig. 50: Separate the AAP housing with the diaphragm

Fig. Fig. 51: Unscrew and pull off the sight glass with the retainer and O-ring

Fig. Fig. 52: View of the float and needle valve components

Fig. Fig. 53: Ensure the power piston moves up and down smoothly

Fig. Fig. 54: Test the opening and closing action of the power valve

Fig. Fig. 55: Connect the leads of the throttle solenoid to a battery to test for proper operation

Fig. Fig. 56: Test the resistance of the coil housing

Place new gaskets and O-rings throughout the carburetor assembly.

Fig. Fig. 57: Place the secondary throttle valve components in this order during assembly

Fig. Fig. 58: Place the check balls in this order

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