See Figure 1
The model 4300 and 4350 4-barrel carburetors are composed of three main assemblies: the air horn, the main body, and the throttle body. The air horn assembly serves as the fuel bowl cover as well as the housing for the choke valve and shaft. It contains the accelerator pump linkage, fuel inlet seat, float and lever, booster venturi, and internal fuel bowl vents.
The main body houses the fuel metering passages, accelerator pump mechanism, and the power valve.
The throttle body contains the primary and secondary throttle valve and shafts, the curb idle adjusting screw, the fast idle adjusting screw, the idle mixture adjusting screws, and the automatic choke assembly. It is used on V8 engines.
ADJUSTMENTS - Float
See Figures 2 and 3
- Adjustments to the fuel level are best made with the carburetor removed from the engine and the carburetor cleaned upon disassembly.
- Invert the air horn assembly and remove the gasket from the surface.
- Use a T-scale to measure the distance from the float to the air horn casting. Position the scale horizontally over the flat surface of both floats at the free ends and parallel to the air horn casting. Hold the lower end of the vertical scale in full contact with the smooth surface of the air horn.
The end of the vertical scale must not come into contact with any gasket sealing ridges while measuring the float level.
- The free end of each float should just touch the horizontal scale, if one float is lower than the other; twist the float and lever assembly slightly to correct.
- Adjust the float level by bending the tab which contact the needle and seat assembly.
- Invert the air horn assembly and remove the gasket.
- Measure the distance from the floats to the air horn rim using a T-scale. Position the horizontal scale over the flat surface of both floats at the free ends, parallel to the air horn casting. Hold the lower end of the vertical scale in full contact with the smooth area of the casting, midway between the main discharge nozzles.
Do not allow the end of the vertical scale to contact any gasket sealing ridge while measuring the float setting.
- The free end of the floats should just touch the horizontal scale. Float-to-air horn casting distance should be 29 / 64 in. (11.5mm). Bend the vertical tab on the float arm to adjust the distance.
Initial Choke Valve Clearance
- Remove the choke thermostatic spring housing.
- Bend a wire gauge (0.035 in. [0.89mm] diameter) at a 90° angle about 1.8 in. (45.7mm) from one end.
- Block the throttle open so that the fast idle screw does not contact the fast idle cam.
- Insert the bend end of the wire gauge between the lower edge of the piston slot and the upped edge of the righthand slot in the choke housing.
- Pull the choke piston lever counterclockwise until the gauge is snug in the piston slot. Hold the wire in place by exerting light pressure in a rearward direction on the choke piston lever. Check the distance from the lower edge of the choke valve to the air horn wall.
- Adjustment is done by loosening the hex head screw (left-hand thread) on the choke valve shaft and rotating the choke shaft.
Fast Idle Cam4300 MODEL
- Loosen the screws on the choke thermostatic spring cover and rotate the housing 1 / 4 turn counterclockwise. Tighten the screws.
- Open the throttle and allow the choke valve to close completely.
- Push down on the fast idle cam counterweight until the fast idle screw is in contact with the second step of the cam and against the high step.
- Measure the clearance between the lower edge of the choke valve and the air horn wall.
- Adjust the turning the fast idle cam adjusting screw.
- Return the housing to its original position.
- Run the engine to normal operating temperature. Connect an accurate tachometer to the engine.
- Disconnect and plug the EGR and TCS vacuum lines.
- Position the fast idle screw against the first step of the fast idle screw. Adjust the fast idle screw to give a reading of 1600 rpm.
- Return the linkage to its normal position, unplug and reconnect the vacuum lines.
Choke Unloader (Dechoke)
- Open the throttle fully and hold it in this position.
- Rotate the choke valve toward the closed position.
- Check the clearance between the lower edge of the choke valve and the air horn wall.
- Adjust by bending the unloader tang on the fast idle speed lever toward the cam to increase the clearance and away to decrease the clearance.
Do not bend the unloader tang down from the horizontal. After adjustment, there should be at least 0.070 in. (1.78mm) clearance from the choke housing with the throttle fully open.
Accelerator Pump Stroke
The accelerator pump should not need adjustment as its stroke is preset in compliance with exhaust emission control standards. If for any reason the stroke must be altered, it may be done by repositioning the link in the desired holes.
Fast Idle Speed
The fast idle speed is adjusted with the engine at operating temperature and the fast idle screw on the second step of the fast idle cam. Adjust by turning the fast idle screw in or out as required.
To adjust the fast idle speed, you must plug the EGR valve vacuum line and disconnect the TCS solenoid wire.
Some carburetors are equipped with a dashpot to prevent stalling. The dashpot adjustment procedure for these carburetors is as follows:
- Be sure that the throttle valves are closed tightly and that the diaphragm stem is fully depressed.
- Measure the clearance between the dashpot stem and the throttle lever with a feeler gauge. For the proper clearance specification, see the chart.
- If the clearance is not correct, adjust it by loosening the locknut and rotating the dashpot until the proper clearance is obtained. Tighten the locknut.