Circuit Information

Float circuit

- The purpose of the float circuit is to maintain an adequate supply of fuel at the proper level in the bowl for use by the low-speed, high-speed and accelerator pump circuits. A spring loaded intake needle is used to give better control of fuel level, especially in rough road or vibrating conditions. Float adjustments must be made with the bowl cover held inverted and with the free weight of the float resting on the intake needle. The float is adjusted by bending the float arm. To avoid placing unnecessary strain on the float, do not grasp the float shell when bending. The bowl vent is calibrated to provide proper air pressure above the fuel bowl at all times. To meet evaporative emission requirements a two way vent valve, controlled by a solenoid, is used. When the ignition switch is in the on position, the bowl is vented to the carburetor air horn. The pressure in the air horn and the pressure in the bowl are the same and this is referred to as a "balanced vent". A restriction in the intake air flow, such as a dirty air cleaner, will not upset this balance. However, it would effect volumetric efficiency. Any time the bowl cover is removed a new gasket should be used and the cover screws properly torqued. A leak at the bowl cover would increase the pressure in the bowl resulting in a mileage complaint. When the ignition switch is turned off the solenoid is de-energized closing the inside vent and opening the vent to the canister. All vapors are routed to and held by the charcoal canister. When the engine is restarted the vapors are purged from the canister to the intake system. If the solenoid should fail the inside vent would remain closed, venting would then be through the charcoal canister which would seriously effect carburetor operation.



Low speed circuit
Fuel, for idle and part throttle operation, flows through the metering jet to the idle jet. When the fuel reaches the area above the idle jet it is mixed with air from the bypass and feedback circuits. This air/fuel mixture is then "squeezed" through the economizer and then flows into a low pressure area, which speeds up the flow of the mixture. At this point, additional air is added by the idle bleed, which helps to finely atomize the fuel and further mixing of the air and fuel takes place.

The amount of fuel which is delivered by the low speed circuit is partly determined by the pressure difference between the air in the fuel bowl and the air in the area above the idle jet. The air pressure above the idle jet can be varied by opening or closing the frequency valve.

When the frequency valve opens, additional air flows through the feedback circuit, reducing the "vacuum signal" on the idle jet. Less fuel is "sucked" out of the idle jet and the mixture is leaned out.



Part throttle circuit

At part throttle, a high vacuum is present in the venturi. This vacuum will draw a mixture of fuel and air from the high speed discharge nozzle. The amount of fuel which is drawn from the nozzle depends upon two things; the position of the metering rod and the difference in air pressure between the nozzle tip and the high speed well.
The air pressure in the high speed well is varied by the frequency valve. When the frequency valve is open the "vacuum signal" is reduced and less fuel is delivered by the high speed circuit.

The frequency valve has only two operating positions; open or closed. Both the low speed and part throttle feedback circuits are controlled by the frequency valve. There are two separate air passages, however.

The metering rod, which has an hourglass shape, controls the amount of fuel which leaves the float bowl.

When the throttle is closed, the metering rod is forced down. In this position the largest diameter of the rod is in the metering jet and the amount of fuel flowing from the float bowl is restricted.

As the throttle is opened, the metering rod moves up. A smaller diameter of the metering rod will be in the metering jet and more fuel can flow through the jet.






Accelerator pump circuit

- The accelerator pump circuit is used to supply fuel during sudden acceleration from lower speeds. The pump circuit consists of the pump diaphragm, intake and discharge checks, pump jet, drive or duration spring, pump bleed and the operating cam and linkage.

As the throttle is closed and the pump operating cam forces the cam follower and diaphragm downward creating a low pressure area above the diaphragm. At this time, the discharge check valve is seated and the intake check valve is lifted off its seat to allow fuel to flow into the pump chamber. When the throttle is open, the pump operating cam moves upward allowing the pump duration spring to lift the diaphragm. As the diaphragm moves upward the intake check valve is drawn to its seated position and the discharge check valve is forced open allowing gasoline to be discharged through the pump jet into the bore of the carburetor.