Carburetor Parts Information

Idle solenoid


The idle solenoid is used to prevent "dieseling". When the engine is turned off the solenoid is de-energized and the plunger moves inward allowing the throttle valve to close. The engine air supply is cut-off and the engine will immediately stop running.


Bowl vent solenoid valve

In order to prevent an excess of evaporative emissions from the carburetor float bowl when the engine is shut off, the solenoid valve blocks the bowl vent and allows the bowl to vent fuel vapors directly to the charcoal canister.


Deceleration switch

When the throttle is suddenly closed, cutting off the engine's air supply, the overly rich air/fuel mixture may cause the oxygen sensor system to suddenly lean out the mixture. This sudden leaning of the mixture may cause engine stalling. To prevent this problem, the deceleration switch causes the oxygen sensor system to go to a non-regulating mode for about 1/2 second, before the system begins to regulate the mixture.


Frequency valve

The frequency valve acts as a variable air bleed and regulates both the low and high speed circuits in the carburetor.

During normal operation the pulse solenoid opens and closes once each cycle. The solenoid makes about 10 cycles per second and a dwellmeter is used to measure the "duty cycle".

Duty cycle readings are the opposite of those seen on fuel injected cars. 100% is the "lean limit" of the carburetor system.


Purge valve

The purge valve will open and allow fuel vapors to flow from the charcoal canister to the engine whenever the speed of a warm engine is above 1300 RPM. The purge valve is closed when the engine is cold and when the engine is idling.


Cold running controls

When the engine is cold, the coolant temperature sensor is closed and the control unit will complete a ground connection for the manifold heater relay. Power is then supplied to the manifold heater throttle flange heater and the cold running vacuum switch.

When the engine is cold, the coolant temperature sensor resistance will be high and the control unit will complete a ground connection for the manifold heater relay. Power is then supplied to the manifold heater, throttle flange heater and the cold running vacuum switch.



Idle solenoid


- The idle solenoid is used to prevent "dieseling". The idle RPM is adjusted at the solenoid. When the engine is turned off, the solenoid is de-energized and the plunger moves inward allowing the throttle valve to close enough to virtually shut-off the air supply. This causes the engine to stop running immediately.



2 Stage throttle positioner

- The Rabbit with A/C uses a two stage idle solenoid, which is controlled electrically and by vacuum. The electrical portion is used to prevent dieseling when the engine is turned off. The vacuum diaphragm is actuated any time the A/C is turned on to increase the idle speed to compensate for the added engine load.