Choke Plate: Description and Operation

Choke Operation:

Variable venturi carburetors are equipped with a dual stage electric choke cap and a choke control diaphragm. The all electric choke cap uses a two stage ceramic heating pill to heat the choke bimetal coil. The first stage of the pill is always on when the engine is running which applies voltage across the cap terminal and ground. The second stage can only be turned on when the ambient is above 12~C (54~F). With the exception of the first stage, operation of the electric choke is similar to previous years.

The VV carburetor also utilizes a choke control diaphragm in conjunction with the all electric choke cap to modulate the cold enrichment system. The fast idle cam is also modulated by the choke control diaphragm to position the throttle opening. For fast idle operation, the fast idle lever contacts the fast idle cam at the highest step to provide increased throttle opening during start-up. After engine start-up, manifold vacuum acts on the choke control diaphragm which has a built-in time delay (2-5 seconds). Manifold vacuum acting on the diaphragm retracts the choke pulldown rod which allows the fast idle cam to engage the fast idle lever on a lower step.

The choke control diaphragm modulates the cold enrichment system as follows:

The accelerator pedal is pushed during the cold start sequence, thermostatic bi-metal coil tension rotates the thermostatic lever. There is a direct connection between the thermostatic coil and the choke shaft lever, both levers will rotate together. Choke shaft lever rotation sets the fast idle cam, raises the CER to the upper limit of its travel and lowers the CVR. When the engine starts, manifold vacuum is applied to the choke pulldown diaphragm. The diaphragm assembly has a build-in time delay which allows a gradual retraction of the choke pulldown diaphragm. The time delay function prevents high cam lean stalls which would result from a sudden reduction in enrichment fuel. The intermediate lever, which is linked to the choke pulldown diaphragm, is rotated counterclockwise and contacts the choke shaft lever pin. Contact between the lever and pin lowers the CER to the run position. As the engine warms up, the thermostatic bi-metal coil unwinds until the CER is seated and the CVR raised to the warm run position.