Carburetor: Description and Operation

The Aisan two barrel carburetor, used on the Chevrolet Nova, is of the downdraft type with a vacuum operated secondary throttle plate.
The carburetor operating circuits consist of the float, primary low speed, secondary low speed, secondary high speed, power enrichment, accelerator and choke.
An Auxiliary Acceleration Pump (AAP) system is used during cold engine operation to supplement the main accelerator pump system. During times of high engine vacuum, the Thermostatic Vacuum Switching Valve (TVSV) is open, allowing intake manifold vacuum to draw the AAP diaphragm inwards. As the AAP diaphragm is drawn in, fuel is drawn into the AAP chamber, where it is retained by a check valve. When manifold vacuum drops, the AAP diaphragm is forced inwards, forcing the fuel in the AAP chamber through a check valve and into the accelerator pump nozzle. When the engine warms up, the TVSV is closed, blocking intake manifold vacuum to the AAP.