Auxiliary Acceleration Pump (AAP) System

Not all engines incorporate this system, refer to application charts for usage.
The AAP system provides additional fuel during cold engine acceleration to overcome lean mixture conditions. The auxiliary accelerator pump consists of a vacuum operated diaphragm, and a fuel chamber with inlet and outlet ball check valves. When vacuum is applied to the AAP, the diaphragm compresses its return spring, and causes suction. This draws fuel from the carburetor float bowl, which is then held in the AAP by the inlet ball check valve. As vacuum is reduced, the diaphragm return spring pushes against the diaphragm, forcing fuel out of the AAP through the outlet check valve. The fuel enters the carburetor venturi through a discharge tube.
The application of intake manifold vacuum to the AAP is controlled by a ported vacuum switch. The ported vacuum switch can be either a thermal vacuum switching valve, or a bi-metal vacuum switching valve, depending on year and model application. In either case, the switch is sensitive to engine coolant temperature, and is on below a specific temperature (applying vacuum to AAP), and off above a specific temperature.