When the engine is cold, an Auxiliary Acceleration Pump (AAP) system unit in the carburetor is operated to squirt extra fuel into the acceleration circuit in order to prevent the mixture from becoming too lean.
A thermostatic vacuum valve (warmup-sensing valve), which is threaded into the intake manifold, controls the operation of the enrichment circuit. Below a specified temperature, the valve is opened and manifold vacuum is allowed to act on a diaphragm in the carburetor. The vacuum pulls the diaphragm down, allowing fuel to flow into a special chamber above it.
Under sudden acceleration manifold vacuum drops momentarily, allowing the diaphragm to be pushed up by spring tension. This, in turn, forces the fuel from the chamber through a passage and out the accelerator pump jet.
When the coolant temperature goes above specification, the thermostatic vacuum valve closes, preventing the vacuum from reaching the diaphragm, which makes the enrichment system inoperative.
- Check for clogged, pinched, disconnected, or misrouted vacuum lines.
- With the engine cold (below 75°F), remove the top of the air cleaner, and allow the engine to idle.
- Disconnect the vacuum line from the carburetor AAP unit. Gasoline should squirt out the accelerator pump jet.
- If gas doesn't squirt out of the jet, check for vacuum at the AAP vacuum line with the engine idling. If there is no vacuum and the hoses are in good shape, the thermostatic vacuum valve is defective and must be replaced.
- If the gas doesn't squirt out and vacuum is present at the
vacuum line in Step 4, the AAP unit is defective and must be replaced.
On later models, a diaphragm kit is available.
- Repeat Step 3 with the engine at normal operating temperature. If gasoline squirts out of the pump jet, the thermostatic vacuum valve is defective and must be replaced.
- Reconnect all of the vacuum lines and install the top of the air cleaner.