Vacuum Deceleration Valve: Operation

Normally the primary, advance diaphragm of the distributor is connected to a vacuum port on the carburetor. During deceleration, when intake manifold rises above a preset value, the deceleration valve closes off carburetor vacuum and provides direct intake manifold vacuum to the advance diaphragm. This permits maximum ignition timing advance to prevent popping in the exhaust system. At idle, the deceleration valve shuts off intake manifold vacuum and opens carburetor vacuum to the distributor.