Distributor Advance Unit: Description and Operation

Vacuum Advance R&R:

Mounted on the side of the upper body of the distributor housing, is the vacuum advance control unit. This unit consists of a plunger attached to a diaphragm, and is connected to the breaker plate.
It controls the ignition advance by using engine manifold vacuum to monitor the load conditions. As vacuum increases, the diaphragm pulls the plunger rotating the breaker plate. This advances or retards the ignition timing as needed.