Distributor Advance Unit: Description and Operation

Distributor:

A vacuum advance unit changes the timing, depending on engine load (intake manifold vacuum), by moving the ignition pick-up mounting plate. The unit is mounted through the side of the distributor

CONSTRUCTION
The vacuum advance unit consists of a spring loaded vacuum diaphragm, the spring, diaphragm rod, and vacuum connection. The diaphragm rod is connected to the ignition pick-up mounting plate. The vacuum for the advance unit is taken from the throttle body and intake manifold.

OPERATION
As manifold vacuum increases (high load), the diaphragm compresses the spring and moves forward. This moves the diaphragm rod and changes the position of the ignition pick-up mounting plate. This change is in opposite direction of rotor rotation, the pick-up signal is now triggered earlier and therefore the spark occurs earlier, advancing the timing.