Distributor Advance Unit: Description and Operation

PURPOSE
Due to the wide variation in engine speed and load conditions.

OPERATION
Ignition timing spark control is controlled by both mechanical and vacuum advance units.

Mechanical Advance
The mechanical advance mechanism moves outward from the distributor shaft as engine speed increases. The amount the weights move outward is in direct proportion to the distributor shaft speed. As the weights move outward, they turn the upper distributor shaft in relation to the lower distributor shaft, thus advancing the ignition timing.

Vacuum Advance
The vacuum advance unit acts on the breaker plate which is free to rotate in the distributor housing. As the throttle is opened vacuum is applied and the breaker plate is pulled in the advance position. As the engine load increases or the throttle is closed, vacuum decreases and the vacuum advance unit internal spring tension pushes the breaker plate to the retard position.

CONSTRUCTION

Mechanical Advance
The mechanical advance mechanism consists of two weights pivoted by two springs of different tension.

Vacuum Advance
The vacuum advance unit is a one piece metal housing with an internal diaphragm and spring.

NOTE: The vacuum advance unit is not serviceable and must be replaced as a complete unit.