Distributor Advance Unit: Description and Operation

Centrifugal Advance:

Ignition timing must be variable due to the wide variation in engine speed and load under normal operating conditions. When accelerating or climbing hills, the engine load can be high and the amount of spark advance required is not necessarily as much as it would be on level ground at an equivalent engine speed. Ignition spark advance is achieved by two separate systems: mechanical advance weights and a vacuum diaphragm unit.

The mechanical advance mechanism consists of two weights pivoted so that they move outward from the distributor shaft as the engine speed increases. As the weights move outward, they turn the breaker plate in relation to the lower distributor shaft, thus advancing the ignition timing. Each weight is restrained by two springs of different tension, giving a progressive advance action. The amount the weights move outward is in direct proportion to the distributor shaft speed.

Vacuum Control Mechanism:

In the vacuum control mechanism, one side of the diaphragm is linked to the movable breaker plate and the other side is connected by a vacuum line to a port just above the throttle plate. A spring is fitted between the vacuum unit connection. The vacuum applied at the diaphragm, combined with the action of the diaphragm spring, gives correct spark advance by moving the breaker plate in relation to the distributor shaft. Spark advance will vary according to the load placed on the engine. As the vacuum advance does not operate at idling speed due to the throttle plate being almost closed, the main action will occur as throttle opening, load and manifold vacuum varies during operation.

A two stage vacuum diaphragm is used. The secondary diaphragm provides limited ignition advance at closed throttle conditions such as idle and deceleration.