The Lucas Distributor Vacuum Unit

by Norman Nock

Originally published in the Austin-Healey Magazine


If your vacuum unit is not operating, the engine temperature and fuel consumption could be higher. Other than that, you would not notice much difference in performance.


When operating correctly, the spark will occur farther advanced when cruising (high vacuum), but as soon as you step on the accelerator, vacuum will be lost and the distributor vacuum unit will retard the spark. If the spark does not retard when you accelerate, you get detonation inside the combustion chamber due to a rapid rise in gas pressure.


If you suspect that poor engine performance or high gas consumption is the result of a vacuum advance unit that is sticking or has failed, check the pipe running to the intake manifold for cracks. Also make sure that the joints are tight. A damaged diaphragm in the unit is also common.


Figure 1: A cut-away distributor cap allows the performance of the vacuum advance to be checked. If the unit is operating properly, and there are no cracks in the pipe leading to the intake manifold, the contact breaker plate will move as engine speed changes. You can also see timing changes using an ignition timing light. Drawing shows how cap should be cut away.


To check this, use a distributor cap with a cut-away section as shown in Figure 1. If the advance unit is operating properly, you will see the contact breaker plate move as engine speed increases.


Figure 1

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