top of page

The Lucas Distributor Vacuum Unit

by Norman Nock

Originally published in the Austin-Healey Magazine

Healey Distributor

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

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

8 views0 comments

Recent Posts

See All

The Negative Spark

By Norman Nock Originally published in the Austin-Healey Magazine Coils are normally wound to give a positive earth spark – that is, the spark plug insulated electrode is negative with respect to the

Sprite Distributor Bearing Modification

by David Vizard Originally published in the Austin-Healey Magazine, March 1991 This Tech Tip was borrowed from the February issue of "Healey Trails" the newsletter of the North Texas Austin Healey Clu

Ignition System Troubleshooting

by Rich O’Hara (From Healey Motor News, 5/80) Those of us, who have owned a Healey, or any other British automobile for that matter, have learned to deal with the occasional and persistent electrical


Commenting has been turned off.
bottom of page