16.02.2006 - Vacuum unit test
Owners might be interested in a simple test suggested to me by an 'old timer' about 50 years ago.
Ideally you need a quiet area but it is not essential. If testing for leaks with the vacuum advance unit out of the distributor just push the linkage in against the internal spring, put a thumb over the suction pipe fitting and let go of the linkage. No movement of the linkage after the initial settling should be visible. I just count 10 slowly and then remove my thumb, listening for the air to rush in and the linkage to drop back to it 'off' position. If this happens then the unit is most likely OK. If air movement can be heard or the linkage moves out during the test; the unit is almost certainly faulty. In very bad cases the unit will not hold vacuum at all.
It is possible to do this test with the unit still in the distributor or car. The vacuum pipe has to be disconnected and the distributor advance plate rotated carefully using a screwdriver.
(by John Harper of England, Secretary Austin-Healey 100 register)
(published with kind permission from John Harper)