Vacuum Distributor Test

To determine for sure which vacuum line is working and which isn't:

1. Put the timing light on and rev the engine up to a steady spead (1500 2000 rpm). Then 'blip' the throttle open and let it go so that the revs don't change much but the load does. If the advance vacuum line is working, I should see an immediate change in the advance, before the engine revs change much, and then a rapid change back again as the throttle is released. If the advance vacuum line is NOT working, there will only be a steady change as the revs increase the advance won't 'blip' with the throttle movement itself.
To put it another way, if the engine revs alter by say 500rpm as I 'blip' the throttle, I should see a timing change of maybe 2 3 degrees caused by the centrifugal advance. But if the vac line is working, you should see it rapidly advance by maybe 8 10 degrees as the throttle arm is moved.
2. Then run the engine at 3000 3500 and check the timing. If my theories (and Rob's) are correct, It should be AT LEAST 30 degrees or more (30 degrees if only the centrifugal is working more if the vac advance is working too).
3. If it's less than 30 degrees, then I'll disconnect both vac lines at the carby and plug them, then adjust the timing for 30 degrees at 3000rpm (this means setting it using just the centrifugal advance), then reconnect both vac lines.
4. Then drop the revs back to idle and look at the timing again. It should be 5ATDC or thereabouts. If so, the retard vac line is working OK. But if its somewhere from 0 to 10BTDC at idle, then I would guess is that it's the RETARD line which is not working.
5. Having confirmed that the retard vac line is working, then I will gently open the throttle with the timing light still on the timing should change rapidly from 5ATDC to some positive figure as the retard line stops working.
6. Re time the engine properly after doing the test (5ATDC at idle).

Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.

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Last revised 6 May 2004.