[FC] vacuum advance question

[Chris & Bill Strickland]

> > when I rev  the engine that the timing retards very far very fast (from 14 btdc to -4 btdc),  and kind of "catches up" to where it should be over a few seconds. < <

and in addition to previous good suggestions, it 'could' be working just fine.

what is the timing with the vac disconnected? does it stay the same when you reconnect the vac?

when the engine is "revved", you open the throttle, which drops intake vacuum which should result in a loss of vacuum at the advance, causing the timing to retard, as you describe. as the engine comes up to speed, and the throttle opening is reduced, the vacuum will build and the advance will "catch up", as you describe, advancing the timing.

you can hook your vacuum advance up to different sources on a Corvair, depending on just what you have -- ported vacuum or manifold vacuum -- and the results can vary.

the best way to test is check the mechanical advance is with the vacuum disconnected and rev the engine -- should be pretty much a straight line advance [okay, not really, but to the naked eye] -- it shouldn't retard if everything is okay.

check the distributor vacuum advance with a mechanical pump so you are absolutely positive of your vacuum source, then check your vacuum source with a gauge to see what it does -- you could have a leak or the wrong source.

Bill Strickland