Originally posted by jthuma94:
There should be no vacuum at idle but as acceleration begins the carb opens and vacuum accumulates advancing the timing. As the motor rpms increase the vacuum decreases and the timing retards.
This is incorrect. The "no vacuum at idle" is ok, but vacuum should only be present at part-throttle. It has nothing to do with engine speed. Advance for engine speed should be performed by the centrifugal advance weights inside the distributor. In most cases, vacuum will drop to near zero at WOT, something people familiar with vacuum-operated windshield wipers will be painfully aware of.
The only reason for vacuum advance is to increase mileage during part throttle situations. It doesn't have to even be used. Many hot rodders don't even run a vacuum advance can.
Here's a good write-up on vacuum advance:
http://www.classicchevytrucks.20m.com/vacuum.htm
