Tom,
I have found that with different engines, vacuum and RPM don't reach their peaks with the same amount of advance. Sometimes one value will peak ahead of the other, then drop off if you continue to advance the timing as the other continues to increase. I haven't found a predictability to that occurance, but I found that advancing timing until reaching peak vacuum alone resulted in timing that predictably was well advanced from the mfr's specification, regardless of the manufacturer. I found the best compromise is to use the tach and vacuum gauge together, stopping further advance when one of the readings reached its peak, then retarding for the 1"Hg loss of vacuum. The "best compromise" setting was not necessarily right on spec, but typically it was pretty close- within a reasonable tolerance, maybe slightly toward a retarded setting toward an than an advanced setting. But with that, I would still use a timing light as the first choice, and use the tach/vacuum gauge when I couldn't use a timing light, as when the outer ring of a harmonic balancer has turned on the hub. All this was arrived at experimentally, checking quite a few engines with various methods and comparing with mfr's specs.
Hope this helps explain my method.
Harvester
