[Excaliber]

One problem with vacuum advance is it goes away (retards) faster than it advances. When you go deep with the throttle the vacuum signal drops to zero or close to it in a heart beat. Depending on the return spring of the vac advance unit that means fairly instant retard, back to whatever mechanical advance was/is in play.

I use a hand held vacuum pump to check the vac advance myself. There fairly inexpensive and that way you can tell just how much advance at what vac signal your getting. How fast it responds, how fast it drops out, etc.