[mr0077]

Most vacuum advance (particularly on warmed over motors) is what is called "ported" vac advance, that is, it goes to a port that is effectively shut off at idle, and only receives a vacuum signal above idle.
This is useful with a "lumpy" cam to help stabilize the idle, since the vacuum signal would fall and rise due to the cam, and thereby increase and decrease the amount of advance given to the ignition...the idle slows, vacuum drops, advance decreases, makes engine run slower, then engine surges, vacuum rises, advance increases, etc.