Originally Posted by oldbogie
Ported vacuum with-holds vacuum advance at idle, no vacuum will be applied till the throttle blade moves past the port thus exposing it to the pressure inside the manifold. As the throttle moves toward WOT at some point the manifold pressure and atmospheric pressure become nearly the same, thus no vacuum is present with a normally aspirated engine. So the vacuum advance amount becomes less and less as the throttle is opened.
I was going to try and stay out of this misinformed topic, but if what you say is correct, why does one have to remove the vacuum source to set base timing at recommended curb idle?
Researching the Venturi Effect will also answer a few misconceptions regarding vacuum signals.