[mr0077]

Hart, maybe I beat Ernie to it...if you check the vacuum signals on ported and unported sources on the carb (or unported direct at the manifold), you will find that with ported vacuum you have zero (or very close) at idle. As soon as the throttle plates move off-idle the vacuum signal appears, low at first until the butterfly moves above the port in the throat of the carb (fully ported). At that point, the vacuum signal is high and fairly strong, under no-load conditions, and is similar (but not identical) to the unported vacuum signal. At cruise both signals are strong, giving max vac advance; as you load the engine (e.g., let the clutch out in gear) both vacuum signals drop, and when you go WOT both signals drop to near zero.