That's the way they were designed- - - -no vacuum at idle, a gradual increase in vacuum on the advance unit through part throttle, and almost full vacuum at light throttle cruise speed. As the throttle is opened for passing or hill climbing the vacuum advance goes away to avoid detonation with the power valve open in the carburetor, leaving only centrifugal advance for pulling.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!