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I did a long post on one of the other vacuum threads, but short version.
Throttle has a flat plate inside, when it open from idle the front goes up and the rear goes down, this creates "control" vacuum ports located with small opening just above or just below the front or rear portions of the plate.
Front port has vacuum at idle, as throttle opens the pressure goes up to air filter no vacuum at all.
Rear port at idle has no vacuum, as throttle opens the pressure drops to manifold vacuum.
Main port is far enough above the plate to always have manifold vacuum.
Big breather port goes to base of throttle almost no vacuum, just tiny drop from air going through air filter etc.
There is also a thermal vacuum switch on the 1/4 head that either opens or closes when it warms up.
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