[Rev]

There are two sources for vacuum. One is sourced from below the butterflies of the carb. That's called manifold vacuum and that's the one you see at idle caused by the intake suction from the pistons going down on the intake stroke with the butterflies nearly closed.. That vacuum falls to near zero as you open the butterflies to WOT. That vacuum is also what draws the gas in through the idle/transition circuit and it also operates the power valve when vacuum falls to 3-6 in./HG.

Ported vacuum is the vacuum caused by the ventouri in the the carb bore and is added to by the booster ventouri. That vacuum is at about zero at idle because there is very little air flowing through those ventouri. As more air goes down through the ventouri, this vacuum inceases. This is the vacuum that actually sucks the gas in through the main jets and sprays it right into the booster ventouri and down through those butterflies as you open those butterflies more. That allows more air through the ventouri and therefore more gas making more power. This ventouri vacuum is also what makes the secondareis open up on a vacuum secondary carb. This makes the secondary butterflies open only on an as needed basis.

Hope this explains the difference in manifold and ventouri (ported) vacuum and how they each function in an engine.

Rev