Truckernix - most modern automotive carburetors have several different fuel delivery systems. We will exclude the pump circuit from discussion, as it is primarily used in transition.
Idle (and off-idle) circuit - provide fuel at low RPM when venturi velocity is not sufficient to activate the main metering circuit. Normal range up to maybe 1000 - 1200 RPM
Main metering circuit - provide fuel for more or less constant RPM from just below the end for the idle and off-idle circuits up to wide open throttle. Generally calibrated to provide engine needs at high vacuum conditions such as normal cruising.
Power circuit - provide fuel for low vacuum (WOT) conditions only. On vacuum operated power circuits, the power valve consists of some combination of a tensioned spring opposing engine vacuum. If engine vacuum is higher than spring tension the valve is closed - no fuel. If engine vacuum drops below spring tension the valve is opened and feeds fuel into the engine.
So from the above, when the carburetor is at WOT both the main metering circuit and the power circuit are providing fuel.
If the carburetor has a vacuum power circuit, then the engineer designates at what value of vacuum the power valve will be actuated.
On the type B carburetor, the vacuum signal goes through the center of the carburetor and is sealed at the bowl/airhorn seam by the airhorn gasket. If the airhorn is sufficiently warped such that this vacuum passage is not sealed, then the vacuum "seen" by the spring is too low, and the power valve can be opened prematurely, causing a rich condition. This is quite common with the B.
A similiar circumstance will occur with the addition of a "large" camshaft, which creates less vacuum; or the use of carburetors originally designed for use as a single carburetor in a multiple carburetor system. One can "tune" for both of these conditions, but tuning for a vacuum leak is a bit iffy.
This is true for any carburetor, not just the B. The manufacturers of aftermarket performance carburetors help the tuners in that they make available different tension springs (Carter) or power valve assemblies (Holley). Often jets are incorrectly changed when the real problem is vacuum control. As Rochester never really went after the performance aftermarket, Rochester did not provide the selection of tuning parts available from others. This simply means the tuner has to do more fabrication to use the Rochesters.
Often in the case of multiple carburetor set-ups, the novice tuner when confronted with the vacuum problem will put much smaller jets in the carburetor, and depending on his driving habits this can work. If a certain amount of fuel is necessary at a given RPM, the engine doesn't care if the fuel is coming from a large main jet, or a small main jet plus a power valve. If the owner simply drives to cruises, car shows, etc., and never uses WOT, this stage of tune will work fine. A more experienced tuner, in this circumstance, would leave the main jets alone, and plug the power valve. Again, calibrating the carburetor for cruise (at a lower vacuum), but not WOT.
Hope this helps, and doesn't muddy the waters.
Jon.
