The problem is probably not a float level issue. The "power valve" is controlled by a spring loaded vacuum piston that is held up by manifold vacuum, through a series of drilled passages in the cast iron throttle body and the aluminum main carburetor casting, then through the top casting. A vacuum leak at either of the two gaskets will allow the piston to drop, and hold the ball bearing valve open, bypassing the main jet. A Rottenchester B carburetor is about the worst excuse for a carburetor that's ever been invented- - - -all because the GM bean counters got too cheap in the early 1950s to keep buying Carter carburetors and decided to start making their own carbs in-house.
A blocked vacuum passage can be caused by using the wrong carburetor to intake manifold gasket, incorrect or damaged gaskets in the carburetor, or a vacuum leak due to warping of the top casting caused by overtightening the 4 screws that hold it to the carburetor body. There's a tech tip somewhere on the forum that covers straightening a top casting, using shim stock, a steel plate, feeler gauge blades, and a toaster oven. Polishing the brass power piston with a felt buffing wheel and jeweler's rouge is also a good idea. After you cover all the above bases, try lowering the float level about 1/32" at a time below the factory setting.
Jerry
