The Stovebolt Site Forums The Shop Area Engine Shop Carburetor gasket

Fred,

I think I understand the theory after reading several Rochester booklets.

Vacuum does hold the powervalve (piston) closed. Under a load with the throttle open, vacuum drops and the power valve opens and gives a little extra fuel to maintain speed...we all know this.

If the powervalve is suspected of leaking, Rochester suggests turning the phenolic insulator over, thus blocking the vacuum passages and rendering the valve 'dead'. Your question was that this also means the valve opens and dumps extra fuel into the system and might make the test NFG.

With the car idling, the throttle plate is closed and fuel is delivered through the idle circuit below the plate.

The fuel the powervalve adds is delivered above the throttle plate, in the venturi, and isn't a factor when the engine is idling with the throttle closed. That circuit is dead until the plate opens and air flows to syphon fuel through the tube.

Does that make sense...it does to me. The thrust of this test is to see if a leaking power piston is feeding fuel through the carburetor into the small vacuum port at the base of the carburetor. Flipping the phenolic spacer over blocks those ports mechanically and can answer that question.

Stuart