A couple of caveats about Power Valves:
Power Valves manage (control) the enrichment circuit in this type of carburetor. Their design is to add fuel under loads and WOT (this is the purpose of all carburetor enrichment circuits). With Power Valves the rating number on the PV indicates the vacuum level at which it opens and supplies fuel to the venturi boosters. Manifold vacuum holds the PV closed, as vacuum drops with throttle opening the PV opens and supplies the necessary additional fuel needed.

Power Valves (or any carburetor's enrichment circuit) can be and should be used to tune WOT because the secondaries take a little time to activate before they can supply fuel. While Power valves come in different vacuum ratings, they also come in different flow rates, standard, high flow, 2 corner, 4 corner and stages (single or two stage), with the two stage adding a slight amount of fuel at medium throttle levels (usually not needed). PV enrichment circuit's passage restrictors can also be increased or decreased in size to further tune the circuit (not advised for un-experienced).

Also, you will hear people say "Power Valves can not add fuel at idle because they are a venturi driven circuit (above throttle) and with the throttle closed there is no venturi vacuum to draw fuel up into the boosters". And this is true, EXCEPT, when the PV diaphragm is damaged or housing/gasket leaks. When this condition exist (damaged PV), manifold vacuum pulls the fuel through/past the PV into the manifold and adds fuel EVEN at idle. And JMHO, why it should be one of the first things checked when a rich idle condition exists.