I had a 7.5 in hg power valve installed. I bought a 6.5 in hg power valve to help the stumble off idle some. But then I do some reading I will include below. Basically the power valve is closed until the set point is reached. So wouldn't I want a larger number valve to open sooner not a smaller. Manifold vacuum is high when the throttle is closed and drops as opened. Sorry for the book below:
When engine load increases (such as climbing an increasing grade on a hill), and during high-speed operation, the fuel/air mixture must be enriched to increase engine power. The enrichment system supplies additional fuel to meet that demand. The enrichment system responds to changes in manifold vacuum. The enrichment system uses an enrichment valve (commonly called a “power valve”) to open an additional fuel metering path from the fuel bowl to the main fuel wells when manifold.
High manifold vacuum (such as at idle or low load, part throttle operation) pulls the power valve diaphragm against an internal spring and closes the valve so that no fuel can flow through it. At low manifold vacuum (such as at high load or high speed open throttle), the force of the spring overcomes the vacuum to open the power valve and provide additional fuel to the main fuel wells.
The power valve is installed on the bottom of the carburetor, with its valve opening in the bottom of the fuel bowl. When the power valve opens, it allows fuel to flow through restricted orifices in the carburetor main body, into the wells just below the main jets. From there, the fuel is drawn into the main fuel wells where it is added to the fuel drawn through the main jets to enrich the mixture produced by the main metering system. In effect, the power valve temporarily “enlarges” the main jet orifices.
Anything is possible if you don't know what you are talking about.