All gasoline power valves flow exactly the same. The number (4.5, 6.5, 7.5, etc.) is the point, in inches of vacuum, where the power valve opens. This is to help get the fuel flowing in the primary circuit, when the throttle is suddenly opened.
You will not notice any difference in the air/fuel ratio when changing the power valve number.
Where you will see a difference is, sorting out the power valve will help you overcome a momentary hesitation when you suddenly mash on the throttle. We can address this once you get the air/fuel ratio sorted out.
I'm assuming that you running pump gas. The additives in pump will make it a challenge. Do you have an otoscope or a 10x magnifying loupe?
Do you have an open spacer or has the divider in your Airgap manifold been modified? An unmodified Airgap will have a small window between the two plenums to help even out the vacuum signal on the carb venturi.
A true dual-plane manifold, with isolated plenums, will pull harder on each carb venturi than an open plenum because each cylinder is exposed to only 1/2 of the carb. An open plenum draws from all carb venturi. Spacers and plenum divider modifications can reduce the vacuum signal that each venturi experiences.
With all of that being said, there is a way to determine over/under carburation using a vacuum gauge, but I don't remember off hand how to do it and I can't put my hands on that information right this moment. My gut feeling is that 2" at WOT is not producing a very strong vacuum signal at the venturi.
In order for air flow to occur, there has to be a pressure differential. That pressure differential between the atmospher and the cylinder pushes air from atmospheric pressure through the carb venturi and ultimately into the cylinder. It is this airflow through the carb venturi that produces a low pressure area that the fuel in the float bowls are exposed to through the primary circuit (jets, power valve, et al). Atmospheric pressure present in the float bowls is greater than the pressure present in the venturi and fuel is pushed from the bowl in the venturi where it is atomized and homogenized with the air flowing into the intake manifold.
Just how much fuel flows out of the venturi is relative to the velocity of airflow through the venturi. When an engine is over-carbureted, the velocity is reduced along with a reduction in booster signal. To compensate, overly large jets need to be used. Don't be shocked if you have to substantially step up in jet size.
Just thinking now... could the spark arrestor be restricting the bowl vents? If you have any kind of open spacer under the carb, try it without the open spacer.
Good luck,
Joe
