[Smittys67]

If the vacum is 6 then use a 4 here is barry grant sheet. The power valve, located between the two main
jets, offers a boost in available fuel to the engine
in high load conditions without creating an
over-rich idle or part throttle. Power valves are
marked with a vacuum rating. This rating is the
point at which the valve will open. The lower
the number, the more the vacuum has to drop
for the valve to open. When the engine is under
a big load, it produces decreased manifold
vacuum along with an increased demand for
fuel. When the manifold depression is less than
the rating on the power valve, it opens and
allows extra fuel to enter the main circuit of the
metering block. It is a good idea to have a
power valve rating that is lower in number than
the vacuum produced by the engine at idle.

Hope this helps.