From the Holley Power Valve Tech Article
"The power valve is a key component of the power
enrichment system of Holley performance carburetors.
The power enrichment system supplies additional fuel to
the main system during heavy load or full power situations.
Holley utilizes a vacuum operated power enrichment
system and a selection of power valves is available
to “time” this system’s operation to your specific
Each Holley power valve is stamped with a number to
indicate its vacuum opening point. For example, the
number “65” indicates that the power valve will open
when the engine vacuum drops to 6.5" Hg, or below.
An accurate vacuum gauge, such as Holley P/N
26-501, should be used when determining the correct
power valve to use.
A stock engine, or one that is only mildly built for street
use, will have high manifold vacuum (17" to 21" Hg) at
idle speeds. To determine the correct power valve, the
vehicle should be driven at various steady speeds and
vacuum readings taken. The power valve selected
should have an opening point about 2" Hg below the
lowest steady speed engine vacuum observed. Holley
has a 6.5" Hg power valve, P/N 125-65, which usually
works out well for most driving situations."
Basically, the main jet size provides the tuning for regular
driving, the power valve size provides tuning for heavy load
enrichment, and when it comes in, and the idle adjustment screws take
care of idle.
If you put in a 6.5 power valve like they mention in the above article,
the enrichment would occur sooner than with the 5.5. The manifold
vacuum drops as the throttle is opened, so a lower power valve number
requires more throttle opening before it starts adding extra fuel. At
any vacuum levels above the power valve number, the power valve is not
adding fuel, it is up to the main jets to meter the correct amount.
Using a good vacuum gauge as a tool is important. If you notice that
the engine seems to bog around 5.5" Hg, (the point where the power
valve opens), then it might be due to the extra fuel which is not yet
needed. Swapping to a 3.5 or 4.5 will delay the enrichment. Keep in
mind that if the carb's jet sizes had been picked to work with the
power valve, changing the power valve will mean you have to compensate
for the change by adjusting the jet size.
Also, on your idle enrichment screws, if they still are not affecting
the idle, it could be due to the idle speed being set too high. If the
throttle blades expose too much of the idle transfer slots, the main
circuit will start adding fuel, and this overwhelms the idle mixture
screw's ability to control the mixture. So make sure that only
.040"-.050" of the slots are showing under the throttle blades at idle.
The slots allow the carburetor to transition from idle to main circuits
by providing a bit of fuel as the throttle starts to open. You may have
to adjust the idle speed down, and then try adjusting the idle mixture
screws to accomplish this.