Rich idle, would this help?

My car has ~8 in of vacuum at idle and has a 10.5 power valve in it. No rich condition at idle. How can that be?

I've setup lots of cars with PV's with opening point above the observed vacuum.

If the engine is set up properly, the PV will have no effect at idle. A PV that is ruptured so it leaks fluid though the PV itself, may introduce fuel to the engine through the vacuum port where the blow out protection is located.

PV plugs come in very handy to prove that vacuum levels at idle aren't an issue when the rest of the systems are set up properly. If the carb is running off the main circuit at idle, you got otrher issues to deal with.