Right, because I doubt you have 9 or 10 inches of vacuum at a cruise and to unscrew the power valve timing requires the rest FP the car to be fixed.
12many did an in-depth post in this forum about correcting power valve timing and his real world results. Worth the read if you look it up.
I’ll go out on a Lomb and say your carb is nowhere close to being tuned if you are using a 6.5 power valve.
You are crutching the overrich condition by opening the power valve very late.
Which is one of the reason guys get hideous fuel mileage with a Holley. And the blame lies directly with Holley for allowing just one of many idiotic tuning issues continue for DECADES.
