The power valve reading has to do with cruise vaccum NOT idle vaccum. The power valve is closed at idle due to low booster signal. Holley has had this screwed up for years.
And if the vacuum signal isn't high enough close the power valve circuit it runs rich. Same as if it is blown from a backfire. I have air fuel gauges hooked up I saw first hand the rich mixture with a 6.5 where a 3.5 was needed. If you are idling off the primaries with a large cam you are into transfer port to far.
