Stock power valve is 6.5, on all Holleys except maybe Dominators, etc. so I believe. If you have good vacuum at idle and part throttle, say up around 17-18", then a 6.5 will come in too late, and will be even more noticeable if you've jetted down a bit from factory for a bit more economy. You'd need to go to an 8.5 or so to have the correct fuel enrichment come in under load. The rule of thumb on a street car is take your idle vacuum and halve it to get your PV rating. On a more performance-oriented tune, say with a lumpy cam with poor idle, go 2' less than your idle vacuum. This is because your cruise vacuum will likely be more than your idle.
As a point of interest, the 5.0 Mustangs in the 80s used a specially calibrated Holley that had a dual-stage power valve. The first stage cut in at around 11, and second stage was at around 8.5. this allowed nice lean jetting for cruise (emissions, etc) but was able to supply some enrichment at part throttle without drowning it.
I had a similar story where I bought an off the shelf 600 vac sec to replace an old one and and things went backwards bigtime. I transferred the jets over amongst other things but with my limited knowledge i couldnt get it anywhere near as good as the old carby. I eventually gave up and took it to a dyno tuner who got the timing 100% right and diagnosed an incorrect power valve. I cant remember the exact size that went in but it wasnt a small adjustment. I couldnt believe how far out the carby's stock power valve settings were at the time for a mild 302C I just assumed it would perform like the old one? The engine purrs like a kitten now it was the best thing I ever did.
nolo180 got a reaction from gerg in 302c fueling issue??