Originally Posted by Tod74
just an FYI
You asked about the 81's being too big....
I looked on Holley's website and most 750 cfm 4150 's are jetted around 70 on the primaries..depending on which model it is..may have been 69 or 71 that is with a power valve so yours is 11 or 12 bigger without the PV. So you are on the fat side of the recomended increase in jet size when the PV is removed.
I have found this too be accurate too. I just replaced one that had 72's in the primaries. I have had the same issues as you describe. Although the others here are more knowledgable than I, I have found that if you are on the street at all, you will benifit from a primary PV... like someone else said, at approx 2 steps lower than inches of idle. Another thing is allowing the the idle circuts to do thier job may require more air too (by drilling small holes in the primary butterflies). Also, make sure your floats are set correctly & needle & seat is not damaged. BTW: I have found the carb I have now works fine on the street w/a secondary PV. Scooter