I am not an expert either, but I have been through what colorado is saying & find it to be accurate. I ended up drilling holes in the primaries once on a 850DP BBC that had very low vacuum (4.5 inches) & a bigger cam... due to the same issue. Prior to that, had ring-wash & had to change the oil allot! The holes were about 1/8" specifically located inside mid primary area if I remember correctly. Ran a primary PV 3.5 & had to play around w/ a few sizes... finally got it all correct & the holes manipulated the x-fer slots being bypassed & dumping unnecessarily fuel @ idle.