I am chasing a part throttle stumble in my 350 http://www.jalopyjournal.com/forum/showthread.php?t=467881 , idles, revs great, timing is on. New plugs now, and added another dizzy and same problem. now I had a 700 dp awhile back and had a simmialr problem, under load. Changed that with a known working 650 dp and still had the same problem, then put my 600 back on and it went away. All is fine until i changed to a 750 vac secondary and it drove fine for awhile like the 700 and then same thing or similar. Now I was talking to Swade41 and it dawned on me that I had a 4.5 powervalve in the 600 and all the others had a 6.5. My vacuum at idle is 10hg, bigger cam, so my thought is that in gear when the rpms drop a bit could the vacuum drop a bit ( i can check this when i get home) causing the pv to add more gas then needed, thus it would not happen at idle or in neutral. If I cruise it is ok part throttle is really choppy and then clears up when floored. Any ideas?
Yes, you answered your own question. The bigger the cam overlap (Lower idle vacuum), the lower PV number it usually takes. The PV tells the carb when to richen your mixture. It senses throttle position by engine vacuum drop off. The lower numbers richen later and later as the numbers get smaller. BTW: those carbs that worked fine, then got worse later....You might have blown the PV causing it to go super rich. Holley has a PV check valve kit for about 12 bucks. It prevents a pop or backfire from blowing the PV. They are a snap to install. The mid range can be further tuned by drilling the byp*** bleeds. MIZ
You also answered your other question on the PV size. Take your engine vacuum and divide it by 2 to get your PV size.
Ok a little late but here goes, took the carb apart and there was some dirt on one of the jets, and replaced the 6.5 pv with the 4.5 pv, and viola....runs fine. Vacuum in gear is less than 10 so the 6.5 pv was causing a too rich mixture thusly causing it to run like ****.
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