What is the stock pump nozzle size on a stock hp500 holley?? I think I have narrowed down my backfire through the carb issue to this either it not large enough, or being to large and dumping to much fuel...

anyone??

Pretty sure it's a 31 nozzle with a 30cc primary and 50 cc secondary pump
The Holley list number is 9022, you can look that up in there master catalog and see what the stock setup was.
And it's an 800cfm not 850

Yep, 31 primary and 31 secondary.
Backfire at the hit is probably too lean, check adjustment of linkage and that you have the correct cam, maybe try moving pump cam from hole 1 to 2 and readjust linkage; don't know the correct color of cam though. I would guess pink pri. and brown sec.