Ive got a pontiac 326 that came with the car and was supposed to have been rebuilt about 15k miles ago, and it looks it because its very clean and all the seals look new and it has the reciept for it. But it backfires when it revs down. It doesnt lag or choke when I accelerate hard also th idle seems to be unstable. The car isnt driving, and this is all info with it in neutral. It has a brand new edelbrok 600 cfm carb that I just put on. I dont think its the mixture or timing because Ive gone through those plenty of times. Any advice is much apreciated. Thank you

Bring engine to operating temp, hookup vacuum gauge, adjust idle mixture for max reading, reset curb idle to your liking.

Retest, still backfire, then ccw on mixture screws 1/2 turn(rich). Should cure it.

Everett 68.350/PG/11.90/115mph

I assume it's backfiring out the exhaust.Make sure there are no exhaust leaks.A small leak will suck some air on decel and ignite the rich decel mixture.BOOM!

------------------
68 z28 ,68 rs 327 ,73 454 vette, 2 goofy kids