I have a 1985 el Camino that has fresh rebuilt 350. It has a mild tv cam, Weiand intake, holley 4160 carb and headers. The garage that installed it really did not do me any favors. Anyway here is my problem. I can only get it to moderately run by advancing the timing to around 34’ btc with about 15 lbs vacuum. Any lower than that it starts but backfires through the carb and diesels when I turn it off and the vacuum is maybe 10lbs. Although it runs but stumbles and runs hotter than I’d like. Which would indicate that it’s too far advanced. I’m thinking that possibly the distributor is a tooth off. Any help would be greatly appreciated.

 

Me, I'd pull the distributor, and start from scratch. Back fire is timing issue. 15 lbs of vacuum is also way low. At 750 rpm, your timing (vacuum advance) should be 8 btdc.