Backfiring when let off gas

[Newtoy78z]

Just purchased a 78 Z28. I had the original exhaust manifolds replaced with headers and replaced both mufflers. She doesn’t miss a beat when idling just backfires when you let off the gas. Sounds like coming from driver side. Also sometimes she starts on 1st try other times takes a couples of tries. Any suggestions? Thanks

 

[bourbon_scotch]

change the timing

 

[tom3]

Sometimes an exhaust leak will cause the backfire too.

 

[mallard]

Backfire or a "rapping" sound? A loose muffler baffle can mimic that sound as well.

 

[ZEN357]

Correct Spark Plugs?

 

[cadillac_al]

Backfire like a rifle shot or minor lean popping?

 

[larrylarry]

Exhaust leak. Air is getting inside the pipe also could be a timing issue to.

 

[Vettenick]

Just purchased a 78 Z28. I had the original exhaust manifolds replaced with headers and replaced both mufflers. She doesn’t miss a beat when idling just backfires when you let off the gas. Sounds like coming from driver side. Also sometimes she starts on 1st try other times takes a couples of tries. Any suggestions? Thanks

Other than the headers, is this car stock with any emission equipment still installed, like EGR ? Did you have this problem before the headers, or did it start right after the headers?