? Back fire on deceleration open headers

My new engine had no issue's on the Dyno , put it in the car and free rev the engine it backfires out of the headers. It seem to do it more on the left bank then the right. Would poor header sealing to the heads cause this? by pulling cold air into the header tube? Or is something else?

mine did that after I switched to E-85 and the first thing a couple of guys told me was "you got a header leak" pulled them off the car this winter and did find some of the slip tubes were a little loose so maybe there is somthing to it.

Headers would be the first place to look

I will have try to reseal them, cannot find correct header gaskets.
Thanks

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Headers would be the first place to look

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My Cuda started doing that after racing it part of the year,big tube headers on a set of eddy RPM heads. I had let one of the end header bolts get loose and it ended up blowing the side of that gasket out Check for warp flanges also