454 backfiring problems need help

[68black_betty]

So I got a 454 with holley 750 dp, air gap intake mild cam. I been having a backfiring problem when I get upwards around 3500 rpm if I let off the gas I get a pop but if I just ease off it its fine. Runs smooth at idle and pretty much most the time.my front two cylinders on each side are running rich but the back two are fine. Timing is at 17 without vaccfuum and like 30 with vaccuum. I have an advance curve timing kit in that is why its high. The wires are good and just replaced header and collector gaskets thinking it was exhaust. Does anyone have any ideas or ran into this problem before?
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