Backfiring issue

Put a vacuum gauge on it. Dig into it deeper and you can Troubleshoot the valves

||Tuning Tips||
The most important thing to do when using a vacuum gauge is to connect it to a constant vacuum source on the engine.

Distributor vacuum is at the metering block on the carb(holley).

Timing affects carb settings. Not the other way around.

Adjust each screw watching the gauge until it doesn't increase anymore, then back it off 1Hg. Air filter on. 15 with shake

Also, make sure that both side idle screws are adjusted the same (ie same # of turns out. - you dont want one at 1.5 turns and another at 3.0).