My 72 has a bad backfire when i down shift to slow down. sounds like a 12 gauge. Not sure what to do checked for air leaks and retightened headers. But what now any help appreciated. the Engine runs perfectly fine.

Man, I remember that problem. I had that happen to me right after I put a set of Cherry Bomb glass packs on, but it ended up being that I was running too rich. I remember a time when I had the doors off and my brother was riding shotgun and we were going down a steep incline and it did that, he thought someone was shooting at us, it was fricking hilarious at the time.

just did the timing this weekend at 13 degrees advanced. also it is fuel injected with the howell tbi with a larger fuel pump. it was a freebee that has double pressure of the stock one. i do not know alot about fuel injection but should be burning it but i know runs a little rich at idle. i will check my mileage this week and see if it has gone down. any ideas would be greatly appreciated