Exhaust backfire form my '31 6cyl. Compression even, timing set, ignition checked and rechecked, original brass bowl changed to BB1, but backfire still continues. I even hotwired a 12v coil to see if that helped, no change. Engine will idle
ok, but once the RPMs increase, backfire begins. Burnt/stuck valve(even compression)? weak fuel pump? I blew out my muffler yesterday. Ideas, Please.
Thanks, Roger.

i had a modified 351 at work that kept on blowing mufflers and it would back fire after ya let off the trottle. i ran the engine at high throotle and ran ATF through it. it smokes like hell but it is suppose to clean up carbon build up on valves and pistons. don't know if it helps just given ideas. sorry 4 the bad spelling.

Weak mixture due to a vacuum leak or clogged carb?

Had a Packard that backfired, turned out to be the condensor.

I had a 53 Plymouth that backfired & blew a hole in the exhaust pipe. I had just tuned it up and had gotten the #3 & #4 plug wire mixed up. That also was a flathead 6.

My 50 Studebaker began to backfire during a short drive. Turned out to be a symptom for a need to have the valves reground on this vehicle which sat in storage for many years. Rust eroded the valves and valve seats.
Martin Lum

Check plug wires and ignition. Do a compression test, low or no compression on one cylinder pinpoints the problem. Or, try disconnecting the spark plugs one at a time until you find the one that stops the backfire, that cylinder is the culprit if it is a bad valve.