440 backfires

Original 440 w/70 k on it. Seems to be one cylinder backfires through the carb and exhaust. Does so intermittently. Compression 120 in all cylinders. Thinking valves are hanging up ? Worn guides ?

I’ve had backfire on exh side only….turned out to be broken valve spring. But, you have both int and ex backfiring.

get some trans fluid and pour it in carb with engine running till it smokes real good then por fast enough to stall it !

let sit for 30 min.

put 3 oz. in fuel tank

this will free stickey valves !

start and drive

Worth a try..Easier than pulling the heads

Friend just went through this and found a bad plug wire!

Put a vacuum gauge on it. low steady vacuum is a problem, or something that affects all the cylinders, worn timing chain, lean fuel etc. . If the needle bounces it is a valvetrain issue, broken valve spring flat cam etc. If it has normal vacuum then it may be electrical.

https://m.roadkillcustoms.com/diagnose-common-engine-problems-vacuum-gauge/#:~:text=Vacuum%20Gauge%20Engine%20Condition%20Indicators%20%20%20,Confirms%20worn%20rings.%20%2012%20more%20rows%20

Has anyone worked on it recently ?
Had something like that when 2 wires were crossed (to wrong spark plugs) on right bank.
Think it was #4 and #6.
Verify dist cap & rotor are healthy & clean (carbon tracking).

Counterclockwise

I had one 440 do that....turned out to be a cam with a bad lobe, ...that was it.

Probably been checked, but make sure one advance weight isn’t stuck. Will backfire on decel.

Fouled spark plug?