Backfire Under Load.

71 225/auto with Hei. Runs great but of late it has developed an odd problem. Starts right up runs great then will backfire under load every now and again and die. You can turn ignition off then turn it back on while it’s rolling in neutral and it’ll fire right back up and runs great. The backfire sounds like a gunshot out of the tailpipe.
My question is, think it’s a carburetor gasket issue or an intake gasket issue? I have double checked and triple checked all ignition parts and timing and it doesn’t change the random problem.
Any help would be great. Thanks.

 

Check the fuel filter and or screen in the carburetor. Also check fuel pump volume and pressure.

 

Filter and pump are relatively new, so is the fuel tank I’m thinking it’s fuel related also, but maybe it drops vacuum around the carb or intake and loads up the cylinders and then it has a backfire…… I don’t know

 

I almost think the ignition is drop out for a second and causing it to load up and backfire, but I have checked that too and everything looks right.

 

What do your spark plugs look like?

 

Backfire usually is fuel in the muffler, it could be a bad plug not firing all the time, just a thought.

 

Just a thought, use ethanol free gas and increase octane rating of gas. I have had problem.

 

I will check them.
Do you think the distributor could be wore out causing it to not fire correctly? It’s a re-furbished distributor from the local parts store.

 

More thoughts, my distributor is original (Prestolite), 52 years old. No issues works well. I do have spare distributor.

 

One thing at a time. One thing at a time.

 

while I don't have Hei
I did have a stumble / back fire / no power while trying to accelerate
did points/condenser/ wires/plugs and the fuel pump . one at a time . while it did seem to drive better missfire was still there
was about to change carburetor . but I just couldn't get it out of my head it was a ignition problem
so I bought a new coil . problem fixed

 

X100

 

x1000

 

Plugs look good
Plug wires are new
Coil is new (first thing I did)
Cap and rotor new.
I totally agree and practice “Change one thing at a time”.
The next thing I’m going to check is the distributor. Going to put it and check for slop in the shaft bushings. I do know that the problem happens under hard acceleration only.

 

A compression test might help. Bad exhaust valve or head gasket...?

 

Vacuum leak at the throttle shaft. Try squirting some starting fluid at the throttle shaft at idle.

 

This was my first thought as well. Check for loose wiring somewhere in the circuit that energizes the coil when the key is on.

 

Had the same problem on my 3B with the F-134 one time. Was an occasional skip and sometimes backfire under load.

 

You can check the distributor shaft grabbing the rotor and see if it moves back and forth, may need a new bushing.

 

But, under hard acceleration/load there would be very little vacuum...?