[jeverts]

One of my sever all 212s just started backfiring at high throttle. It will run smooth as silk at low and mid throttle but as soon as I crank it all the way up it starts sputtering and backfiring like crazy. Am I looking at a carb problem or could it be something deeper? Tearing into it today, so I'd appreciate any pointers.

 

[KHodges]

1st- try adjusting the carb a little. Go rich and see if it gets worse or better, then lean it until it runs smooth at 3/4 to full. If that doesn't work, return it to present setting and....
2nd-look at the points
3-maybe a bad coil.

My 318 would run just fine at idle and up to mid throttle, but would cut out and backfire at full throttle. Turned out the coil was bad.

 

[Herminator]

I am guessing points, condenser or coil but usually a coil starts stumbling when they are hot.

 

[PaDave]

Sticky and or burnt exhaust valve. Would be my first guess. Pull the valve out and run a 5/16" drill bit in the guide BY HAND.
That brings back some memories along with my 2¢.