[Tommy]

Expand on your description of "backfire." Has it ever blown back through the carburetor? If so, Hersh's idea about the power valve may be right on target. If you mean it sounds like it is missing on one or more of the cylinders on one side of the engine only, then I agree it is likely ignition related. ... After my last engine rebuild, it was running spark plugs that were too cold and they would foul out much sooner than expected. That caused the engine to sound like it was running very badly. Replacing the plugs is relatively easy to do to rule out that as a cause.