Motor is popping back through the carb... Help!

Tom, you're not going to like it. Something is holding an exhaust valve shut. This will not cause a skip, but reroutes the exhaust back up through the intake port and out the carb. Although it does not skip, it certainly does disrupt an otherwise good running engine, doesn't it? Things to look for on the good side: Broken rocker arm, bent or broken pushrod. Things on the bad side: Wiped camshaft lobe. I said you wouldn't like it.