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

StrokerScamp said
If the intake is hung open, it will not pop.

Disagree, I've seen it happen. I believe what happens is the open valve sucks out of the intake, then blows it back into the intake, and if the thing is open enough, when the plug "goes off" it fires by chain reaction up through the intake.

THIS IS NOT CAM TIMING I don't believe.

I believe you've either

1 wiped a cam lobe

2 have one or more valve adjusted too tight

3 damaged something in the valve train.