When a engine has a sprung or broken reed valve it will just pop but never run.

Non reed valve engines if they have a crankcase leak or bad seal will do the same thing.

Several engines have the reed valve on the side of the crankcase. Where the carb mounts expands out somewhat where it bolts to the crankcase. This is where the reed valve is.

If the reeds are just sprung, see if you can turn them over to make them tight. If you see a crack in one, it will have to be replaced. If a chainsaw dealer cannot fix you up, maybe you can make one?

Enjoy,

Jim