Backfire through the carburetor.

[SidecarFlip]

A bad cam, loose valve seat, bent pushrod , broken timing key , lean carb.

Lets add bad gasket on the intake side. One has to laugh sometimes. People think (wrongly) that changing the oil and spark plugs cures everything. it cures nothing actually.

 

[sgkent]

under load makes me think mixture. When the mixture goes lean the fuel molecules are farther apart, taking much longer to burn. That allows any remaining mixture to burn into the intake cycle and it spits back into the carb. So mixture is the first thing I look at on a carbed engine - piece of dirt in a jet, weak fuel pump, even old gas. In addition to other causes listed before this reply. another cause can be a backed up catalytic converter if it has a cat. On engines with points sometimes we see a failed condenser.