[rotorwrench]

After fire in the exhaust can be due to an overly rich fuel mixture or leaky exhaust valves. Sticking valves can also cause enough delay in closing that compression gasses leak into the exhaust. A malfunctioning ignition system can sometimes cause a spark at a time when a valve is open to the exhaust but it usually warns the operator in other ways as well. It can cause missing and popping in the exhaust. The jumping sparks can usually be seen while the engine is running at night.

Lean conditions cause back fire through the carburetor throat. Some folks refer to an after fire condition as back fire but it doesn't fit the definition very well. When the exhaust was mentioned, I could tell the meaning though.