What would cause a lawn mower motor to sputter, pop and backfire when you shut it off? Bad gas?

Sounds like it's running too rich and may be carbonned up. Small engines, particularly 2 cycle engines tend to become clogged with carbon after time. It builds up on the cylinder walls, the pistons and the exhaust system. It can make the engine hard to start eventually and has the effect of raising the compressions. If the carb is adjusted to run rich (too much gas to air ratio), then there is excess fuel present. The engine may run great but when the ignition is shut off, the excess fuel ignites due to the high compression (dieseling). You can a few seconds of sputter, pop and eventual backfire when the engine finally stops. Step one- adjust the carburetor fuel mixture to the manufacturer'[s specs. Usually 1 1/2 turns from full off is a good starting point. If the problem persists, then tearing down and decarbonizing the cylinders is the only way to eliminate the excess carbon. One way to verify that this is the problem is to run the engine at full speed at night. You can see little sparks of burning carbon comming out of the muffler in the dark. It's a sure sign of excess carbon.