Engine Pops & Sputters When Turning Off
This problem is most likely dieseling, which means there is un-burned gas in the intake and it fires from the heat when turning off the engine.
Automatic transmission type of engines might have a dashpot mounted on the carburetor. Make sure this is adjusted correctly. See adjustments.
Running rich - Lots of things could cause this. Look at your spark plugs. If burning black then you are too rich. The same things that cause richness can also cause flooding. See flooding. A gas smell coming from the engine when idling also indicates it's running rich.