More than likely, you're going to find that the THROTTLE VALVE SWITCH is NOT closing at IDLE . . .

This is the cause of this problem in the MAJORITY of cases . . .

It is remedied by:

Cleaning the THROTTLE BODY with carb cleaner and checking the THROTTLE BODY STOP SCREW adjustment, and then checking the THROTTLE SWITCH to be sure that the IDLE CONTACTS are CLOSED at IDLE . . .

Also check to be sure that the ACCELERATOR CABLE is NOT too TIGHT causing the THROTTLE BODY NOT to return to IDLE . . .