I am not certain,but I think if you follow the linkage to the injection pump, you will find a solenoid that holds the linkage at a given RPM at idle. Try manually pushing the linkage to kill the engine after you turn the switch off. The solenoid might be clicking but not going far enough. I have had the adjustment be off and just shorten the rod to let it close off the fuel also. Last case, you can disengage the idle solenoid and adjust the linkage to where it will die when you throttle all the way back. Still turn the key off to disconnect the power to keep it from running the battery dead. Let us know what you find.