I haven't heard the way the engine sounds when you go back and forth across the cut-off point. Lean mixture is usually accompanied by a 'lean spit' as you gradually increase to the cut-off, and perhaps a bit of a hiccup when you reduce the throttle and it starts firing again. If you don't have anything like that, there is a possibility that the kill-switch is malfunctioning rather than it being lean mixture. If it is running fast and is killed by the switch, if you let it stop completely with the throttle wide open then take out the spark plug it should be wet. If it died of lean mixture the plug would be dry.
