I have to run a high idle rpm (1000-1100) on my engine, well, I don't 'have to' but I like it up there. If I run 'race gas' it shuts off fine, but that's a hassle and expensive, so I just run premium gasoline from the pump and 'deal with' the run on by using the clutch to kill the motor.
With a high idle speed after shut down by the key, the motor continues to pull air/fuel through the carb(s). This fuel/air is then detonated simply from engine compression/heat and/or glowing embers in the combustion chamber, which will ALWAYS be there no matter what you do. Self detonation is effectively 'deiseling'. Lower octane fuel ignites more readily than higher octane fuel, which is why the higher octane fuel resists self ignition and usually stops engine run on (deiseling).
Lower engine rpm idling, in my case, tends to foul the plugs if it is prolonged (long red light) and can cause a miss fire when I accelerate as the plugs clean themselves off. Thus, I prefer a higher idle rpm to avoid the hassle of a miss fire. An occasional blip of the throttle also works to clear the plugs, but what ever... Higher idle speed and a clutch kill works for me.
