Carb Question

Unless you have the idle set way too high, its probably not the carb. Run-on or "dieselling" is usually caused by excess carbon buildup glowing in the combustion chambers , so when you turn off the ignition, these glowing bits will act like a spark and fire the excess fuel in the chambers, causeing the run-on.

To de-carbon the engine, you can use that SeaFoam stuff, or, ("CheapSkateWay"©)TRICKLE some water down the carb while revving the "p*ss" out of the engine, in Park/neutral, of course. The water turns to steam and really cleans out the excess carbon.