Amer, a couple more thoughts: How do the present set of spark plugs look? Are they a nice light brown color with no deposits? If there are black deposits, they could glow and cause dieseling after shutdown; could also indicate a mixture/carb problem.

Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.