Small Engines (Lawn Mowers, etc.): Toro Powerlite dieseling, compression ratios, combustion chamber

Question
My Toro powerlite (witch i've had about 10 - 12 years) has never let me down and still runs great. I maintain it seasonally.Last year it started dieseling when i turn the key to shut it off. The only way to shut it down is to choke it. It runs noticeably rough after i turn the key, but won't stop running. Its a two stroke Tecumseh, 3 horse. Pull start.My guess is it needs a carb rebuild. Any help is appreciated.

Answer
Excessive combustion chamber deposits can create hotspots and elevated compression ratios that can cause dieseling.  However in your situation i would first check the key switch to make sure it is making a good connection to ground out the ignition.