[Excaliber]

I wouldn't worry to much about the 'run on' condition when you shut the engine off. Thats generally caused by one or two things.
1. Idle speed to high.
2. Octane rating of the fuel to low.

As the engine continues to push the pistons up and down AFTER you turn the key off (which is ignition only) the pistons continue to draw a fuel/air through the carb. Just enough for the compression stroke to 'self ignite'. A small piece of carbon glowing red hot perhaps, acting like a 'glow plug' ignites the mixture. Or it might simply be the 'heat of compression', like a diesel engine with no spark plugs. Higher octane fuel is harder to ignite so it takes a 'real' spark to set it off, thats why higher octane fuel works to eliminate 'run on' or 'dieseling' as some call it. By closing down the carb butteflies, and thus lowering the idle speed, it's more difficult for the pistons to pull in a fresh charge of fuel/air, thus starving the motor to death.

Run on can be a VERY serious problem, until you get that resolved use the clutch with the car in gear to stop the engine at the same time you turn off the key. Run on can cause a timing chain to jump, break pistons and wreck havoc in general if it's pronounced and allowed to continue. In some cases the engine will actually run BACKWARDS, very bad when this happens.