[Excaliber]

The typical cause of 'run on' is low octane fuel. Small red hot deposits accumulate in the combustion chamber of ALL engines. With time, the deposits increase. These 'glowing red hot' deposits ignite the fuel\air mixture STILL present in the combustion chamber when you turn the key off. Because the motor is still running it draws in more fuel\air through the carb, it's ignited and the cycle continues.

By it's very nature and design high octane fuel is more difficult to ignite, it is more resistant to 'detonation', thus 'run on' or deiseling. It takes a 'spark plug' to get it to burn, glowing red hot carbon deposits generally cannot set it off.

If the idle speed is low enough when it's shut down the engine cannot pull in enough fresh fuel\air through the carb to support 'run on'. High compression ratios also contribute to the problem, but generally it's a high idle speed that is the culprit.

My engine idle rpm is best around 1100, but this is 'just fast enough' to support a bit of 'run on', sometimes it does, sometimes it doesn't. I'm in the habit of leaving the engine in gear and letting out the clutch (while holding the brake) when I turn the key off to BE SURE 'run on' won't happen. It can be very damaging to your motor under the right conditions.

With an automatic trans another 'trick' is to turn off the motor with the car in DRIVE, when the idle rpm is at it's lowest point at shut down time.