Run-on; Engine doesn't shut off

Check your timming. It might be to far advanced.
You also might have a just have a bad tank of gas.

http://www.answers.com/topic/dieseling

Dieseling or engine run-on is a condition which can occur in sparkplugged engines whereby the engine keeps running for a short period after being turned off.

Potential causes

This condition can occur for a multitude of reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, for they can glow red. It has also been suggested that an improperly rated sparkplug can retain heat and cause the same problem.

A carburetor that does not close entirely can contribute to running once the engine is off, for the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase and provide ample fuel for dieseling.

Incorrect timing.

An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.

Idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running.

It's been my experience that in most cases the "run on" is caused by too much fuel at idle.
- If your car idles at over 800rpm, try turning down the idle. Also Try adjusting your carb's idle mixture screw so that the mixture is a little leaner at idle.

Also: Timing should have no effect. Once your key has been turned off, your sparkplugs no-longer fire...thus timing is a non-issue. However, turning down your timing can cause a slower idle speed which may help..but the proper way to control idle speed is via carb adjustment, not spark timing.