One of the most common causes of "running on" is a high carbon build up in the combustion chamber which at times the carbon glows red hot and acts as a ignition source even when the ignition is turned off, the engine usually chugs along at slow revs for a few seconds then stops or will actually run backwards at a higher RPM just prior to stopping, check your plugs to make sure they are clean,there are products you can spray into your carb or add to your fuel tank that will breakdown the carbon,you will see a lot of brown crap coming out your exhaust when you use it, or pull the head off and de-coke it
Russ