Running-on is when the engine continues to turn after you've switched off the ignition because the fuel/air mixture is being ignited by a hot-spot in the combustion chamber. Furthermore, because the mixture is often firing at the wrong point in the cycle, the engine does not run smoothly but jerks and rocks badly on its mountings (and sometimes backfires). The problem can be so bad that the engine runs on for a minute or more if left alone. Read more
Causes of running on
Running-on occurs when the fuel/air mixture in the cylinders ignites without a spark. This is known as the dieselling effect because it is caused by the fuel igniting spontaneously in the combustion chambers, which is what occurs (deliberately) in a diesel engine. There are several causes.
Check all the possibilities shown here, starting with the easiest ones.