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
White deposits on the end of the plug indicate that the fuel/air mixture is too weak.