Any idea what would cause the engine to keep running after it is turned off? It finally quits. Seems to be worse if the car sits at idle for a while and then the key is turned off vs turning it off immediately after driving it for any length of time. I just got the carbs professionaly rebuilt and tuned on the car. Timing is factory set. Points, plugs, wires, condensor, cap, rotor are all new. Dwell set to factory spec's. All factory ignition parts used.