Dieseling will make some clattering sounds as the engine struggles to continue running. A big difference in the way it sounds if the ignition system is still working due to alternator feedback. If you turn the ignition switch off and the engine clatters and runs terribly, then it most likely is dieseling. If you turn the ignition switch off and there is no drastic difference in the way the engine is running, then check for something feeding power to the coil even though the switch is off.
Ignition timing can contribute to this. If it is a bit too slow, sometimes the idle speed is adjusted to correct for slow idle, opening the throttle too far.
Bumping the timing up where it should be will tend to speed the idle up a little, and then you can idle it down to the correct speed.
So do some basic checks on the ignition timing, point gap and make sure is truly is "dieseling" rather that just running on due to ignition being fed power.
