the engine that continues to cough, puke, knock and sputter after the ignition is turned off, may be doing so because some hot spots within the combustion chamber continue to ignite the air/fuel mixture. But don't rule out an electrical issue that may cause the ignition to get an intermittent charge due to a worn ignition switch or some other wiring issue.
engines with mechanical fuel pumps and carburetors, only need the ignition system to ignite the spark. but an engine with carbon build up in the combustion chambers, or even nearly melted areas on the piston from detonation (pinging) and keep things going after the ignition is turned off.
ignition timing set on the advanced side can also cause this internal heating to be excessive enough to cause the dieseling.
A couple of things to try, is to rule out an electrical issue, but you seem to have done that by disconnecting the coil with the ignition system still on. other ideas is to temporarily retard the timing back 10 degrees or more, to a more cooler operating range. Another is treating for carbon build up. I did this once to an engine spraying a fine mist from a garden sprayer through the carburetor while giving it lots of throttle to keep it running. But heavy deposits built up over time may require chemical or, removing the heads to clean mechanically from the affected areas.
Don't forget the "italian tune-up" method. I've used it on a few diesel cars, too. It involves going wide-open-throttle on a steep hill incline, placing the engine under heavy loads and upper RPM ranges, to flush out that carbon. But be careful, if the ignition timing is pretty far advanced, this can make things pretty hot.
I'm thinking a really rich A/F mixture may also contribute to the dieseling for similar reasons, there is heat in the engine igniting the mixture after the ignition has turned off.
