heat, load, air, fuel.
decrease heat, air and/or fuel
or increase load(leave it in drive when you shut off key)
Timing is not part of making an engine with the key turned off stop. Once the key is off spark ends.
Dieseling occurs as CI (compression ignited)combustion, contrasted to SI (spark ignited).
Retarding the timing lowers the rpm(and air/fuel flow). Also makes it run hotter(oppoisite of desired).
Just venturing a guess.....Probably running on ported advance and have the butterflies open too far to get decent idle rpm to compensate for retarded timing. If so, go to full man vac adv and then you can close butterflies down and get back in the idle circuit, reducing air and fuel flow and also cooling the engine. Or get the dam 190 stat out of it and put a good ole 160.