Teddy, up untill a couple of years ago, I would have agreed with you totally, ignition timing can not cause dieseling because the spark is shut off.
I proved myself wrong when working on an old V6- 478 GMC truck. This old critter always had a terrible dieseling problem when shut down warm. It had an Allison automatic transmission, so I could not put it in gear and kill it with the clutch like you do with a manual transmission.
I had to idle the engine as slow as possible and shut it off with the automatic in gear, after a few minutes idle, but still it liked to crack and pop along,dieseling with the ignition off.
The truck always ran a little warm and had the dieseling problem, but I just wrote it off as the nature of the beast.
I noticed the old truck seemed to run the exhaust system very hot, as wiring and hoses near the exhaust manifolds tended to suffer from excess heat. I got to thinking, late ignition timing is usually the cause of high exhaust temp, might the timing marks be off on this engine ??
I checked and yep, the vibration damper outer ring had slipped and the timing marks were more than 10 degrees later than the marks indicated.
I repaired the timing marks, correctly timed the engine and noticed it ran cooler, had more power and the exhaust ran much cooler. Another amazing thing happened after fixing the timing problem, the old GMC no longer dieseled when shut down warm ?? I knew the ignition timing was shut off, so it could not be igniting the fuel, but it finally sunk in that the late timing had been causing the exhaust valves along with the exhaust manifolds to run very hot, this providing the "hot spot" to ignite the fuel and cause dieseling. Correct ignition timing allowed the exhaust valves to run cooler, so no more dieseling.
I guess what I learned from that is to never say never, as LATE ignition timing on this engine did indeed cause dieseling. :)
Cant hurt to recheck timing on the old farmall, it might be part of the problem, along with too fast an idle.