Farmall c Won't shut off

I just Finished a complete overhaul on my 1950 Farmall c. I used 1/8th inch overbore sleeves and step pistons. After I shut it off it keeps running very rough and slow. The engine is"t even hot yet. I know there is no carbon build up it is all new. Could timing cause this, or is the fuel/air mix too rich, or do I need to get a higher grade fuel?
Thanks.

Is it 6 volt or 12 volts with an alternator? With an alternator it may be back feeding voltage to the ignition. I think I would use the 89 octane gas. Hal

Howdy
Could be----Idle too fast and/or timing off
Bob S.

Timing has nothing to do with it. Once you shut off the ignition there is no spark at the plugs. Idel it down slower to prevent dieseling.

Be sure to use a cold spark like D-15 or D-16 in Champion or same heat range in other brands.

"Timing has nothing to do with it."

Huh? Timing can be everything to do with dieseling.

Please explain! I would like to know how.

c-man's sounds like a new rebuild on the motor so I think it's safe to eliminate hot chunks of carbon or the burrs at the top of the cylinders being hot enough to make his engine diesel.

Cam timing can be off. Though it's still out there as a possible cause, I'll assume for now he got that right or never monkeyed with it at all.

Running an engine with the ignition timing retarded can easily cause it to run sufficiently hot to diesel at shutdown, especially if it has been run up to speed. In my exerience with older, timing is the notthe only, but the most common cause of his problem, and is only more common with the lower octane fuels we have these days as compared to what was available in the fifties.

That is not to discount your advice about his idle speed. If his valves are at all leaky, the angular momentum left in the motor could cause it to turn far enough after shutting down the igintion to draw in more fuel that would ignite. But I tend to doubt that a new rebuild with good compression and tight valves in proper time would run on that far.

My exception was not to your advice about the idle speed. It was only to the statement that timing might have nothing to do with it.