Yeah Bob S - it's called dieseling because ignition occurs as within a diesel engine - ie. without the aid of spark. A quick review:
Running can be caused by one of 2 fundamental causes -
1 - Ignition spark continuing after the switch is shut off
2 - True "dieseling" - fuel/air charge ignited by something other a spark at the spark plug.
Causes for #1 can be the ignition coil being backfed from another 12 volts source - usually the alternator on an improperly wired 12 conversion - or (occasionally) by a bad kill switch on a magneto-equipped tractor.
Dieseling OTOH is caused by something in the combustion chamber remaining enough between power strokes to spontaneously ignite the fuel air mixture when the piston is near the top of the stroke - even though no spark is present.
Distinguishing between the causes is usually very easy. An engine with backfeeding coil or bad kill switch will generally continue to run pretty much unchanged (ie smoothly) after hitting the switch.
A dieseling engine however will run erratically, usually missing on a few cylinders and with a distinct metallic knocking after the ignition killed.
Common causes of dieseling is foreign matter (a bit of carbon or a piece of head gasket protuding into the cylinder) that becomes heated red-hot during engine operation.
Other possible causes are using spark plugs several heat ranges too hot, having the idle RPMs set too high, not fully closing the throttle before hitting the kill switch, use of low octane gas in a high compression engine (not a problem with a tractor engine!), low coolant causing a grossly overheated engine, etc.
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Incidentally - and contrary to the opinions of some - changing ignition timing has NO effect whatsoever on dieseling. Very simply there is no spark to time when the ignition is shut off. Consequently "ignition timing" becomes a meaningless term.
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