I will assume with your self diagnosis that you do not have a carburetor? That is the only way you could say it's not dieseling.

Before we get the necessary info/history/back story and motor accessories, the first answers will be dieseling or too high of an idle....or a fancy "hot" spark plug. There are other heat related causes. Lean/rich/carbon/plugs/heat riser/exhaust leaks/timing.....also a bad ignition switch or wire or a miswire.

1. Is this a new motor install? or rebuild?
2. Does it have headers? Do they leak?
3. Been running ok until just recently this started happening?
4. Have you just completed any motor work/replacement of any parts. Done any timing?
5. Carburetor? Carburetor model? Hot rod motor parts?
6. Any wiring done recently?
7. Spark plug part number? Stock coil?
8. Mileage on motor?
9. Happens after short runs and long runs? If you start it up cold and run for two minutes, does it still run on?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn