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Dieseling is caused by gas still making it's way into the engine, and igniting even when the ignition is off. I've heard that in a hot engine, carbon still left in the combustion chamber can be hot enough to act as an ignitor, and ignite the fuel still being dumped in. Other places that can be a snag and remain hot enough to ignite fuel are sharp edges of the combustion chamber or head ports caused by head rework. A small burr with very little volume will heat up very quickly compared to the rest of the metal in the head, and could be red-hot during operation, but still ignite fuel when ignition is killed. Other causes I've heard of are the incorrect heat range plugs not sufficiently burning the fuel-air mixture and a lean fuel-air mixture which will burn at a higher temp than a rich mixture. Timing is generally not the culprit here; timing that is off will lead to knocking under load, not dieseling. The first places I would try are the plugs and the fuel-air mixture. Any head rework done before? Just some places to check.
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