Given your high idle setting, the most likely cause is the throttle plates are openned too far on the idle speed adjustment. Dieseling usually is a combination of the throttle plates not being closed sufficiently, and high cylinder temps, hot spots in the cylinder or spark plugs that are retaining too much heat. The high heat could be due to carbon buildup, vacuum leaks or lean mixtures and insufficient cooling. But often, if your vac advance is not operating correctly, the temptation is to raise the idle speed by screwing in the throttle plate idle stop which smooths out the idle. Retarded timing at idle will cause higher cylinder head temps. When the ignition is shut off, the plates are open enough allowing air to carry more fuel into the engine, where it is spontaneously igniting due to the high cylinder temps. Check to make sure the timing at idle is correct, and that the vacuum advance cannister is working properly.
Try adjusting timing and & idle speed. These two things will lower temps, lower the engine rpm's by closing the throttle plates more.
Doug
