What causes "dieseling"

Dieseling is usually caused by a combination of things:
- high idle speed. Rotating assy has more momentum at higher rpm & tries 'to keep going'. Lower the idle speed. See no reason for 800 rpm, 4 speed, & stock engine.
- retarded timing. Timing being 'late' helps the engine to run on; it also causes hotter engine temps which causes hot spots. Try advancing the timing a few degrees.
- hot spots, hot carbon particles in the combustion chamber, usually with high mileage engines. Engine at fast idle, try pouring water slowly down the carb bores to steam-clean the chambers.