I am experiencing an overrun on revs when turning off fuel tap. recent BVF carburretor clean and tune gives steady tick over with electronic ignition system at around 1,200 revs, but on turning off fuel tap revs speed up automatically up to c.6,000 revs unless I turn off at key switch – any ideas what’s happening here?
Sounds like you are burning an accumulated amount of fuel built up in the crankcase. Big industrial two strokes can do this leading to a ‘runaway’ engine. I would be looking at seals, gaskets and float chamber levels.
Some fun with a RAP single cylinder semi-diesel. It fires, runs away despite fuel being shut off and rags stuffed over the crankcase air intake, but away it goes!