Hi MF,
Something I thought of, is it possible, particularly if you're over-reving, and then switching off the mower, that it's flooding due to heavy load just prior to shut-down? That would cause excess fuel to be entering the combustion chamber but the spark plug would never spark and would become wet.
Something I normally do is to, after cutting, turn off the fuel supply and give the mower a 6-10 count before switching it off, that way extra fuel in the engine and carby is used, trying to prevent that exact issue. Maybe give that a shot?
Cheers,
pau13z
