Air leakage into the intake pipe or crankcase is the usual cause of excessive no-load engine speed with crankcase induction 2 strokes. You said you have checked for intake pipe leaks, but you did not say you have looked for bad crankcase seals. On some of those Victa engines one of the crankcase seals sometimes works its way out of the housing, allowing suction leaks into the crankcase.
If it is not a suction air leak, there seem to be two main possibilities: the governor is malfunctioning, or you are expecting a lower idle speed than the governor can give you. I know people fairly often complain about the minimum possible idle speed of the Powertorque engine. It is high by design on that engine, because it needs to be. I do not know whether the same complaints arise with the full crank engines that had the black plastic carburetor, though.
