Thanks aussietrev, I'm not familiar with the carburetor that Mitsubishi is using, and at the moment I'm a bit mystified about what could cause it to be lean when you aren't touching it, but OK when you hold the throttle open. I agree with both AVB and you that one of the standard things to do with a strange engine that misbehaves is to give it a proper tune-up, including cleaning the carburetor and inspecting the gaskets and controls. Even if the problem is not in the carburetor, that process will lead you to work through the external linkages etc, and that could lead to finding the problem.
I'm conscious of the experiences Mark Electric has had with the notoriously finicky Suzuki M120X 2 strokes, which seem to have to be set up by a combination of black art and sheer perseverance. Perhaps you've found another engine like that, with a linkage system that is both so arcane and so unstable, that only long experience (or advice from someone with long experience) with that particular engine is likely to solve the difficulty.
