Unless I am reading Adam's post wrong, he said he had the jet needle all the way down with the circlip at the top, which is the first position.That's why I suggested he raise the needle to the third position, which is the nominal position. With his needle dropped as it is he's very lean in the 1/4 to 3/4 range.
Another thought is on a machine that old it could very well be a worn jet needle & if so, no amount of adjusting will fix it. I had that happen on a Suzuki I had one time.