I have had this with my Saito 56. When leaning the HS needle what seems to be a peak is reached i.e. no rpm increase with further leaning. Thinking I had maxed out the engine (7700rpm or so) I left it and flew, engine was obviously rich so after landing I leaned the HS needle more (pinching the fuel line helps to confirm that the needle is still rich). After another (several) clicks the rpm starts increasing again. This is where you need dto go slowly - 1 click at a time and allow several seconds between clicks to let the adjustment take effect. Almost a full turn leaner the rpm was up to over 10 000rpm, now I richened back to 9700rpm checked the transition and been great ever since, minor adjustments for temperature only.