For an engine to run at those sort of revs its got to get air from some where,If its got a manifold leak between carb and head, it would run lean, as it cant pull much fuel from the carbs.running lean can add to the higher revs.It would have to be a big leak for it to run with the chokes closed so do a manual test by blocking the carb intakes with your hand when its running, No air, engine should die.If still runs its serious leak.If it dies, then its mostly an adjustment problem,so check your idle screws are wound out so the slides are all the way down to the bottom of the carb throat,then start re adjusting from there. There's also needle hight to consider, if thats been played with at all, it should be almost fully down in the main jet when the slide is down.when thats checked the idle mixture will need setting may be 1.5 to 2.5 turns out from closed (usually the fully screwed in position is closed) Sorry i cant give you specifics for your correct settings, this is general carb info.someone may have more detail for you. Its been a long time since ive worked on minuki carbs and It may not be very acurate description of the adjustments. Steve.