Sounds like an air leak.My rmx 250 would do something similar - poor idling, low power, it would idle really high, until put it in first and slipped the clutch a bit.
After lots of head scratching, it turned out to be a slight leak in the base gasket and a decent one in the flywheel side oil seal.
Neither of those were detectable with carb cleaner, nor did the flywheel side seal leak oil like people usually say they do.
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