I've never had that problem with my NSR300... then again I've only ridden on the street. Mine never feels like it would flip. If I let the clutch out below 6000rpm then it just really bogs down I have to get it around 7k and slowly let the clutch out feeding in throttle. If I do it right it will take off rapidly but only slightly come up on the front. Are you really dialling in the revs/being abrupt or is it THAT powerful? Mine gets 65hp on the dyno. Same gearing too.
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MC21 Honda NSR250. TYGA 300 conversion.