My rebuilt 300 is slow to take off. If I go to take off slowly, without letting the clutch out above 5,000 rpm, it almost dies. Before my rebuild it seemed to go a bit better taking off calmly. Jetting hasn't changed, seems like it could be a powervalve thing, but I've got them set correctly though I'll double check that tomorrow. Can't think of what else it could be.
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MC21 Honda NSR250. TYGA 300 conversion.

even though you haven't changed your jets have you checked that they aren't blocked or gummed up? no air leaks or improper seals from assembly?