what xt have you got? x, r or z?

I think what your really talking more is not response but free revving under load, as the xt motors spin up freely when not loaded but on a standard engine, in gear under load it takes a while to rev the engine trough the range. I think you will be looking at the basics of air filter replacement, fuel mods (either kevs or PC's) and exhausts. each will help the engine take in more air, make more of a bang and allow the fuel to exit the engine better

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Plough on