have you taken the carb apart? cleaned it at all? check the main, your floats, etc... only other thing i can think of is you got a bent exhaust valve, or your cam is 180 degrees out. easy enough to do on them motors. but then comes the question, do you have any compression?
1985 KXT 250 Tecate - kx cylinder, kx ignition, bassani headpipe, answer silencer, v-force reeds, 38mm airstyker carb, +2 franks swinger, raptor rear shock.
1996 YSF240 Blaster - maxxed out!!!
Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting "HOLY SH-T, what a ride!"