Any 2stroke gurus?

I have a dirt bike (rm250 1999) that has plenty of power in gears 2-5 but in first it sputters through the power-band. I would say 80% of the time it sputters and the other 20% it rips right through the gears. This happens when the bike is warm and I've already checked the reed valves which look good to me.

Thanks in advance