Dirtbike sputters bad?

i have a 2000 rm 250 it sputters bad it starts in first gear when the powerband is suppose to kick in then sputters constantly in the rest of the gears i replaced plug and put 90 octane gas in it it still sputters why is it doing this? oh and the bike will not idle on its own at all i dont think its topend it starts 2nd kick