250-500cc Restricted wr250????

Hi everyone! i`m new to Huskies but have had many dirt bikes in the past..I recently bought a 2010 wr250 uk, the power is fine up to 3/4 throttle where it just goes flat! it revs out put does not make any more power, I changed the main jet from 440 to 480 it pulls a little harder around mid range but still flat at top end, standard needle on 3 clip, powervalve connected and opens when revved, do I need to go bigger on the main or am I missing something! all comments welcome