01 f4i mid range breakup problems
 

i resently bought an 01 f4i stuntbike, im getting a midrange breakup, but it has low end power and after prob 11000 rpms, i changed the plugs and the fuel an air filter, the fuel filter was hard to blow thru, so i thought i found the problem but nope its still breaking up, its not even like a spudder, its more like a lack of fuel, it feels like a two stroke with a nasty powerband, any ideas would help thanks, also the bike is totally stock except a slip on