Honda cr250 91 problem?

hi i need help real bad my cr250 it runs for about 5 mins and bogs out and it wont rev at high rpms it dies down i have cleaned my carb put fresh fuel and oil in still doin the same can some1 out there help me plz!!!!and my plug is always wet and black when i take it outi have already dun that my mixture is 50:1 and my mixture screw is 1 and half turns outwud it bog if my spark wor sparking at high rpms it might be breaking down ???????