My 04 125 Which I have had no trouble with till now is fouling plugs. Its running a standard jet. 40-1 yamalube 2r. Had a new plug and fouled it after 40mins of trail riding. It had sat for 4 months (with fuel in carb) I drained carb, new fuel off I went. Done 22 hours on piston. Its not been nana riden either. Carb overhaul?