get a new plug, mine did the same thing, plug looked the same as yours (minus all the rust and mud and stuff) put in new plug and it ran perfect. i would also bump the pilot up to a 42 works wonders on these ol girls.
then, if that dont work clean the carb good and adjust the valves
2005 400EX
99-04 plastic, 155 main jet, 42 pilot jet, HMF Sport Series Slip-On, UNI air filter, LSR PB Block off plate, TAG bars, AC nerf bars, 15/39 gearing, CST Ambush rears, stock Ohtsu fronts
1973 Yamaha DT360A Enduro
hooker headers/expansion chamber no other pipe
98% of the teenage population does, will try or has tried smoking pot. If you're one of the 2% who hasn't, copy & paste this into your signature.