Finally joined TT after several weeks of reading posts. Trying to help a friend here with a 1999 YZ400. Bike idles high for a long time before fianlly comin down then it stall easilly, pipe gets red hot real quick, plug is white (all tell me it's lean) but it back fires like it's running ritch and if you pull the choke while hot it dies like it's flooding out. Need some ideas, pilot jet is a stock 65, turning fuel screw out doesn't help, turning it in didn't do much either (didn't kill it), cleaned the starting, pilot and main jets as well as the corresponding air jets and passages, clean air filter, new plug. Oh it also has a pretty bad flat spot just off idle, cleaned a badly mudded up accelerator pump and this made it better but still very noticable. One thought, could a bad coil give any of these symptoms? Thanks and great to be here.