I HAVE A 2000 400EX AND I HAVE MODIFYED THE STOCK MUFFLER AND AIRBOX AND PUT IN A 158 MAIN JET. THE NEEDLE IS IN THE STOCK POSITION. I CHANGED IT AND IT DIDN'T HELP ANY. MY PILOT IS STOCK 38 AND I'M GOING TO CHANGE IT THIS WEEK. MY PROBLEM (IF YOU WANT TO CALL IT THAT), IS MY PLUG IS BLACK AROUND THE THREADS AND TAN ON THE TIP AND CERAMIC. I'VE TRIED A 155 JET AND THE NEEDLE MOVED BUT IT RUNS WAY TOO LEAN( RED HEADER PIPES). IT RUNS VERY GOOD ALL THROUGH THE POWERBAND BUT IS HARD TO START IN COOLER WEATHER. ANYONE HAVE ANY SUGGESTIONS?

 

I can't offer jetting help, but some 400EX owners have trouble starting in cold weather. I think bumping up the pilot jet does the trick. I haven't had this problem with mine, and I've ridden it a few times here in Minnesota.