Hey guys just purchased my first 400EX. Got a really good deal on it, but it needs a little tuning. so i came here for the experts advise! Ok it fires right up, runs great as long as you dont get into it wide open, then it starts to cut out. you can go as fast as you want, just cant hammer on it all at once. i know the rings are shot, so i figure part of it MAY be the plug is nasty?? think it may be a carb prob? HELP!! i know nothing of these things, excpet i really love it!!

 

does it have a aftermarket pipe? maby no air box lid? Juist a guess but it could be running to lean wich would also make it run hot....wich if thats the case you could fix it by putting on a air box lid ,and/or installing a jet kit in the carb ....my 300ex is piped and jetted and it runs perfect but if you hammer the throttle from idle it will stall becouse it will dump to much fuel ,but if you let it start to rev before you hammer down it runs fine ......and when you say wide open you may be hitting the rev limiter wich will make the enjine stutter like its misfiring this is normal ,and if u want it to last try to shift before you red line it ....if it is doing the same as my 300 id say theres nothing wrong,but replace the plug only 2 dollars or so.. if your blowing white smoke it will eat plugs quick ,and id recomend rebuilding before you blow it up ..it will be alot cheaper if it hasnt gone to peices like my 250x wich i juist bought a motor for ( becouse i dintend rebuild in time)

 

No is definatly not the rev limiter. it does it just as soon as you take off, not even close to taching it out. i dunno much about it, but i am assuming it has a jet kit in it. it does have an aftermarket whitebrothers pipe. it was used for racing so it has alot of aftermarket stuff. i dont know if it is bored or not, but there is a weisco piston sticker on it. i dunno, i am hopin just a clogged jet in the carb

 

i jus picked up a 04 400ex and it had idel problems..but i seemed to work it out..but the other day i was ridin for about 3hours on tight trails and it started doing the same thing...ne thing past half throtle and it would bog but if i kept it at 1/4 it was fine all the way home...i jus toke the plug out and clean it and it ripped up the street fine...idk if it was jus over heating or what..

 

my 440EX did the same thing, my ckoke was begining to stick