OK, i just purchased this 2001 400ex, and know next to nothing about it. the kid i got it from, didnt know about it either, he had only rode it one time. but it was DEFINATLY used for racing. The guy that owned it first owns a race track a few towns away, if that tells you anything. Anyway. it had a White brothers pipe on it, that was SHOt. i mean broke and bent and everything. unfixable. so my brother still has his stock silencer so i put it on. Ran great (just quiet!) but after just 15 minutes or so racing around a gravel lot, the header pipe was cherry red. HOW DO I FIX THIS??!! like i said, i am assuming it is probably bored, but i dunno for sure or how big. PLEASE HELP!!!!!!

 

Going from a high flowing aftermarket pipe back to a stock one, should make you run richer not leaner.....

 

yea thats what i would guess. but i also never rode it until i put the stock pipe back one. i did take out the baffel on the stock pipe but didnt get to ride it much to see if that helped. like i said with the stock pipe one, riding laps in a gravel parking lot for 15 minutes, i came back, and the header pipe was cherry red. what do i need to do???

 

The spark plug will tell you if you are lean or rich, white or pale tan, lean, dark brown to black, rich. An air leak in the intake could cause you to lean out, also, running a quad hard,especially an air cooled one, will get the pipe heated up pretty fast unless you are going fast ,moving a lot of air past the pipe and motor. Do you notice any missing or hesitation when accelerating? does it pop or back fire when you coast to a stop? things like this will tell you if you have a jetting problem, but pull the plug and check that first.

 

yea the air box was off alittle bit, got that fixed, and yes it does back just a little bit. just need to jet it down you think?? and it is hard to tell by the plug cause it needs rings too, so it is always a little black

 

Back fire is lean, you need to open up the low speed (idle mix) screw about a 1/4 turn at a time until it stops popping when you coast. If you are oil fouling the plugs, it will be hard to get a good reading, unless you buy several new plugs, warm it up on the old plug, put in the new one, run it hard, hit the kill switch and pull the plug for a reading, hopefully, you won't foul the new plug that quick, if it is burning that much oil, I would forget rejetting until you rebuild the top end and get it runnig right !!!