I am planning on going to colorado to ride in the moutains. and i live in kansas so there will be a n elevation difference. and it says in the manual to change your jet, i know carburators well but i am not familiar with this one. and i think that it will run ritcher up there(i might be mistaken). And it runs really good and i didnt want to mess it up. so will the elevation really jack with the preformance or will the changes be minor? any advice will be appreciated

 

i had to update my prifile i had just bought this 400 ex used to have a 350x (Still Do though they are fun as hell to ride)

 

Youre correct that the motor will be much richer at altitude.
How high do you plan on going? Is it just for this one ride?
One of the easier things to do, especially if youre not going to do this steady, is to add venting holes in the air box lid. And while riding in lower elevations, keep them sealed up with duct tape. Then as you go up, and it starts to show symptoms of running rich, start pulling the tape off, one at a time, until it runs better. If you really cant get to lean enough, then remove the lid completely. You could put like 6 or 8 ,1" inch holes in the lid.
Then as you come back down the hill, retape the holes.
Running too rich, really wont hurt the motor, but running too lean can,
Heres an article on jetting, and it has the symptoms of rich and lean, at the bottom.
jetting

 

Hondabuster has a good system.
FYI, I just rode last week in Utah at around 6,000 feet. I removed my airbox lid on my stock 400EX and it ran great. Colorado can get up there....10K to 13K feet. Removing the airbox lid at that elevation definitely won't hurt-it still might be rich.

 

ya just remember to rejet

 

pushed enter to fast ^

but im sure you will half to rejet it, but if you go there just have someone that is specilized in atv's to give u a hand

 

Yes you will have to rejet it i just got back from riding the mountains in colorado. I lost power on this one mountain that was 14,230 elavation so i had to take the air filter lid off to get up the rest of the way. Then when u get back from the trip you will have to rejet it again.

 

If you stay below about 9000 ft you should be OK. If you go higher you will need to rejet for sure. When I went out there I changed both the main and the pilot jets. Dropped the main down 3 sizes and the pilot down to the 38 for the high places. Don't know anything about the airbox lid being that I don't run a lid. Most people only change the main, but the pilot needs to be changed also if yuou are going realy high (10,000+). You will still fell a lost in power after the jetting.