i just installed a kn filter on my 04 400ex and didn't touch anything else. i went to start it up and it wouldn't revv (barely run) with no choke. half choke it would run a little better but wouldn't revv too high. so i assumed that the air/fuel mix was too lean so i adjusted the air/fuel screw (i think it is, it's the smaller one next to the idle screw). i turned it clockwise for a little more fuel and when i got it where i wanted it fuel was pouring out of the overflow hose thing. i just turned it back to where it was originately and no more leak. i stopped running it after it was pouring fuel because i didn't want anything to get messed up. do i need to rejet? how many turns exactly do i need to go from original to get to where the high flow filter and carb agree? any help and info would be great. lol i was just hoping that i would install the filter and have no problems just more usable power... thanks in advance...

 

It sounds like the screw you were turning was the drain for the float bowl. It should be tightened all the way. If all you installed was a K&N then I wouldn't think you would have to rejet. Try turning your idle screw up a little and see if that helps.

 

so i wouldn't even have to adjust the mixture? i'm so confused with this jetting and rejetting it's like a whole new language i never have to deal with this drag racing...

 

Shouldn't. Maybe a 1/4 turn out more but for the most part if ALL you have is the K&N with lid on and no pipe I'd think you would be ok. The air/fuel mixture screw is the brass looking one that screws up into the carb from the bottom and really hard to get to with the carb on the bike.

 

thanks a lot man, it's great being able to put up a Q and have it answered quick by people who know what theyre talking about...

love this site.

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me too!

 

My experience with K&N's is that YOU NEED A REJET, even with the stock muffler and airbox lid on! Just adjusting the screw, isn't likely to do it. You will be RUNNING LEAN, and YOUR ENGINE WILL BE TOAST. Take the bike to a qualified mechanic, and get a proper rejet.

 

There are 4 circuits in your carb. Starter, idle, mid and main. The mixture screw will only adjust the mixture on the idle circuit.
Those 4 circuits work somewhat independant of each other, so while one may be too rich...another can be too lean.
Usually the jetting can be figured by drivabilty issues. Like bog, missing, backfiring, smoking and so on.
most likely the main jet will need to be bigger, by one or two sizes up.
Heres an article on jetting, and symptoms of lean and rich, and dont be afraid to jump into this and do it yourself, we all had to start somewhere.
jetting
It will help if you can find someone to help you get thru the first time you take it apart, maybe someone from an atv club, or maybe a knowledgable riding buddy.
Its better to be too rich than too lean, being too rich just wastes gas. Being too lean may cause engine damage.
After changing to a freer flowing air filter, youre gonna be leaner than it was before.

 

OMG!......................Anyone I have ever known who installed a K&N and nothing else has never experienced ANY performance gain. And they sure as heck didn't TOAST their motor. Putting a K&N on a stock motor does absolutely nothing. Installing a slip on will change the flow a lot more than just an air filter of any kind. He would actually gain more flow by removing the lid than just adding a K&N or any filter for that matter. I say save the jetting changes for when you get a slip on and remove the lid!!!!!!!

 

OZARK-Not my experience at all! If I put a K&N on a stock 400EX, even with no other mods, the thing will hardly run! JUST READ THE ORIGINAL POST, he says it "barely runs", and you say it does "absolutely nothing"????? He has refuted you right there (OMG)...........!

There are posts in here all the time by somebody who wants more air, but doesn't want to be bothered with a rejet. They heard somewhere more air is good, but they aren't really sure how or why, and they don't understand the air/fuel mixture principle. What everybody needs to understand is that if you don't need a rejet, you have gained nothing, and all you have done is spend you money for a useless mod!

One bit of advice for NOSLIW. If you are going to make other mods in the future (like the airbox lid, header, or a muffler), just wait and do it all at once. If you do it piece-by-piece, you will be rejetting for each one. Just do them all together at the same time, and have it properly rejetted once and for all, and then be done with it!