400ex k&n air filter
 

Some people say you should re-jet if u put a k&n air filter on.do u have 2? how much extra horsepower will it add?

400ex k&n air filter
 

To me, it's not worth just doing a K&N without doing the header and muffler, and venting the airbox. I do know that if I slap a K&N on an otherwise stock 400EX, it will hardly run, which means you definitely need a rejet. So, it does something, but by itself it won't be dramatic.

400ex k&n air filter
 

i have just a kn and no airbox lid and it runs fine, no rejet needed

400ex k&n air filter
 

I dont wanna take off my airbox lid. Where i ride it would get very dirty. What about putting a k&n airbox lid. Would i still have to rejet?

400ex k&n air filter
 

i didnt need to, i suggest taking the lid of cause it sounds alot deaper and an increase in hp but you do need to clean it alot, where i ride it is very dusty.

400ex k&n air filter
 

reconranger is right. You can't just put on an air filter and run good. Oh, it will run, but you aren't getting close to top performance. Especially if you take the airbox lid off. If you add a pipe, rejet, and add filter, you have the complete package. Each one complements the other.
think about it, your bike is jetted to run what the stock fuel flow is. If you push more air, you need to push more fuel. you accomplish this by running a bigger jet in the carb.

I can run the bases with one shoe off and one shoe on, but when I put both my shoes on I run much faster and without a limp!

400ex k&n air filter
 

Quote;i have just a kn and no airbox lid and it runs fine, no rejet needed .

The Honda 400EX comes from factory jetted on lean side for emmisions. Altitude and temperature are other factors that effect jetting. So for Tyson the only way his quote makes any sence would be if his riding area is at high altitude &/or hot temps. With his set up at sea level and cold temps he should have problems. Better to error on the rich side than take a chance damaging motor by running lean. I have this notion that in early stages of running lean the motor could be damaged before the rider has a clue from riding performance.

400ex k&n air filter
 

So your saying that if we're in high altitude, then it would not matter about the lid being off or is it the whole filter in general? Right now we're in the mountains of Idaho about 30 miles from the Canadian border. But when we go home we have to go back to about almost 1000 feet in Washington. What would we ahve to do? I also asked the honda dealer if I would have to rejet it if I had just a filter and he said no, he said maybe if I had the filter and a slip on and definently if I had a full pipe and a filter. So how are you backin this situation up, just by your own knowledge? or by what other people are saying? Thanks

400ex k&n air filter
 

Ok.. I think i will wait till i get a pipe to re-jet. Thanks

400ex k&n air filter
 

there was a write a a couple years ago on pipes and jetting and how the manufactures spend a lot to get them very close to on then people change pipes and such and slowly get worse and worse. not all just the ones who don't know.