i have an 07 400ex with a removed spark arrestor stock exhaust, all else is stock. i removed the airbox lid to see the performance increase and there was an increase in performance but it made the exhaust VERY loud in the mid-high rpms and almost sounded like it was running super lean, but i doubt that a removed airbox cover can make it THAT lean with stock filter. why is it so much louder with no airbox lid?

 

Just keep riding it like that, and when your piston melts, you can come back here and whine to us about it! These are jetted lean at the factory (to meet emissions standards!), so you do some airbox or exhaust mods without a rejet, and you are way lean. Remember...a lean engine will often run better than a properly jetted one....so folks fool themselves thinking they have done something "good". But, it will run hot and might even toast your engine in the end....

The reason it is louder with the lid off, is that sound comes back up out the carb and into the box. That is why stock boxes are usually very well sealed.

 

your quad is already running lean from the dealer to meet emissions by you removing the air lid it is super lean.it is sucking in a lot more air so the motor can run better so its gonna be louder.yea its gonna be faster but your doing damage to the motor.

 

ya, i know how lean works and all, jst suprised me that it got that lean that quick, thanks.

 

my 400 is bored to 426, 11.5:1 piston, K&N filter, White brothers pipe, and has the air lid removed. Is that bad that the air lid is off?

 

Welcome to the forums.

Since this is an old thread, you may want to start a new one.

But since we're here...

With all the work done on yours, I would have to think it's been rejetted. If not, it's probably running like crap. I guess the question would be, was lid taken off when all the other work was done, or later on?

And a lot of people run without the lid. Just remember you can't go through the deep water.

 

My advice on running your atv without a air box lid would depend on your riding area and filter. if you have a k&n fabric filter with no airbox lid and most of your riding is in sand i wouldn't recommend that. in sand i would choose either the fabric filter with lid or foam filter without lid. also if you want extra protection on a fabric filter other than keeping it clean and well oiled would be a pre-filter it slides over top and blocks extra particles above and beyond the normal filter. also if you change something keep in mind that you may need to change the jetting too.