I have a 2000 400ex with K&N filter, white bros slip on muffler and a big gun rev box. The quad sputters when I open it up it doesnt do it unless its been running for at least 20min. I have to choke it about half way for it to run right. Any suggestions

 

I think with the mods you have done you haved leaned out the car. Take a look at the plug after riding for about 20 minutes, if the plug is white and ashy you are lean, which can cause your problem.

 

So i assume a jet kit will fix this problem.

 

Jet kit should take care of things.