[MiamiTrail]

I have a stock 2007 400ex, and I was planning to upgrade it by pulling the airbox cover and adding some jets. Exhaust is stock. Air filter is UNI. I ride mostly at the 4000' foot range and higher.

Suggestions where to start? Thanks.

 

[Here 2 help]

You start with stock jets and make changes as required

Best guess? You might go up about 2 sizes on the main jet for top end and if it doesn't idle right (after taking the top off the box) back the fuel screw out 1/4 to 1 full turn to smooth the idle - chances are the idle will not be affected

Of course you are using 93 octane without ethanol, right? Jetting is affected by the fuel you use also.