My friend has a stock green one. Above 5000 ft it runs terrible. Spits and sputters unless it's held wide open.

Dealer sold him some main jets that are leaner. We put in the leanest one and no change. What about a pilot jet? Any other ideas?

THX

 

We're at 6,000 ft here in New Mexico. The Yoshi pipe works great with stock jetting...go figure.

 

Yesterday I rode my stock KFX400 at 6000+' and it ran very well. It was a little rich but still ran very well.

 

So nobody has problems with the bike except my friend. Weird.

The bike is pretty much unrideable over 5000ft.