I live at about 1300 feet above sea level (according to my Garmin GPS). This weekend my son and I packed up our bikes (he's 13 and has a bone-stock Honda CRF150F) and went to western Maryland to ride the trail in Green Ridge and Savage River State Forests. His bike ran fine. Mine, which I believe is running just slightly lean at 1300 feet, sputtered and coughed at 6000 RPM when we were on the trails, at roughly 2500 feet above sea level. His Honda ran fine. I could understand it if I was borderline-rich to begin with, but going from slightly lean to overly-rich doesn't make sense. Anyone have any experience with this?

I'm running a Bill Blue Mikuni with a #140 main jet and the jet needle on the richest setting. Airbox open or closed made no difference.

Side note: the Green Ridge trail (our first trail-riding experience) was terrific! Savage River's Meadow Mountain and Poplar Lick trails were L-A-M-E...basically a dirt road through the woods.

 

My bet is that with your setup you're not running lean at any elevation, certainly not not at FL2.5