Trout,
I live in Utah and ride between 4,000 & 5,000 feet. I have always heard that for every 1,000 feet in elevation you will lose a certain amount of power that you can't get back. About the only thing you can do is bump compression to help make up for it. Also my local dealer says they normally take whatever main jet is recommended for sea level and multiply by .95 to come up with the right main for our elelvation. So a stock TRX450R has a 118 main and when I checked mine after dealer setup it had a 112 (or 118 x .95 = 112). So there are some things they try to do to compensate for elevation but any bike should not run as strong at a higher elevation as it will at sea level unless modifications are made. I am not sure why the Octane at our elevation is only 91 at the pump and sea level guys can get 93. It must have something to do with elevation. Take Care Scott in Utah.
04 - TRX450R - Front tires ITP Sand Stars 22x8x10 On ITP C Series Polished Wheels, Rear tires Skat-Trak 8 Paddle Haulers 20x10x10 On ITP C Series Polished Wheels - HRC kit installed - Armadillo Six Pack Rack
86 - ATC250R - Port/Polish, FMF Fatty, Delta V-Force Reeds, ITP Sand Stars 22x11x10 on Mitchell Gold Anodized Aluminum Wheels.