I just got back from Colorado. I live in Georgia and my stock jetting has worked well for my enduros around here, including some elevation changes in the North Georgia mountains and the mountains in NC.

I called Apex sports in Colorado springs to ask what I should do with my jetting and followed there advice. I changed the needle, went to a 168 main and a 45 pilot. I had no bottom end(at low rpms), but my mid and top end were fine. As long as I kept my rpms up on hill climbs, I was fine, but when I had to lug a little, it just died. All our bikes did the same thing with the exception of a ktm400. It seemed fine.
I tried from a 158 to a 190, tried a 42, 45 and 48, tried everything between 1/2 turn to 2 turns on my air screw. I have a powernow in the back of my carb. Does that make a difference? My plug was almost a silver color. I figured I was running too lean. I finally gave up and rode with what I had. It made it almost impossible to climb steep hills.

We are putting together another Colorado trip for next year, but I would like to be more prepared with my bike. Can I get my 03 yz250f to act like a 250f at 12,000 ft? It was so dissapointing riding a dead bike.......
If I had any idea jetting was going to be that much of a problem, I would have done a lot more research beforehand.

Thanks for your advice....

At high altitude the jetting needs to be leaner, but not so much that it falls flat. Your mid and top end were fine, so the bottom is where you needed to focus.
I don't think the bigger pilot(45) was the right choice and would have thought a 40-42 would be more advisable. The needle clip helps, but taper also may be a source of problems. The KTM 400 uses a needle with less taper(stock) and will give the bike an advantage for higher elevation riding, less adjusting is needed. Jetting gets very complicated when switching tapers around and I'm working on making a kit specific to the YZ/WR250. Hopefully you will have an answer for next year's ride.

James