My buddy has a 01 cr125 that has horrible low end bog and thought maybe somebody would have a suggestion for jetting at 800-1000 ft and 75 degrees. Or could it be the power valve?
Well I'm not experienced BY ALL MEANS yet, but since your friend has a jap bike, a Mikuni carb, and lives at 1000' in 75 weather, he is like me except i have a 250.
If it is a jetting problem, then he needs to go leaner on the pilot circuit probably and play w/ the air screw too.