Went riding this weekend and ran into problem that I need help to solve. XR 650R with HRC Tip and uncorked. 68s pilot, 170 main Stock air filter Riding between 6000 to 8000 feet at 75 degrees Valves adjusted about 150 miles ago and no problems running for last 2 weeks. Rode for about an hour and everything was fine on mostly logging roads. Bike started to backfire occassionly and started to really blubber and finally came to stall out. Kicked it over and started after several minutes and would run good for about ten minutes and then same thing would happen. I repeated this scenario for next 20 miles to get home. Bike would run through gears fine and accelerate good until it ran for about 10 minutes then wouldreally blubber then stall. Let it cool for 5 minutes and away again. Any thoughts would be great.