last summer i went on the paiute trail in utah. awesome trip. elevation ranges from about 5000 on the low to almost 12000. both my 03 ranchers seem impervious to anything including altitude. my 05 foreman always ran rich even at sea level and would become hard to start and severe power loss above 6000 ft. i rejetted with the dynajet kit and set it up according to their recommendations for 3000 ft. the atv runs perfect up to about 9500-10000 ft. but shows some power problems above that. the suzuki vinson on the utah trip was having major problems above 7000 and a brute force wouldn't start at 5000.