My 2002 summit 700 is completely stock except for new v force reeds. the carbs have not been touched since new (other than being cleaned). At an elevation of 2200 ft, it runs good except for a slight hesitation between 4700-5000 rpm, or at about 40km/hr on the trail. However at higher elevations, this bog becomes extremely bad. The plugs seem to be a nice brown colour, what they are supposed to be. all other rpm ranges the sleds works perfect. any ideas what needs to be adjusted?