I have a 1998 summitx 670 that has no top end power. On a stand at 4000 feet elevation it runs great, pulls 8100 rpms. At 5500 feet elevation and higher where I ride, this sled will only
pull 6200-6400 rpms and run about 50-55 mph on flat trails wide open. It has no hill climbing
torque and the longer the hill the worse it runs. I have had it worked on by two different shops
but it still runs the same. The first shop replaced both drive clutch halves and pulled the driven
clutch to clean. He also changed the carb jets, we then disconnected the dpm tubes to the carbs
but made no difference in performance. The second shop aligned the engine to the clutch, tuned the carbs to each other, removed inline filters to the carbs and checked compression. The engine has 135 lbs compression on both cylinders. They also pulled the head to check the top end and said it looks great. I purchaced the sled used last year, it has around 3100 miles.
Any ideas on what else to check?

If the sled runs good at low elevations then the first thing I would check is to see if it has solid pins in it, after 5000' you need to use hollow pins and a little tighter converter. -- Bill

If you don't have the shop manual you should get one because it has the high altitude setup in there for you.

http://www.snowmobileworld-s2.com/forums/iB_html/non-cgi/emoticons/smile.gif If anyone else has this problem, I replaced the cdi computer and that fixed the problem, the
sled now pulls great at 5000 ft and up.