I have a 1987 cougar 500 fan cooled. The sled only has 2100miles. I put in fresh gas, new plugs and plug boots. everything else is bone stock. When I took this sled out yesturday, it ran great for the first 10 miles, then the top ends drops off almost in half. Then Rpms seem to be up in the 5500-6000 range, but the sled will not get over 45. The secondary clutch buttons look a little worn, so I am going to replace them. What else could be the problem?