More than likely you clutch is full of belt dust and that is causing it to hang up and not shift correctly, lean the clutches and check for worn parts while it is apart, the spring should be fine I have 7000 miles on my Formula 500 and the springs weren't sagged like I though they would be for that much time on them. Make sure to clean the clutches both drive and driven. If your sled has RAVE valves and they are stuck and not opening, that will cause the engine not to rev out all the way so if it has them check them also.
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