just bought a 2001 summit 800 with 488 miles, and at idle the engine seems to vibrate excessively, give it some gas and it stops also has alot of noise coming from the clutch.

broken or loose motor mounts

Mine does the same. The engine just rocks at idle and then rev it up and its smooth. My primary also sounds like it rattles when it is at idle. Once it warms up it goes away. Its not too loud though. Maybe just the way the Summit 800 is. I have 1000 km on it so far with no problems. To stop the rattle, hold the brake when you just start it and engage the sled. This will warm the primary and in 1-2 minutes any noise is gone on mine. Dont smoke the belt, just do it gently. Works on My Summit.

My summit shakes pretty violently as well. Same thing with the rattle, although I would call it more of a whistle. I can't find anything wrong and have been told that is the way they are.