Loosen the axle nut and rotate the chain adjuster to the same position on both sides of the swingarm until you obtain the correct chain slack, then tighten the axle nut.
it is self adjusting .
By removing a link in the chain itself.
Yes, you could. Is it a good idea? Probably not. If the tensioner is worn so must be the chain.
owners manual, page 122.
To change the clutch on a 2002 Yamaha 660 Raptor, start by removing the clutch cover bolts and taking off the cover. Disconnect the clutch cable from the lever and remove the pressure plate by unscrewing the bolts. Take out the old clutch plates and replace them with new ones, ensuring they are properly aligned. Reassemble by reversing the steps, making sure to adjust the clutch cable for proper tension.
Here are instructions for a 2004. There should be little, if any difference.
buy a Yamaha yz 125 if that's to slow for you
2002
Oil capacity for the gearbox on 2002 Yamaha YZ85 is 500cc, after overhaul 550cc according to my book.
It has a Chain.
A Yamaha ATV that small is carbureted , not fuel injected.
2002