Parts and drawings of assemblies can be found here:
http://www.yamahagenuineparts.comhttp://www.canns-bilco.comhttp://www.powersportspro.com/pages/parts/viewbybrand/2/Yamaha.aspxThe tractors were made between 88-90, and many parts are now obsolete. Most people don't know Yamaha made a yard tractor. The 6700 is most like the 6800, and I believe the deck size was the only difference, otherwise the engine, tranny, etc were all the same.
For cutting problems, you have to make sure your deck is level. Get a deck leveling tool, and check from the blades to the level surface. Should be equal from side-to-side, and front-to-back. The procedure is the same for all yard tractors, you can look it up. The adjustments are on the pieces that attach to the deck.
The engine and hydro both use 10w30 motor oil. The powersportspro site above has detailed parts drawings and is pretty useful. The engine is 18hp.
Rattosh51
YT6800