Join Date: Jul 2012
Location: Louisville, KY, Southern Indiana, & Montgomery, AL
Well, I haven't had alot of personal experience with the '85 500, but it seems to me in the back of my mind that there was an issue with the shaft drive splines on them. They have carburetors which I like AFTER I get them cleaned, synchronized and generally working correctly. Those jets gum up pretty quick. Try "mechanic in a bottle" if you need to try to ungum them.
Those old bikes are very easy to work on. There might have been a petcock vacuum issue, but it probably has been resolved. Also, seem to think there was a first gear issue with that year. Do a google search for issues and I am sure there are lots of posts about common problems and repairs.
All old bikes should have their fork seals checked, compression checked, check the regular oil and shaft hypoid. When was it changed last? If you can, go to the Yamaha xs11 forum and look for a thread (might me a permanent one) about what to check on an old bike prior to buying it. I seem to think it was a pretty detailed list of what and how to check, starting with the BRAKES. . .
I'd sure check for frame integrity with all that northern salt.
All those older bikes were extremely well balanced. Buy a manual for easy instructions on all the basic repair things. Just remember to have FUN with it!