85? Wasn't that a 700 Nighthawk or something? I'd love to own one of them. DOHC makes more power than our SOHCs. Unfortunately, they just don't look like "classics", as they don't have the chrome and such that our older SOHC bikes have.
Personally, I think the 77 and 78 K models are the best years for the CB550.
The later F models are similar to the prior model years with the exception of the beautiful 4->1 exhaust from the factory, and the kickstart arm and footpeg changes that the 4->1 exhaust required.
Through re-jetting and cleaning my carbs, I really learned to appreciate the more refined carburetors on the 77-78 K models.
Otherwise, aftermarket support seems to have been better for the earlier years. Seat covers are available from Travelcade, but you have to order them directly from Saddlemen, because, for some reason, none of the catalog shops (JCWhitney, Dennis Kirk, etc.) seem to list the seat covers for the 77 or 78 K (they are slightly different both from each other and the earlier models). These seat covers are EXCELLENT and will last many years - much longer than an OEM replacement seat, if you can even find an OEM seat anymore.
Side covers are nearly impossible to find for the 77 and 78 K models, so make sure you get them with your purchase or are prepared to go hunting for them. I think Meier still makes them for the earlier models and the F models.
The clutch mechanism was refined and improved over the years, with 77 and 78 having the least problems and requiring the least "fiddling".
Engine refinements reached their pinnacle in the 77 models with the much-improved rocker arm shaft design.
77-78 K models also incorporated an automatic fast-idle cam that raises the idle when the choke is engaged, so no more fiddling with the throttle stop screw was required.
Enjoy!
