I love both these bikes, and for a long time I was doing a solid job with upkeep, but I've run out of time and space to work on them, so the last time each of them had a significant issue, it got parked. The bummer is that it was about a year ago for each of them, so they'll both need some extra love.
The 1981 GS550t has a GS650 (673cc) top end on it, so it rips. It has the original cam still – I'm told that if you can get the right GS650 cam it brings the power band slightly down the rev range. It was running great until it just stopped one day mid-ride and wouldn't start again. Could be anything. I imagine if you have a garage and a free weekend you can figure it out.
The 1978 GS750 was in incredible shape when I got it. I'm the second owner and the first had only put something like 10k miles on it in 40 years. I replaced everything rubber and it ran like a champ, until it started having electrical issues. It blew headlights a few times, so I suspect the harness is getting old and high-resistance. I fixed it by running an LED, which was fine. Around a year ago everything electric stopped working. I did some searching on the GS Resources website and the recommendation was the swap out the reg/rec and main fuse for more modern equivalents. I bought the pieces, but never had the time or space to get in there. I do think it would benefit from a whole new harness. That plus a cleaning and a reupholstered seat and it's basically still a new bike.
This whole process makes me pretty sad. If I had a garage and a workbench I'd still be ripping around town on these, but they've been slowly transitioning into scrap in my carport and if they complete the transition I won't be able to live with myself.
Make me an offer for either or both. I have a few extra bits that I'll also give you, mostly for the 550 (engine pieces, transmission pieces, the OEM exhaust). I'm keeping my lil' Shoei fairing with the horns on it, though.