OK, get your mind out of the gutter, it's becasue of the stupid ultra stiff throttle return spring and the strap on my seat.
I spent about 4 hours on the bike over the weekend getting to know/road testing my resurrected '77 550k including a fair amount of highway miles at 65-70.
The good: runs smooth. Got good gas mileage (~46mpg), no sign of oil leaks or burning (well, except for the left fork leg).
The Bad: You have to WORK to hold the throttle open for long periods of time. I didn't have this issue with my Enfield or my FJ. I may test out disconnecting the spring before long rides just to see how it works. Also, being long of leg (6-4) the most comfortable spot places my tail bone right on top of the seat strap. That bastard is definitely coming off for the IL-IN-OH legs of the rely I plan to ride starting Saturday.
Finally, and this has nothing to do with the CB in particular, I was reminded how much I dislike spending lots of time at highway speeds without a windscreen of some sort. Fortunately, a good friend had one on an old '81 Z1 Classic he owns that he hasn't ridden in several years. So I picked that up to install.