My 1975 CB550K ran just fine, except in the rain. About a month ago, I installed a Dyna-S ignition, which corrected my goofy timing issue instantly. It does, however, require a source of 12v power, which is tapped off at the line leading into the coil, a rather exposed spot under the tank. I had not ridden in rain since installing the unit. As soon as light rain caused a mist in the air, I'd lose power and eventually stall, or sputter on and off, depending on the amount of water kicked up by the front tire. The engine resumed normal running as soon as the wiring dried out. My fellow rider ZanVooden suggested simply wrapping the exposed connection with plastic from a grocery bag, which I did at our lunch stop. We didn't encounter any more rain, but I bet that simple fix will work next time I'm caught out in wet weather.
This was the bike's first long day trip, and it handled 60mph cruising on state highways like a champ. My best mileage was almost 55mpg, running ethanol-free 91 octane.