Hey David,
I had a similar problem with my '84. It would start ok at first, but then not have enough juice to start the bike after a short (one hour) ride. The battery looked good and would charge up fine on the charger. It wouldn't loose a charge over night when left on the bike. It seemed like the only problem was that it didn't seem to be charging while i was riding. I thought that it might be the stator, but since I'm not very mechanical I decided to try swapping out the battery before I got into something that I didn't know a whole lot about. It ran fine with the new battery and I haven't had a problem. I figure that the battery was done for even though it looked good. Don't know if this helps, but it might be a simple fix. Good luck.
Dana.