Hub routine maintenance. Just do it!!
My tail end started getting squirelly on me the other evening. (Last day of the workweek and almost home, by the way. It really is a valiant little bike.)
It started throwing the chain. Did it once and I put it back on on the side of the road. Rode about ten feet and it threw it again. Replaced and made the remainder of my uphill section of commute on foot. Started on the downhill and it threw the chain again. Coasted the rest of the way home.
My suspicion was a broken axle. Everything felt a bit 'soft'.
What I found was no broken axle plus the coaster brake mechanism seemed to be okay. I then turned to the drive sprocket, of course, but found no problem there. I still don't have any explanation for that trouble. I can only suspect that my axle shifted forward in the dropouts and loosened the chain.
What I did find, though, was that there was still some grease inside that hub. But not as much as, I think, I put in there and it was clearly dirty and ready to be changed.
From about a year ago until now I've been inside that hub four times. The first two times I didn't pack just as much grease as I could into it and wasn't surprised that there wasn't much there when I opened it up. Number three, though, I spread it in there pretty generously. But there wasn't a lot in there the other day, all the same. And there wasn't a hint of red in it, either. It was dark grey. It was only three or four months old.
I'm beginning to see the handwriting on the wall; hubs, and the other bearings, need to be re-greased often. Probably with the change of season. If not more.
I've never had to do it this often with a pedal bike. But I guess conditions are different with a motored bike.