Originally Posted by
cny-bikeman
The sagging chain is due to freewheel friction. Rinse it out with solvent then lube with a fairly heavy oil, letting it drain before reinstalling on the wheel.
I hope I can do so since that would make everything much easier and cheaper, but it's not entirely clear how I'd rinse it out. People generally seem to advise not taking apart freewheels since it is so difficult to get the bearings back in, and I don't see any obvious way to get a solvent (or oil) inside the freewheel mechanism. It's a 4-prong SunTour freewheel, if that makes any difference.