But what surprised me, is it shifts to and from the large (34T) cog without complaint, and it's the small one (13T) it won't play with nicely - chain skips all but the lightest load. Is it just a matter of chain length in this case? The chain is really loose when it's running on the smallest cog. (Standard 52/39 chain wheels up front)
The other thing that surprised me is when it's running on the big cog, it seems to still have some give in the spring; as if it's not completely maxed out at 34T.
It's a 1980ish Shimano 600 Arabesque medium cage RD. Original freewheel it came with was a 5 speed 14 -28 (worked fine with that one); the one I'm experimenting with here is a 6 speed, only one tooth smaller on the small cog (13 - 34).
I'm pretty inexperienced with tuning RDs, having only changed them out on a few bikes, and know the long-cage ones seem to always soak up any range I try.