I spent an unsuccessful 4 hours today trying to tune an indexed-shifted rear derailleur.
First I set the H and L limit screws. I verify that the chain goes over all the cogs, but doesn't go over the lowest and highest cogs. But then the chain shifts erratically between gears. So I use the barrel adjuster to adjust the cable tension so the chain shifts on clicks. Usually this is all I have to do.
But now for the strange part: after that barrel tension adjustment the thing won't shift to the smallest cog. Even weirder, if I put it in high gear, reach under the downtube and pull on the bare shift cable, and then snap the cable to let it go, it shifts into the smallest cog. But if I shift out, it usually won't go back.
I'm going nuts.
Could this be caused by a bad or binding shift cable? Or do I need a new derailleur spring? Or could this be something else? The cables haven't been changed in about 10 years (about 6 of those it was unused). The derailleur itself is an old Alivio MTB model.