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Installed an Alivio RD-4000, brand new, nearly same issue occurs. Upper and lower limits are seemingly perfect, cable tension has been fiddled with, and it just won't shift down to 4th and won't shift up to 8th speed. I bought the derailleur from a bike shop, mechanic guy clamped the bike on a stand, eyeballed the hanger and said it should be straight—I don't see any sort of deformation either. Brand new shift cable and housings btw.
Shifter is crisp and feels great in general, but is rather old and used, can by any chance that be the issue?
vpCogworks-
Shifter is crisp and feels great in general, but is rather old and used, can by any chance that be the issue?
If the indexing detents were worn, I wouldn't expect it to affect the upshifts at all, and probably not the downshifts either unless you short stroke it.
You haven't said which cassette you're using. Is it in good condition? Is it of the kind where you might have accidentally slipped an 8-speed spacer into the stack instead of a 9-speed one in one or more positions?
gbj_tester
Or it could be as simple as the slightly wider pulley cage of the 8-speed RD not playing nicely with the narrow 9-speed chain when it's not pushed to either one side or the other