"Adriad Marshall" wrote in message
om...

I'm trying to adjust the rear derailleur on my bike (Shimano Acer X in
the rear, Gripshift shifters in the front, 21 speeds).

. When I fine tune it so that
upshifting is perfect, the downshifting develops problems; and vice
versa.

There are even times when everything seemed perfect, shifting was
flawless in both directions, on all sprockets. But after cycling
through all sprockets a few times, shifting problems developed as
described above.

2 common culprits: bent rear derailer cage or hanger, or friction in
cable/cable housing. Friction usually reveals itself by slow upshifts when you
have the downshifting set properly, the spring in the derailer can only
overcome so much friction.

Low-end indexed components like those are pretty marginal at best. They're
hard to get perfect shifting from even when they're new and clean, when they
get worn a little and dirty, they get temperamental.