I recently was given a very nice Miele Mercury mountain bike. I'm fairly new to indexed shifters, and I'm having a bit of an issue with shifting on this bike which I cannot immediately see the solution to.
I did some work on the weekend to get it up to ride-able condition (replaced brake pads, cleaned and lubed the drive-train, etc.) and noticed that the derailer would not move inside enough to drop the chain onto the smallest cog. When riding it home from picking it up, I also had the chain drop off the outside of the large cog once or twice while testing the shifters, and it would not shift into the largest cog on the wheel.
I started by adjusting the rear derailer stops to allow it onto the bigger ring, that was pretty basic - problem solved for the rear, shifts great now.
I then addressed the chain-jump issue by adjusting the appropriate screw to stop it from pushing the chain so far off the cogs. I tried to shift to the small cog to make adjustments for that stop - it wouldn't. It stopped at the middle one. Checked the screw - not even touching yet, so it's not stopping the derailer. Shifted to the big ring, OK, clicked the shifter back again, middle cog. Further clicks in that direction have no effect, the derailer just doesn't want to go that far inside.
I checked the mechanism, and it looks like that's as far inside as it wants to go. My first instinct was to just ignore the problem - I live in the suburbs, the biggest hill I can find can be tackled with the middle cog and using one of the larger ones in the back. Oops, now the chain drops onto the small cog all on it's own under a load, randomly, if the rear is on an inner ring as well.
There's something about a self-shifting chain that won't allow itself to be ignored, so I appeal to the gurus of bikedom - what's happening here? Please let me know if you need further details - I can't post a photo as my brother has my digital camera for a while.
My thoughts are that maybe the derailer has been bent outwards? It doesn't appear damaged, and I hesitate to muscle it inwards if that's not really the issue. What leads me to this thinking is that it won't shift inwards and originally had a problem of shifting too far outwards, but since this is an indexed shifter and not a manual one I suspect there's an adjustment to be made elsewhere that I'm missing.