Originally Posted by
CliffordK
Perhaps some more details about the shifting problems.
Upshifting? Downshifting? Small, Middle, Large chainring?
Shifter? Derailleur model? Photos?
I worked on an older cheap Shimano MTB derailleur a while ago. It tended to trap the chain when upshifting from the middle to the big ring. The solution was to move the derailleur up enough that most of the shifting was with the flat part of the derailleur.
A bent derailleur cage can also cause problems, and might be able to be straightened if done with care.
Look carefully at the derailleur cables and cable housings. New slick derailleur housing might be helpful.
+1
Sometimes you need to raise the front derailleur to get it to work right. This is especially important if you changed chainring size. If you went from a 46t to a 42t large ring, (really necessary if you drop a chainring and just run middle/small), the front derailleur may not want to shift up if you set it at 2mm. I've raised a FD till I found a sweet spot. It does make it infinitely easier if you have a bike stand.
John