Technical FAQ: Vintage derailleur shifting problems
I have an older steel trek with all Campy Nuovo Record components. I recently did maintenance on the rear derailleur including new cable, housing and general clean-up.
After replacing everything I find the shifting to be overly sensitive and it now takes a lot of adjustment to seat the chain completely. The adjustment is usually a longer throw than the movement required to shift initially. As all of the repair books/manuals document Ergopower and STI, and have no coverage of older derailleurs do you have any suggestions as to what the problem may be?
I think you have too much friction in your cables, even though you just replaced them. Perhaps you put a tight bend in the housing to the rear derailleur? Those derailleurs originally came with an unlined steel coil housing there within which the cable did not slide particularly well. If you did not already, you should replace that with a piece of lined derailleur cable housing. Clean and grease the cable guides at your bottom bracket, too.
Another cause can be a worn chain that is laterally too floppy to move over a cog when the derailleur pushes on it.
Another possibility is that the chain is too long so the gap between the upper jockey wheel and the cogs is too large.