One of the main causes of poor shifting performance in a bike that had formerly shifted pretty well is excess dust, dirt, and contaminant in the derailleur housing.
This video demonstrates a technique on how to lubricate a bicycle derailleur cable without having to use any Special Tools. The trick is basically that you run the gears up to the point that the derailleur is at it’s maximum extension under tension (on the biggest cog for standard, or down on the smaller cogs for a reverse, or “Rapid Rise” derailleur) and then, without pedaling, you release the shifter to the other end of the spectrum. By leaving the derailleur at it’s full extension, but releasing the shifter you create a bunch of slack in the cable (because the chain on the cogs holds the derailleur in place at full extension. With the cable slacked out you can remove the derailleur housing and cable from it’s guides (if the guides are slotted which is the standard on most decent bikes.) Then with the cable housing able to slide up and down on the cable itself, you can lubricate the cable, inspect for kinks, and clean.
The best part of this technique is that you can do it quickly, you can do it anywhere, and it does not mess with the derailleur adjustment at all.
Duration : 0:11:20