This instructable will help you to tune up a rear derailleur.
Step 1: Tools
You will need:
Cleaner( I use Pedros Oranj Peelz)
Step 2: Clean
First thing, put some cleaner on your rag and clean the outside of the derailleur.
Step 3: Pulley Crud
Next clean the crud off of the pulleys. I just pedal the bike while running the rag over the pulleys. The inside of the top pulley can get filthy...
Step 4: Clean the Chain
Next clean the chain.
I put cleaner on a rag, then turn the cranks backwards while holding the rag over the chain right at the bottom of the derailleur cage.
Step 5: Lube the Pivots
Drop of few drops of chain lube on the pivots of the derailleur, the most important are 8 spots on the parallelogram.
Wipe off any excess.
Step 6: Lube the Chain
While we have the lube out, lets lube the chain, I use white lightening, it has worked well for me for years.
I drip lube on the chain above the cassette.
Then use your rag to remove any excess lube from the chain, you don't want the lube to be on so thick that it gets dirt and grime caked on.
Step 7: Grease the Bottom Cable
If you have been using this cable for a while, crud can build up in the housing down by the rear derailleur.
What you can do is shift all the way up to the biggest ring in the back.
Once the chain is in the biggest ring, do not pedal, but shift to the smallest ring.
Step 8: Clean the Cable
At this point you can pull the housing and cable out of the last cable stop on the frame.
Step 9: Slide the Housing Up
Slide the housing up, if the cable has any corrosion on it you can use some green scotch bright to rub it off.
Step 10: Lube the Cable
I put a little grease on my fingers and lube the cable before putting the housing back on and in to the cable stop on the frame.
Step 11: Lower Limit
The outer limit makes a big impact with shifting performance.
Shift to the small ring in the back, look at the upper pulley, the teeth from the pulley should be perfectly aligned with the teeth of the smallest cog.
If it is sitting too far in towards the frame then we will adjust the outer limit.
Step 12: Outer Limit
You will turn the screw marked "H" in to move the pulley in, or the opposite for out.
Step 13: Inner Limit
At this point shift up to the biggest ring in the back.
We want to grab the derailleur by hand and see if we can push the chain over the cassette and in to the spokes. If you can you want to turn in the limit screw marked L. This chain popping in to the cassette can ruin a ride as it tears through your spokes and leaves you walking home.
Step 14: Barrel Adjuster
Now its time to fine tune the derailleur.
You will want to turn the barrel on the back of the derailleur in or out to line up the pulley with the smallest cog.
When it is lined up then try shifting through the gears.
If it shifts up to bigger gears fine but down to smaller gears not so well then you will want to tunr the barrel adjuster in slightly. Keep playing this game to you have it shifting right.