It’s often difficult to know where to start with gear adjustment. Sluggish shifting can result from a combination of factors, both constant and intermittent. The rear derailleur, in particular, relies on everything being set up perfectly so that all the components work together.
It’s also tricky to adjust gears because they behave differently under pressure. Gears that feel perfect when you’re trying them out in the garage can be disappointing when you try them out for real. Occasionally, the opposite situation occurs: you can’t get the gears to shift properly at all in the shop, then you go for a ride anyway and unexpectedly they feel fine.
Adjust cable tension
If you’re unhappy with the shifting, the most sensible place to start is with the cable tension adjustment. Click the shifters all the way into their neutral position (high gears for standard derailleurs, low for rapid rise) and then shift over into the neighbouring sprocket. If the chain doesn’t sit vertically under the sprocket or doesn’t shift crisply, you have an adjustment problem – see cable tension.
Check hanger alignment
Shift into the big sprocket and look at the chain from behind the bike. The chain should make a straight vertical line down the back of the sprocket and around the jockey wheels. If the jockey wheels are tucked in towards the back wheel, you have a hanger alignment problem.
Replace or clean cables and casing
If your cable tension and alignment are correct, but your shifting is still sluggish, your gear cable may be dirty, kinked or corroded. In particular, check the section of outer casing that connects the rear derailleur to the frame as it is vulnerable to getting squashed or kinked.
Cables (the wires) are among the least expensive parts of the bike, so changing them doesn’t break the bank. If you normally cycle in conditions where you need to clean your bike after a ride, think about changing cables at least four times a year. The rear derailleur cable is the bike part most susceptible to contamination because it transfers a very precise signal. Of all the repairs in this section, fitting and adjusting your rear derailleur cable is the best one to know. You can get all sorts of fancy cable sets that are designed to enclose the inner wire and keep it free from mud, but, in my experience, soil has an unrelenting urge to find its way into the piece of outer casing between my frame and derailleur where it lodges, specifically to make my gear changing feel like rubbish. Changing the cable is not a complicated job, and the reward is instant: an improved bike and ride. The first part of the procedure depends on what kind of shifter you have, so we’ll go through each of those in turn. Once you have the cable installed in the shifter, the procedure for adjusting the derailleur is identical. Start with the shifter that’s most similar to yours, then hop to the adjustment section.
Before you fit any new cable, you need to remove the old one. Cut off the cable end and undo the pinch bolt that clamps the cable onto the derailleur. Thread the cable back through each section of outer casing in turn, leaving the casing on the bike. Make a note of the cable route because the new one will have to go back the same way. If you replace the outer casing as well (which is not a bad idea – especially the last section that leads to the rear derailleur), then go to the section on outer casing. Cut the cable so that about 15cm (6 inches) is left poking out of the shifter, then follow the instructions for your shifter to get the old cable out and the new cable in.
Clean or replace your rear derailleur
Your derailleur will work much better if it’s clean and oiled. Give it a good scrub and oil it. Hold the bottom of the cage, near the bottom jockey wheel, and rock it gently toward and away from the wheel. Knocking, clicking or moving more than 4mm (around 1⁄8 inch) sideways indicates that the pivots in your derailleur are worn out.
If none of these works, check that your shifter is sending crisp signals. Shift into a large sprocket, then click the shifter as if changing into a small sprocket, but without turning the pedals. This creates slack in the cable. Pull the section of casing that joins the bars to the frame forward and out of its cable stop. Slide the casing toward the back of the bike. This exposes the cable as it enters the shifter. Take hold of the cable and pull gently away from the shifter. Operate the shifter, checking that, as you shift in either direction, the shifter pulls through little chunks of cable, and then releases them neatly, one at a time. If the shifter slips or misses clicks, service or replace it.
“Your gear cable may be dirty, kinked or corroded”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Rear hanger alignment”