A rear mech that doesn’t index correctly can ruin a ride, this can happen for a number of reason’s. The most simple and common cause is a loss of cable tension, get the tension right and the gears will work like a dream. Here’s how:
Make sure that your in your highest gear with the chain sitting closest to the frame as pictured.
Shift up into the next gear, this should happen as you here the indexed “click” in the shifter. If you here the “click” but the chain hasn’t shifted up to the next gear, cable tension is too low.
The barrel adjuster is used to increase or decrease cable tension. If when in top gear once the shift lever is operated the chain does not move up into the next gear, you need to increase cable tension. Likewise if the shifter is operated and the chain jumps up two gears, cable tension is too high and needs to be decreased.
To increase cable tension, the barrel adjuster must be undone – turned anti-clockwise. To adjust it correctly make sure the chain is sitting in the highest gear. The lever is operated but the chain doesn’t shift up to the next gear, keep turning the pedals whilst turning the barrel adjuster until the chain correctly moves up into the next gear.
Continue shifting through the gears, one “click” one gear. If the shifting quality deteriorates as you go through then just increase cable tension (anti-clockwise) by a half turn.
NOTE: Gears are always adjusted by shifting into an easier gear. Once they are correctly set in this fashion they will also index correctly as you drop back down into a harder gear. If you are in a situation whereby the gears will not index correctly when dropping back down into a higher gear, the most likely cause is friction in the outer casing. The solution is to replace both inner and outer cables.