Adjusting the gears on a Shimano Tourney derailleur on a regular basis will make your chain and gears last longer as well as provide consistent, smooth shifting. This easy process will keep you in the pack instead of walking home on the next group ride.
Check the cable to ensure it is tight and remove any slack you find. Look for broken or kinked strands on both ends and replace if necessary.
Place the bike so the rear wheel spins freely while allowing easy access to turn the pedals by hand. Use a bike stand if one is available.
Shift to the middle front cog. Shift to the smallest rear cog, continue to the middle and large cog. Return to the middle cog and inspect the chain for stretching. Replace the chain if signs of wear is visible.
Turn the tuning barrel to the right as far as it will go with an allen wrench.
Crank the pedals while turning the tuning barrel to the left. Count each turn until the chain jumps to the next cog.
Divide the number of turns you counted by 2 and turn the tuning barrel by that number. For instance if the chain jumped on the second turn, reverse the tuning barrel one turn.
Use very small turns on the tuning barrel until the bike is shifting smoothly.
The tuning barrel on a Shimano dérailleur should turn easily, do not force it or the screw can break off.