Two methods control Shimano bicycle gear shifters. The first method deals with cable tension that operates a spring in the derailleur. The second method, which is often overlooked, is the limiter adjustment. The limiter adjustment keeps the chain on the gears. If the limiter adjustment is not correct, the chain will drop off the gears on either side or prevent the bike from shifting. The high and low adjuster screws are two small Phillips head screws on the back of your rear derailleur. The screw on top is the high gear adjustment. The screw directly below that is the low gear adjuster.
Put the bike in a bike stand. Turn the pedals with one hand as you shift the bike chain to the biggest gear using the handlebar gear lever. Shift the bike to the biggest gear in front and the smallest gear in back.
Turn the pedals with one hand, and reach up with your free hand to push the handlebar shifter to change gears to the second smallest gear in back, then immediately shift into the smallest gear in back. Watch the chain as it is moving, If the chain hesitates to drop onto the smallest gear or chatters before it drops into the smallest gear, turn the top limiter screw counterclockwise one-quarter turn.
Listen and observe the chain. Turn the high limiter screw clockwise if the chain chatters on the outside of the smallest gear, this means the derailleur has travelled too far to the outside of the gear. Tightening the high limiter screw restricts derailleur travel. Loosening the high limiter counterclockwise allows more travel of the derailleur. Repeat shifting from the second to the first gear and adjusting the screw until the shift is smooth and quiet.
Turn the pedals as you move the handlebar gear shifter to move the chain to the smallest gear on the front and the biggest gear on the back. Push on the shifter even farther after the chain has shifted to the biggest gear. If the chain drops off the gear to the inside of the wheel toward the spokes, turn the low-limiter screw one-quarter turn clockwise. This will limit the inward travel of the derailleur.
Shift the bike back down to the second biggest gear, then immediately to the biggest gear. If it does not go immediately to the biggest gear, or is hard to push up to the biggest gear, turn the low-limiter screw counterclockwise until the chain moves smoothly up to the biggest gear. Repeat shifting between the two gears, adjusting the low limiter until the bike shifts smoothly.
Do not turn adjuster screws more than one-quarter turn at a time. They are very sensitive and only need small adjustments.