German cuckoo clocks are popular timepieces from the famed Black Forest. Still made according to centuries-old traditions, they are powered by a system of weights. As the weights descend, they advance wheels inside the cuckoo clock. These wheels, in turn, move other gears that control the hands of the clock and also the cuckoo. The time itself is regulated by the movement of a pendulum. As the pendulum swings from side to side, it advances a wheel that controls the speed of the other gears. Often, repairing a cuckoo clock made in Germany can be as simple as adjusting the pendulum.
Watch the swing of the pendulum and listen to its sound. The movement of the pendulum should be even and regular, and its sound a steady "tick-tock."
Move the pendulum bob downward if the cuckoo clock runs too fast. The pendulum bob is the weight at the bottom of the pendulum. On cuckoos, they are often shaped like leaves.
Check the time on the German cuckoo clock again after it has been running for a little while. If it still noticeably fast, adjust the pendulum bob downward once again. Keep adjusting until the clock appears to be keeping accurate time.
Move the pendulum bob upward if the cuckoo clock is too slow. This has the effect of "shortening" the pendulum and making it run faster.
Check the German cuckoo clock's time again in a little while. If it is still too slow, shorten the pendulum again by raising the pendulum bob.
Once the German cuckoo clock seems reasonably accurate, keep checking it at the same time each day. Adjust the pendulum bob if necessary. Ideally, cuckoo clocks should be accurate to within three minutes a week. Check the clock only once a week, once this accuracy has been achieved. A pendulum that swings too fast will make the German cuckoo clock run fast, while one that swings too slow will make the clock run slow.
Make sure the German cuckoo clock pendulum can move freely. The pendulum must not hit the wall, or the chains that hold the weights. The clock itself must also be completely level for the pendulum to swing correctly.