An adjustment system for a rotary valve having a drive wheel with drive wheel teeth that can turn about a first axis of rotation to drive a driven wheel with driven wheel teeth that can turn about a second axis of rotation that is substantially parallel to the first axis of rotation. The teeth extend in each case around only part of the circumference of the respective wheel. The driven wheel can be adjusted by the turning of the drive wheel from a first rotational position, in which the teeth are mutually engaged, into a locking position, in which the driven wheel is held in place when the drive wheel turns further. A rotary valve with such an adjustment system is also described.