Abstract: There is provided a rotary valve system for an internal combustion engine which has the capabilities of advancing of retarding the openings of the valves. The rotary valves comprise a cylindrical valve body rotatably disposed within a valve sleeve. The valve sleeve is rotatably disposed within the head of the internal combustion engine. Both intake and exhaust functions have a rotary valve. Under normal operation, the valve sleeve and body rotate at the same speed opening intake or exhaust ports at the appropriate times. Advancing or retarding of the valve timing is accomplished by varying the rotational velocity of either the valve body or the valve sleeve which in turn rotates the valve sleeve relative to the valve body to either advance or retard the opening of the appropriate port. A second embodiment has a valve sleeve which is normally stationary but is also selectively rotatable to effect the valve timing.