An adjustable arm assembly is drivably coupled to a control shaft and is moved by a link that has a centered position. The arm has two portions that are releasably clamped together. A first arm portion is connected to drive the control shaft. A second arm portion is connected to the link. The adjustment includes a clamp that will releasably clamp the two arm portions together in a fixed position, and an adjustment screw is used to adjust the relative rotational position of the first and second arm portions to move the first arm portion to position the control shaft in a centered position relative to second arm portion, when the second arm portion is in a spring return position. The arm portions are then clamped together.