Abstract: A control device, which is interposed between a modulating motor and a valve, includes a control arm which is mounted on the shaft of the valve and a roller follower that is on the arm. In addition, the control device includes a bracket which is mounted on a fuel line adjacent to the valve, a cam which rotates on the bracket about an axis that is parallel to the shaft and an operating arm which rotates with the cam and is connected to the modulating motor, so that the modulating motor moves an air damper and cam in unison. The cam has an arcuate element which curves eccentrically about the axis of rotation and screws which are threaded into the arcuate element such that their ends form a cam profile. The roller follower bears against this profile, riding over the ends of the screws as the cam rotates.