An HVAC (heating, ventilating and air conditioning) damper includes a series of pivotal damper blades that are opened by an air pressure differential across opposite sides of the damper. In some embodiments, a fan discharging against the damper blades provides the necessary air pressure to open the damper. To control how far the damper opens, an actuator moves a variable position abutment that obstructs the damper blades to limit how far they can pivot in the open direction. The position of the abutment is infinitely adjustable to provide the damper with infinite adjustment over a range of open positions. In some embodiments, the abutment applies a unidirectional torque to the damper blades regardless of whether the damper is opening or closing.