Abstract: An incinerator chimney stack valve mechanism which normally prevents the flow of fluid from the chimney stack. The incinerator chimney stack valve mechanism is mounted upon the chimney stack adjacent an opening in the chimney stack. A closure member is mounted upon an arm member and is movable toward and away from the opening in the chimney stack. An engagement member is mounted upon a second arm member. The second arm member is connected to a motor member which moves the second arm member for engagement of the engagement with the first arm member for movement of the closure member with respect to the opening in the chimney stack. The weight of the engagement member is sufficient to move the first arm member and thereby move the closure member with respect to the opening in the chimney stack if the motor member should fail to function properly.