90 has a linearly moveable member rigidly attached to fulcrum 65 whereby said actuator means can linearly move the fulcrum to a predetermined position altering the drive lever ratio a gaseous fuel under a predetermined pressure is introduced into fuel inlet passage 5 b through a fuel inlet fitting 80 most components are identical to the preferred embodiment and unless explicitly stated otherwise the numerals and corresponding definitions are identical to those in fig6 the differentiating factor between the preferred and alternative embodiments is the trim valve geometry and kinematics indeed a linear as opposed to rotary trim valve 57 preferably conically shaped is slidably mounted into the housing linear trim valve 57 controls the flow area of trim fuel passage 5 d and is kinematically linked to variable ratio lever 60 by means of a valve rod 58 referring now to the preferred embodiment in fig6 incoming air impinges on air leading surface 10 a of air flap 10 causing the flap to rotate about its axis in said first angular direction d 1 against the tension of return spring 20 under steady state airflow the return spring tension balances the pressure force acting on face 10 a holding the flap in a position determined by the point of equilibrium between the two forces increasing airflow will move the flap in the direction d 1 and decreasing airflow will cause the flap to move in said second angular direction d 2 opposite to direction d 1 under the action of return spring 25 at zero airflow the spring urges air flap 10 to an initial position against a.