predetermined length from its free length in the neutral position of the throttle valve unit as may be appreciated the left hand and right hand shoulder portions 9a and 9b of the inwardly projecting curved portion 9 function to position the throttle linkage including the throttle lever 8 and the throttle valve shaft 4 at the predetermined neutral position corresponding to a predetermined throttle valve intermediate angular position when the throttle actuator 3 is de activated reference sign 12 denotes a wide open throttle position stopper 12 simply a wide open throttle stop whereas reference sign 13 denotes a closed throttle position stopper 13 simply a closed throttle stop when the throttle actuator 3 generates a driving force acting in the rotational direction of throttle valve opening clockwise direction viewing fig1 and thus the left hand side movable block 11a is rotated in the anti clockwise direction viewing fig1 through the throttle linkage namely the throttle valve shaft 4 the throttle lever 8 and the arcuated arm portion 8a the maximum possible anti clockwise angular displacement of the left hand side movable block 11a or the maximum possible angular displacement of the throttle valve 2 in the valve opening direction is restricted by the previously noted wide open throttle stop 12 on the contrary when the throttle actuator 3 generates a driving force acting in the rotational direction of throttle.