the rotated position of the lever 25 operated by the trigger wire 30 when the number of revolutions of the engine again increases and exceeds a predetermined number of revolutions the vibration sensor 101 again opens and the opening degree of the throttle valve 27 is decreased by the actuator 81 the operation as described above is repeated whereby the speed of the engine is maintained at a level less than a predetermined number of revolutions and the overrunning of the engine is automatically prevented without the need of the operator 39 s control of the trigger wire 30 according to the variation of load in the embodiment shown in fig5 an actuator 181 connected to the upper end wall of the body 35 of the carburetor 24 is actuated by negative pressure applied from a vibrating pump 141 through a vibration sensor 201 members corresponding to those shown in fig4 are indicated by reference numerals to which 100 is added provided in an atmospheric opening 149 of the vibrating pump 141 is a check valve 154 to allow a flow of air from a pressure chamber 146 to the outside on the other hand provided on a passage 152 is a check valve 148 to allow a flow of air from the vibration sensor 201 to the pressure chamber 146 the vibration sensor 201 is designed so that a ball 207 is urged against the end of a passage 152 by means of a spring 204 accommodated in a housing integral with a port.