i u201d for firing the spark plug 45 in a known manner this engine timing control device 54 is powered with electrical power from a battery 57 through a main switch 58 the ignition timing control device 54 includes an electronic circuit 59 constituted by a microcomputer or the like and the ignition circuit 55 consisting of a cdi capacitor discharging ignition circuit and a power circuit 61 the power circuit 61 is formed by a constant voltage circuit for outputting power source voltages for the ignition circuit 55 and the electronic circuit 59 the output from the sensor 49 is transmitted to a rotational speed detection section 62 of the engine system ignition timing control device 54 and specifically the electronic circuit 59 which outputs a signal n indicative of the rotational speed of the engine 22 during each complete revolution cycle in addition the outputs from the leading and trailing edges 52 and 53 of the timing mark 51 registered on the sensor 49 are transmitted to a degree of rotational variation detection section 63 this rotational variation detection section 63 outputs a signal indicative of the speed difference to an output restriction judgment section 64 in the described embodiment the flywheel 46 may be formed of a magnetic material and the sensor or coil 49 faces the rotational locus of the timing mark 51 in this case opposite ends of the timing mark 51 are detected from changes in magnetic resistance in the magnetic path passing through the iron core of the coil 49 alternatively the timing mark 51 may be formed from permanent magnets fixed.