the conventional ignition device and the present embodiment on the same engine is shown in fig1 it will be seen that the fuel efficiency is improved for 2 12 according to the present embodiment in the calculation of the vr value the microcomputer operates to limit the variations in ignition timing so as not to be less than 1 u00b0 as this will provide a smoother feel to the operator and simplify the system by permitting the use of a simpler computer as shown in fig1 the minimum variation intervals minimum increments of the vr value or resolution to 0 625 of the total variation range of the vr value the value of 0 625 corresponds to 0 0125 of the vr value 2 0 u00d7 0 625 100 the load fluctuation in response to the change in the vr value increases in high engine speed and low load thus with a vehicle such as a motorcycle of approximately 100 kgf kilogram force 1 kgf u2248 9 8n in the vehicle weight or the like it is found that the conditions as described above are nearly met if the minimum variation interval of the vr value is determined as the change in an engine load of approximately 7n u00b7 m newton meters also indicated as nm 1 kgf u00b7 m u2248 9 8n u00b7 m thus the resolution 0 0125 corresponds to approx 7n u00b7 m of the load change in the high engine speed and the low load therefore the fluctuation of the load 7n u00b7 m approximately corresponds to the change within 1 u00b0 of the ignition timing as shown in fig1 it is found that the change within 1 u00b0 of the ignition timing corresponds to 7n u00b7 m of the change in the engine.