engine cycle is determined by the amplitude of the trigger pulse from the electromagnetic pulse generating means 30 and the voltage on capacitor 53 at the time the trigger pulse is generated in the present embodiment the voltage on capacitor 53 is determined by the amplitude of the trigger pulse and by the value of the resistance of transistor 125 of timing circuit 100 a trigger pulse upon gating scr 49 charges capacitor 53 to a voltage corresponding to the amplitude of the trigger pulse and in the interval between trigger pulses the capacitor 53 discharges through timing circuit 100 to a voltage determined by the resistance value of transistor 125 a trigger pulse from the generator 30 gates scr 49 on when its amplitude exceeds the voltage capacitor 53 had discharged to in the interval between pulses timing circuit 100 controls the discharging of capacitor 53 by transistor 125 transistor 125 is actuated by switching transistor 120 to operate in its active region as a resistor over a predetermined range of engine speeds so that its resistance is not substantially a constant value during a portion of its operation when the resistance of the transistor decreases and capacitor 53 discharges to a lower level permitting trigger pulses from the generator 30 to pass to scr 49 at a lower voltage level this triggers scr 49 on earlier in the operating cycle of the engine hence advancing the timing the timing circuit 100 is powered from the main tank capacitor 27 through lead 14 in operation capacitor 108 charges.