output of the comparator q1 assumes a near ground potential as shown in curve 5b accordingly a base current may flow through the diode d1 and the resistor r4 into the base of transistor t1 causing it to conduct the collector circuit of the transistor t1 then carries a current i whose value is adjustable by an adjustable resistor r6 and this current i is fed to the remainder of the circuit in the exemplary embodiment of fig3 to the multiplier circuit 308 so as to cause an increase of the pulse length t m and thus an increase of the fuel quantity fed to the engine as already mentioned the fuel quantity is increased by the ratio of i 1 i 2 i 1 the current flowing through the resistors r4 and r5 charges a capacitor c2 connected to the anode of the diode d1 so that at a certain time the base potential of the transistor t1 rises for example to the value 0 6 volts and the transistor t1 blocks again thus the time during which there is a supplementary fuel supply is limited to the time of conduction of the transistor t1 as may be seen this time can be determined or altered by appropriate dimensioning of the resistors r4 and r5 and the capacitor c2 and preferably by the use of an adjustable resistor r5 the time constant of the delay circuit r1 c1 in the input circuit of the comparator q1 is so chosen that the output time constant of the comparator q1 is uninfluenced but if the output voltage of the air flow rate meter 2.