Abstract: The invention is an arrangement for controlling the air/fuel ratio A/F in a carburetor. The arrangement can be connected to a membrane or a float carburetor and controls the mixture ratio by influence on the differential pressure .DELTA.p between the fuel chamber (18) and the venturi (12). The subpressure from some part of the intake system (16) is passed through a magnetic valve (32) activated by pulses of a variable length. In order to equalize the whole or part of the subpressure there is an opening (33) with inlet from the atmosphere to the valve. By varying the pulse length a changeable subpressure is obtained which is connected in lieu of the regular atmospheric ventilation of the carburetor.