A fuel-metering unit adapted for use in an internal combustion engine having an exhaust pipe is described which unit comprises an air-intake suction tube, a randomly adjustable throttle valve therein, an air-measuring device mounted in the suction tube upstream of the throttle valve, by-pass means connected with the suction tube and bypassing the air-measuring device and means for metering fuel in amounts substantially proportional to the amount of air flowing through the suction tube. This unit is improved by providing control valve means associated with the by-pass means and adapted for controlling the by-pass means and thereby varying the proportionality of fuel and air in dependence on characteristic engine data, and by providing an exhaust gasmeasuring probe adapted for mounting in the exhaust pipe of the engine and adapted for controlling the control valve means.