An air flow rate sensor, located in the indication tube of an internal combustion engine, displaces a fuel metering slide, thereby changing the fuel flow and the fuel pressure gradient. This pressure gradient is applied to a differential pressure valve which actuates a fuel flow control throttle until the pressure gradient has been restored to a nominal value, corresponding to a desired fuel-air ratio.In a variant embodiment, an arbitrary change in the metered out fuel results in a fuel pressure gradient which is used to reset an air-flow control member until the nominal value of the pressure gradient has been restored, corresponding to a desired fuel-air ratio.