Controlled mixture formation. This invention pertains to the generation of homogenous mixtures having a freely selectable composition and is produced by using a rotary distribution cup driven by its own motor, with metered quantities of fuel being supplied to the pre-chamber of a rotary fuel distribution cup via a feed pipe extending through a metering valve in such a manner that the composition of the mixture produced in each case can be adjusted, irrespective of the amount of the air, over the entire operating range of the internal combustion engine, with the fuel being radially distributed by centrifugal force, very finely divided and mixed with air in a mixing chamber wherein the quantity and composition of the mixture are adjusted by means of the metering valve in conjunction with a control valve in such a way that the production of the mixture and its conveyance take place at approximately ambient pressure, with a motor performance graph establishing the optimum setting for the metering and regulating valves for mixture production at approximately ambient pressure while improving fuel consumption and the exhaust gas quality, with the invention, in connection with the noted control system being especially suitable for the best possible operation of four-stroke engines with lean fuel mixtures at lambda values of at least 1.5.