Abstract: In order to produce a transition mixture enrichment during acceleration of an internal combustion engine, a carburettor or other mixture generator 1 of the engine with a mixing chamber 2, a main throttle 4 and a choke valve 5, has the choke valve 5 operated by a quick-acting electric drive 6, for example a two-coil rotary setter, which is controlled by a microprocessor 8. The microprocessor is fed with signals which sense the opening of the main throttle 4 and preferably also other engine operating parameters such as temperature and speed. When the main throttle 4 is opened to accelerate the engine, the microprocessor 8 causes the electric drive 6 to close the choke valve 5 abruptly and temporarily. The extent of closure of the choke valve 5 is dependent upon the extent of the throttle opening and on the other operating parameters to give the required mixture enrichment. This arrangement avoids the necessity to provide an accelerator pump.