There are a variety of devices the purpose of which is to evaporate the fuel components entering the intake in the liquid state by preheating the fuel-air mixture. In this manner, a better preparation of the fuel-air mixture is achieved, whereby the presence of constituents injurious to health in the exhaust may be diminished. Preheating of this type is advantageous especially at low load, since it prevents the carbureted fuel from recondensing in this operating mode, with resulting combustion of a leaner fuel-air mixture, thereby reducing the efficiency of the engine. At full load when the engine is hot, on the other hand, preheating is not desirable since losses of fill would result. To obtain controllable preheating, more or less effective depending on the operating condition of the engine, dampers have been proposed, arranged in the intake and/or exhaust line in order to deflect or divert the incoming fuel-air mixture and/or the exhaust flow from the heat transfer wall. Such dampers, however, have the disadvantage of interfering seriously with the direction of flow of the incoming fuel-air mixture by a deflection or partial constriction and displacement of the port area, and may lead to separation of the mixture, while in the exhaust line a resistance is created that undesirably increases the back pressure.