Several embodiments of engines and systems for fueling engines to reduce the emission of unwanted exhaust gas constituents and to improve the fuel economy of the engine. A catalytic converter is provided in the exhaust system and a gaseous fuel is used at least during starting so as to bring the catalytic converter up to its operating temperature rapidly without necessitating the provision of an enriched fuel/air mixture. An oxygen sensor is employed in the exhaust system and a feedback control system varies the fuel/air ratio of the engine during its running in response to the output of the oxygen sensor. In order to test that the oxygen sensor is at its operating condition before feedback control is initiated, a rich fuel mixture is supplied and this rich fuel mixture is discontinued immediately upon the receipt of a signal from the oxygen sensor indicating that a rich mixture exits. Applications of the principal to both air valve type of carburetors and conventional carburetors are disclosed.