An engine control system for controlling combustion of an internal combustion engine equipped with an exhaust gas purifying catalyst. The system controls fuel combustion on the basis of control parameters including air/fuel ratio, ignition timing and intake air flow so as to cause a rise in exhaust gas temperature while an exhaust gas purifying catalyst installed in an exhaust line of the engine is insufficiently warmed or inactive. When a fluctuation of engine speed is detected, the system controls the air/fuel ratio to reach a target value close to a stoichiometric value, for example, a value between 13 and 17, and controls a control parameter other than air/fuel ratio so as to bring the fluctuation of engine speed within limits of stable combustion while the exhaust gas purifying catalyst is insufficiently warmed. The air/fuel ratio feedback control may be commenced immediately after an engine start.