Abstract: A fuel injection system and method of operating it for a two-stroke crankcase compression internal combustion engine. The engine is provided with a main fuel injector that injects fuel directly into the combustion chamber and an auxiliary fuel injector that injects fuel into the inductions system. A throttle valve controls the flow through the induction system. The system includes a detector for detecting an abnormality in the operation of the main fuel injector and shuts the main fuel injector down when this occurs and increases the amount of fuel supplied by the auxiliary fuel injection. When this abnormal operating condition occurs, the degree of opening of the throttle valve is also reduced.