In this study, a hydrogen-fueled spark ignition engine was investigated for UAV operation, with a focus on its combustion region and idle condition. A 2.4l, four-cylinder spark ignition engine was used for experiments, with modification for hydrogen usage instead of gasoline. In experiments, the feasible combustion region and limitations of the combustion phenomena for the hydrogen-fueled spark ignition engine at specific load and speed conditions of 50 Nm and 2000 RPM were examined. It was found that owing to the wide flammability range of hydrogen, the air–fuel ratio could be varied from 1.0 to 2.4. However, misfire and backfire occurred because of the mixing issue and highly ignitable characteristics of the hydrogen–air mixture, respectively, under relatively rich conditions (excess air ratio between 1.0 and 1.5). Unstable combustion occurred under a relatively lean condition (excess air ratio>2.0). Considering important parameters such as the nitrogen oxide emissions, unburned hydrogen, and brake thermal efficiency, an excess air ratio between 1.8 and 2.0 with maximum brake torque timing (MBT) operation was appropriate for this condition. Extremely lean conditions with zero load can be achieved with stable combustion with the excess air ratio up to 3.0, which can almost eliminate nitrogen oxide emissions.