This thesis describes performance testing of 3W Modellmotoren's 100i-B2 which is a two-stroke gasoline engine presently being used to power a commercially produced Unmanned Air Vehicle (NAVMAR's Mako). Since the engine was originally manufactured for use in radio controlled model aircraft, the only performance information provided by the manufacturer is its rated power output of 9.3 Hp at 8500 RPM. However, much more detailed information is required for the UAV application in order to select propellers and engine operating points that maximize the range, endurance, and load-carrying capacity. This thesis reports the first detailed characterization of this engine's performance in the open literature that includes measurements of power output, specific fuel consumption, exhaust and cylinder head temperatures, and exhaust gas composition as a function of engine speed. The measurements show that the peak power output is 9.32 Hp at 8500 RPM with a brake specific fuel consumption of 0.797 lb/Hp-hr. The maximum BSFC of 0.668 lb/Hp-hr is achieved during ¼ throttle operation at 6500 RPM with a power output level of 5.08 Hp. Exhaust gas composition measurements indicate that the carburetor controls mixture ratio effectively across the entire operating range of the engine unlike smaller model engines. A preliminary attempt was also made to simulate the engine numerically in order to identify areas where the engine design could be improved. The simulation suggests that while the engine's performance is near optimal, it might be possible to gain additional power by decreasing the exhaust port duration.