An electronically controlled fuel injection system for controlling the air/fuel (A/F) ratio has been looked forward as a means for improving drivability, output characteristics, and fuel consumption of two-stroke cycle motorcycle racer engines. However, actual installation of such a system on a high output two-stroke cycle engine (which utilizes exhaust gas pressure pulsation effects) has been considered difficult for the following reasons. Fluctuation in the delivery ratio (L) during firing and misfiring becomes great due to effects from the exhaust pipe. Applying the control method used for conventional four-stroke cycle engines (by which the delivery ratio (L) is measured) would necessitate a large and heavy system.The authors have eliminated such problems by developing an electronically controlled fuel injection system, the PGM-FI (Programmed-Fuel Injection) system, which employs basic intake air flow data according to engine speed (NE) and throttle opening (θTH). Compensation for the delivery ratio (L) fluctuation in low throttle opening regions is carried out for depression at engine manifold (PB), while that in high throttle opening regions is carried out for combustion pressure (PI). Compensation is also enabled for ambient temperature (TA), ambient pressure (PA), cooling water temperature (TW), and exhaust gas temperature (TEXH), thus establishing a control method which is uninfluenced by operational conditions.The following paper discusses an example of applying this newly developed PGM-FI system on a two-stroke cycle motorcycle racer engine, whereby all the compensations mentioned above were made effective.