In this paper, an Electronic Control System (ECS) is designed to manage a 125 cc single-cylinder air-cooled motorcycle engine. The aim of this study was to accomplish low cost, high reliability and mainly meet the motorcycle engine emission standard of China Stage III.The intake port fuel injection mode was chosen with a redesigned part of intake pipe. Gathering information of speed, throttle position (TP), inlet temperature and pressure, cylinder temperature, switch-type exhaust gas oxygen sensor and the battery voltage, an Engine Control Unit (ECU) was devised to calculate fuel injecting pulse width, advance ignition angle and control the working conditions of the fuel pump and the exhaust gas oxygen sensor. Additionally, a three-way catalytic converter (TWC) was used to reduce exhaust gas emissions.The basic fuel injection table and ignition advance angles table according to the speed-TP were calibrated accurately through bench test with consideration to engine performance, emission and AFR. Taking account of the cylinder temperature correction term, air pressure correction term, the inlet air temperature and pressure correction terms and the battery voltage term, the AFR in stationery on a feedback control constantly fluctuated around the stoichiometric ratio. In terms of transient AFR control, a nonlinear compensator based on the fueling dynamics parameters x and τ was designed and the x Table and τ Table were acquired through the method of pulse-changing the fuel injection rate and recording the AFR response. As for the fact that either x or τ varies among speed, TP and cylinder temperature, the final value of each other was calculated by timing the MAP-searching value and the cylinder temperature coefficient. For startup conditions, cold or hot, an approach by reducing the enriching fuel gradually rather than the nonlinear compensator was applied to ensure the success of start.The following bench tests demonstrated that the fuel economy, exhaust gas emission and output torque under low-medial load conditions all showed great improvement while less than 5% decrease in maximum power compared to the original motorcycle engine equipped with a carburetor.