In order to improve the air fuel ratio in exhaust manifold of four-stroke 125c.c fuel injection system engine, An event based closed loop fuel controller, which is designed based on Linear Quadratic Regulator. Since the wall weting in the intake charge system, deposit factor, and boiling time constant are varied with the engine operating conditions, a look-up table is established using experimental data for controller. Moreover, Narrow-Band Oxygen sensor feedback signal can only afford limited air fuel ratio information. Therefore, a kalman filter is proposed to estimate the air fuel ratio in exhaust manifold and amount of fuel evaporation. Thus, the fuel controller can adjust the control command based on the estimated results to improve the air fuel ratio control. A 125 cc four-stroke spark ignition engine model, which consists of charging, torque, friction, and engine rotational dynamics, is established using Matlab/Simulink in this thesis to verify the proposed control strategies. The simulation results show that the proposed control strategy can be used to improve the disadvantage of conventional algorithms and achieve better control robustness performance.