In practical gasoline engines it is necessary to achieve a low emissions and low fuel consumption with high load operation. Several techniques were developed for reducing the emissions and fuel consumption from gasoline engines, such as EGR (exhaust gas recirculation) and Ultra lean combustion. Nevertheless, there are only very few data available for reducing the fuel consumption in small gasoline engines (50-125cc). As the first step of this study, experiments have been carried out to examine the influence of oxygen concentration in intake air and fuel concentration on flame speed in small gasoline engine by using carburetor and EFI system. The flame front travel time was measured using ionization probes located at different point of cylinder head. The main conclusions are as follows: 1) the flame speed monotonically increases with increasing the engine rotation at any concentration in intake air; 2) Mean increasing rate of pressure is not affected by oxygen concentration in intake air and fuel concentration without load; 3) It is possible to improve the combustion behavior by using EFI system for small gasoline engine.