This paper reports visualization of behavior of spray, wall film, and initial flame propagation in an SI engine with port fuel injection system for motorcycle in order to directly investigate their influences on combustion and relations among them. Borescopes were used to visualize the flame propagation in the combustion chamber and wall film in the intake port. Various injection systems and injection parameters were tested: injection direction, timing, and size of droplets to investigate the effect of mixture formation. It is concluded that combustion stability under low load condition is greatly influenced by mixture inhomogeneity in the combustion chamber whose evidence is the luminous emission. It is caused by direct induction of considerable amount of liquid fuel with large size of droplets into combustion chamber or too inhomogeneous mixture in the intake port. Luminous emission in the flame was also seen under wide open throttle condition due to direct introduction of wall film into combustion chamber. Finally, great potential of highly atomized spray to keep combustion stability under low load condition and homogeneity under high load condition without defect of transient behavior due to wall film has been shown with injection timing of IVO.