Intake flow rate of practical small two-stroke engines has been controlled by the opening ratio of throttle valve. The purpose of this study is to investigate flow behavior behind a carburetor for different throttle opening ratios in relation with the pressures. Velocities in the intake pipe were measured by a fiber LDV together with pressures at different positions under motoring and firing conditions at 3000 rpm. The results show that the recirculation vortex were formed behind the carburetor and its position and size depended on the throttle opening ratios. The variation of intake volume flow rate with the opening ratio was made clear quantitatively.