A flexible-fuel system for small motorcycles has been developed that can utilize both gasoline, and ethanol as an alternative to fossil fuel. For practicality, we aimed to develop a system that uses a minimum of additional parts. As a method to make both ethanol and gasoline available with the system, a control algorithm that estimates the ethanol concentration by utilizing the output voltage of an OXYGEN SENSOR and selects the appropriate injection quantity is employed. Usually, in automotive flexible-fuel vehicles (FFV), sub-tanks are mounted to ensure engine startability in cold temperatures. However, in small motorcycles, limitations of mounting space must be addressed. In this system, by clarifying the control logic and determining the difficult cases for cold temperature starts due to high ethanol concentration, configuring the indicator to promote gasoline mixture when the start is difficult enabled the elimination of the sub-tanks. This flexible-fuel system has been mounted on 150cm3-single-cylinder motorcycles, resulting in an increase in weight of approximately 500g. The system is suitable for mass-production.