<span class="label">1</span> <div class="htmlview paragraph">There is an increasing demand to improve the fuel economy of motorcycles in recent years. The lean-burn combustion is one effective means of accomplishing this. However, it is difficult to introduce the lean-burn concept while maintaining the high output and the quick response these are important for application to motorcycles. In this research, we have developed a fuel supply system that realizes both the quick response and the stable lean combustion using a conventional injector targeted for small motorcycles. This system intensifies the in-cylinder flow and helps the fuel droplet atomization, much better than conventional system due to air flow generated by the additional the passage. Therefore these improve the air-fuel mixture formation and flame propagation. As a result, it was possible to make the air-fuel mixture leaner while improving the throttle response by shortening the distance over which the fuel is carried.</div>