With the remarkable rise of gas prices and global air pollution, measures to improve fuel efficiency and reduce emissions have become urgently needed in the motorcycle industry, as in the automobile industry. One approach is to improve the thermal efficiency of the engine, and much research and development has been done for many years on this subject.Community-based small motorcycles require both high mobility and fuel efficiency in developed and developing countries. Drivability and emission control of recreation and sports motorcycles are also needed.However, when developing engines for small motorcycles, due to differences in engine speed range, driving load range, devices for driving and emission control, market prices, and infrastructure, some different approaches from those for automobile engines with their many advanced technologies are needed.This report describes mainly techniques for optimizing the shapes of the intake port and its outskirts based on a wealth of knowledge about improving thermal efficiency gained during engine development. Using the results of both combustion analysis and the flow state at a dynamo bench and a flow bench with an actual engine, and forecasts of the internal flow by three-dimensional steady and non-steady flow analysis were adapted to develop the specifications.As a result, the combustion between cycles was stabilized with less fuel by intensifying the turbulence energy around the spark plug, ideally being controlled the fresh air-fuel mixture before ignition and increasing atomization of the fuel. Moreover, ISFC was able to be improved from our previous engines without sacrificing drivability.