Today, environmental issues have become major global issues. Under these social circumstances, restrictions on vehicles powered by internal-combustion engines have become increasingly stringent. While, many models of motorcycle are intended for hobbyists, and enjoyment is a major factor in the appeal of these products. So it is necessary to simultaneously realize improved environmental performance and heightened enjoyment. Therefore, the authors focused on controlling the richness of the mixture, the air-fuel ratio. In the case of intake port-injection engines, it is common these days to control fuel injection using a fuel model that takes fuel adhered in the intake port into consideration, in order to control the air-fuel ratio. When this method of control is used, it is necessary to derive the ratio of injected fuel that adheres inside the intake pipe and the ratio of adhered fuel that is drawn into the cylinder. However, it is difficult to actually measure these values, and the ratios need to be estimated. Therefore, the authors developed appropriate technologies to derive the fuel adhesion ratio and fuel removal ratio statistically using the system identification approach, thereby achieving highly accurate air-fuel ratio control.