<div class="htmlview paragraph">Experimental studies of the intake-generated charge motion (swirl and tumble) and engine combustion were conducted in a 125 cc four-stroke motorcycle engine. In this work, a Variable Inlet Port (VIP) was designed to generate various levels of charge motion in different operation conditions. The static flow test and the engine experiments were performed to study the effects of inlet charge motion on the engine combustion, cycle variation, fuel consumption and exhaust emissions. The results show that the cycle variation decreased, the lean limit extended, the burning rate and the fuel economy increased when the charge motion increased. With this new design of flow control system, the motorcycle can be run with lean mixture and drastically reduce the exhaust emissions and fuel consumption while still maintaining high specific power output.</div>