A control system for the intake flow on 4-stroke engines had been developed. It was named the VIP (Variable Intake Port) system. Such system was proved an effective strategy to achieve fast lean-bum at part load and to increase volumetric efficiency at wide open throttle. Test results showed that the maximum power of the VIP engine could rise more than 9 % in comparison with the original one. Under the ECE-40 test mode, this VIP motorcycle, without catalyst and secondary air system, could improve at least 16 % on fuel consumption. Emissions of HC+NOx was reduced from 1.07 to 1.00 g/km, and CO from 3.69 to 0.75 g/km. Not only the combustion variations at part load were improved, but also the maximum vehicle speed and the vehicle acceleration were superior to the original motorcycle.