Reducing motorcycle exhaust emissions to improve air quality is important in Asia, due to the large number of motorcycles. This study describes and proposes a lean-burn system for reducing pollutant emissions and improving motorcycle engine performance. The lean-burn system, called semi-direct injection (SDI), is comprised of high-swirl charge, injection during intake-valve opening, and liquefied petroleum gas (LPG) injection. A conventional motorcycle engine with port fuel injection (PFI) and spark ignition (SI) was retrofitted by designing a new intake port with a controllable plate to enhance the swirl of intake flow. The swirl ratio was increased to 3.4, enhancing the turbulence of air flow inside the combustion chamber, and the lean limit was extended to 1.7 of relative air/fuel ratio (lambda). The engine was tested at a low-load region which includes most operating points of the ECE-40 driving cycle. A complete engine performance map was produced and a comparison was undertaken between the new design and regular gasoline engines. The results show that brake-specific energy consumption (BSEC) decreased by 19.2%. Brake-specific exhaust emissions of CO2, NOx and CO were decreased by 27%, 47% and 94%, respectively. HC emissions were increased by 4.5%.