Five small two-stroke engine designs were tested at different air/fuel ratios, under steady state and transient cycles. The effects of combustion chamber design, carburetor design, lean burning, and fuel composition on performance, hydrocarbon and carbon monoxide emissions were studied. All tested engines had been designed to run richer than stoichiometric in order to obtain satisfactory cooling and higher power. While hydrocarbon and carbon monoxide emissions could be greatly reduced with lean burning, engine durability would be worsened. However, it was shown that the use of a catalytic converter with acceptably lean combustion was an effective method of reducing emissions. Replacing carburetion with in-cylinder fuel injection in one of the engines resulted in a significant reduction of hydrocarbon and carbon monoxide emissions. However, difficulties with stable operation under transient operations indicated that the combustion chamber should be redesigned for in-cylinder fuel injection. It was shown that acceleration pumps, which are typically used to improve chainsaw engine performance in transient operations, significantly increase HC and CO emissions. On the other hand, oxygenated fuels can reduce those emissions. Overall, a compromise has to be reached between emissions, performance, cost, and durability for a given two-stroke design.