:For a crankcase-scavenged two-stroke cycle S.I. engine, the irregular combustion at light loads results in engine instability and high exhaust hydrocarbon emissions. It is recognized that the poor scavenging process is one of the major contributing factors.From the test data of a single cylinder fuel injected two-stroke cycle engine, it was found that at certain light load conditions the exhaust charge flow could strongly affect the combustion stability. A butterfly valve controlling the passage area of exhaust pipe was designed and installed in the engine to investigate the effect of exhaust charge control strategy on combustion. Test results showed that the exhaust contraction ratio should decrease with decreasing load and speed in order to achieve the improved combustion stability and lower hydrocarbon emissions. Compared with the case without exhaust charge control, the reductions in the coefficient of variation (COV) of IMEP and exhaust HC emissions were very significant by adopting the exhaust charge control device with a suitable contraction ratio. Analysis of the cylinder and crankcase pressure data also revealed the relationship between crankcase reverse flow and the combustion instability. As the engine was operated at further light load conditions, the exhaust charge control device became less effective.