In general, two-stroke engines have a tendency to develop irregular combustion under light load. Under this condition, two-cycle motorcycles have inferior riding qualities. This paper discusses the operating conditions of the engine under which irregular combustion takes place. To analyze irregular combustion, the gas scavenging process is simulated and the adiabatic flame temperature is introduced as a criteria. Auto ignition, which is apt to occur under light load and high speed, is also investigated and is discussed in association with gas temperature and adiabatic flame temperature at the moment of ignition.