The air-fuel ratio of the mixture around the spark plug must be within acceptable limits to ensure good startability. It has been impossible to measure this continuously directly; this paper presents a local air fuel ratio meter which we have developed. The sensor uses the ultraviolet absorption properties of gasoline vapor and a two-path construction to allow for optical fouling. We have developed a mathematical model of the air-fuel mixture formation and distribution, including the effects of ambient temperature and gasoline fuel properties. This model is based on the results obtained using the new local air fuel ratio sensor.