The aim of this study is to investigate the influence of lean combustion on the formation of combustion chamber deposit in a two-stroke cycle engine. The contribution of fuel and lubricant oil to the deposit formation was discussed in this paper. The results obtained were as follows : (1) With making the mixture lean from the rich side, the weight of combustion chamber deposit derived from the lubricant oil was increased, but that derived from gasoline was decreased. (2) The contribution ratio of gasoline to the total weight of deposit was below 5% in the lean side, however, that was 10 to 30% in the rich side. (3) Most of the combustion chamber deposit derived from the lubricant oil was formed from the detergent dispersant added to the lubricant oil, and the metallic detergent formed a larger amount of combustion chamber deposit than the ashless dispersant.
