The tightening of the exhaust emission limits and the desire to reduce fuel consumption determine the direction of research on internal combustion engines. As it is known, the leaning of the mixture in spark ignition engines generally decreases fuel consumption and NOx emission together. Due to this fact, many researchers are focusing on lean combustion. The main purpose of this study is to compare the effect of different chamber geometries on combustion parameters of a spark ignition engine operated at lean and stoichiometric mixtures. One of the most important parameters of the combustion process is cyclic variations. It defines the stability of a spark ignition engine in operating conditions. Cyclic variations are increased with the leaning of the mixture in spark ignition engines. For lean mixture, a new combustion chamber was designed and manufactured for accelerating flow motions through its sickle and bowl shape. This chamber called MAN-Ricardo (MR) type decreased burn durations in the combustion process. While the start of the ignition for MR was later, the burn duration is shorter than that of the other two geometries. Therefore, MR type reduced the cyclic variations compared to other geometries in homogenous and lean mixtures. As a result, the combustion chamber must be designed to increase the turbulence intensity for a spark ignition engine to operate stably in lean mixtures, unlike the stoichiometric mixture.