The research presented in this thesis is an effort to improve small engine combustion through the application of lean combustion. The first part of the research is focused on conducting an experimental investigation into the application of lean burn strategy on a single cylinder OHV utility engine to reduce engine-out emissions and at the same time maintain acceptable cyclic variability in combustion. The parameters of interest to investigate cyclic variability in combustion were spark plug variations, load control and charge stratification. The main findings showed that the spark discharge energy had a major influence on engine performance. It was also found that the engine can be operated at a high volumetric efficiency and very lean AFR at 75% and 50% load by the use of fuel injection. This is especially helpful for small engines operating on the EPA B-cycle. The second part of the research deals with the study of a Flat head, also known as side valve (SV) engine platform. A novel approach to lean combustion in a flat head engine is proposed by directly injecting gasoline fuel into the combustion chamber. The main advantage of the direct injection flat head (DIFH) engine over the conventional OHV GDI engine is its simplicity in design, low cost and, greater flexibility in placement of key engine performance hardware in the cylinder head. To first understand the behavior of the in-cylinder air motion, the air-flow structure developing within the combustion chamber was investigated using PIV techniques. The results show that squish is the dominant turbulence generating mean flow structure in the combustion chamber of the DIFH engine. Although the DIFH engine produced about 8 times more UHC emissions as compared to the conventional spark ignited OHV engines, it produced about 5 times less CO emissions as compared to the OHV engine and showed a 16% improvement in brake specific fuel consumption. The current combustion chamber has a dual chamber design exhibiting different combustion mechanisms in both the chambers, causing complex undesirable interactions between key engine performance parameters. Based on these fundamental studies a new combustion chamber design is presented for better performance.