Small utility engines have been targeted with new exhaust emissions regulations by both the United States Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). CARB emissions standards are already in place, as shown in Table 1, while EPA is still considering legislation. Overhead valve (OHV) engines and those near the upper end of the power limit for small engines, 18.6 kW (25 horsepower), will be able to meet these new regulations much easier than the lower power engines and those of side valve design. Side valve, or L-head engines, have relatively higher emissions levels and will require a greater effort to meet regulations. One of the reasons that L-head engines produce higher levels of exhaust emissions is due to relatively high combustion chamber surface-to-volume ratios compared to OHV engine combustion chambers. There are also areas in the combustion chamber that have difficulty sustaining a flame front and hence, combustion is quenched before burning all of the fuel/air mixture. This results in high hydrocarbon (HC) production.This work was conducted, using Southwest Research Institute (SwRI) internal research funding, to improve HC emissions by optimizing combustion chamber shape. Tests were performed on a 3 kW (4.0 hp), single-cylinder, air-cooled, L-head gasoline engine using special instrumentation to determine flame front arrival at the edges of the combustion chamber. Exhaust emissions were measured with both the stock and a modified cylinder head at a variety of equivalence ratios to document results.