The distribution of fuel-air mixtures in many L-head engines is not homogeneous. If local mixture is too rich or lean, incomplete combustion occurs. This can play a major role in unburned hydrocarbon and carbon monoxide emissions. Fuel-air mixture distribution depends on in-cylinder swirl and turbulence and is directly related to intake manifold configuration, fuel delivery system design and combustion chamber shape. Understanding the spatial mixture distribution may help improve the design of these aforementioned components. Consequently, a more complete combustion process may result, and emissions reduced.A method that measures the emission of CH and C2 radicals via the use of an optical fiber bundle was used in this research to map the mixture uniformity in the combustion chamber. The intensity ratio (IC2/ICH) was correlated to the fuel-air equivalence ratio. The mixture distribution measured was then correlated with the hydrocarbon emission sequence. The exhaust gas was sampled by a port mounted sampling valve and analyzed by a FID. Two fuels, propane and Indolene, were used with a Tecumseh horizontal shaft L-head engine.For both fuels, high hydrocarbon emissions early in the exhaust process were identified as resulting from locally overrich mixture in the valve region of the chamber. For propane, the hydrocarbon content of the gases leaving the cylinder last, was independent of mixture distribution and was likely due to wall quenching. On the other hand, for Indolene, mixture mal-distribution was found to contribute to the hydrocarbon emission late in the exhaust process.