Many recent publications indicate that spark ignition (SI) engines equipped with the conventional port-injection fuel system (PIF) seem to have serious fuel-maldistribution problems, including the formation of liquid layers over the combustion chamber surfaces. It is reasonable to expect that such a maldistribution is an unfavorable condition for the flame propagation in the cylinder.The in-cylinder flame behaviors of a PIF-SI engine as fueled with gasoline are investigated by using the Rutgers high-speed spectral infrared imaging system. These results are then compared with those obtained from the same engine operated by gaseous fuels and other simple fuels.The results from the engine operated by gasoline reveal slowly burning fuel-rich local pockets under both fully warmed and room-temperature conditions. The local pockets seem to stem from the liquid layers formed over the surfaces during the intake period. The (invisible) post-flame oxidation of the rich pockets is observed to continue even after the exhaust valve opens. On the contrary, the same engine run with a gaseous fuel exhibits some predictable and “clean” flame propagations.The new results obtained from the present study suggest that such a late oxidation of locally fuel-rich liquid pockets may be a significant cause for the emission of the engine-out unburned hydrocarbon (UHC). The sluggish consumption of the fuel there may also be a factor for reducing the thermal efficiency of the engine. A parametric study of this observation is performed to obtain a better understanding of the findings.