A previous study by the authors has shown an efficiency benefit of up to Δηi = 10 % for stratified operation of a high pressure natural gas direct injection (DI) spark ignition (SI) engine compared to the homogeneous stoichiometric operation with port fuel injection (PFI). While best efficiencies appeared at extremely lean operation at λ = 3.2, minimum HC emissions were found at λ = 2. The increasing HC emissions and narrow ignition time frames in the extremely lean stratified operation have given the need for a detailed analysis. To further investigate the mixture formation and flame propagation und these conditions, an optically accessible single-cylinder engine was used. The mixture formation and the flame luminosity have been investigated in two perpendicular planes inside the combustion chamber. By quantifying the equivalence ratio using the laser-induced fluorescence (LIF) technique a correlation between the presence of an ignitable mixture at the spark plug and the indicated start of combustion (SOC) was shown . Through the investigation of the flame luminosity, the main reason for incomplete combustion and the according emissions was revealed. With this cause-and-effect comprehension, the combustion chamber has been modified to enforce an improved charge stratification. A geometric modification of the piston head ensures deflecting the ignitable mixture to the spark plug and prevents the injection from reaching the inner cylinder wall and crevices. Optical investigations were used to develop a design that improves mixture formation.