The effect of local fuel concentration on cyclic variability in combustion and engine performance in a lean burn stratified charge engine has been investigated. The fuel concentration in a plane close to the spark plug was measured for a large number of cycles using laser-induced fluorescence (LIF) and simultaneous measurement of in-cylinder pressure in an one-cylinder optical research engine. It could be shown quantitatively that the fuel concentration in a small region close to the spark plug has a dominating effect on the subsequent pressure development for lean mixtures. Variations in the mixture concentration in the vicinity of the spark plug contribute significantly to cyclic variation in combustion. Measurement of the flame area in the same plane 20 °CA after ignition revealed that the direction of growth of the established flame is not significantly influenced by the stoichiometry.The issue of non-uniformity in residual gas concentration prior to ignition was addressed by measurement of the nitric oxide (NO) concentration in the unburned charge. A LIF method was developed to measure average and instantaneous NO concentrations in the same one-cylinder research engine. The results revealed that the residual gas is not homogeneously mixed with the air and that significant cyclic variations in the local residual gas concentration exist. These fluctuations are large enough to contribute significantly to fluctuations in mixture concentration.