An experimental study is carried out to investigate the combustion process in a Wártsilá 34SG spark-ignited lean burn four-stroke large bore engine (bore 340 mm) by means of optical diagnostics when operating on natural gas. The main focus of this work is to gain knowledge about in-cylinder combustion phenomena when igniting a lean air/fuel mixture with pre-combustion chamber induced jets. Especially the origin of cyclic variability is of interest. The flame propagation process in a single cycle was captured using a high speed video camera. The analysis is based on apparent heat release rates in the pre-combustion chamber and main chamber, in order to find correlations with the imaged phenomena. The results show that the flame propagation inside the main chamber starts at the end of the pre-chamber combustion heat release and that variation in main chamber heat release does not correlate with variations in the pre-combustion chamber.