In this work the pre- to main chamber ignition process is studied in a Wärtsilä 34SG spark-ignited lean burn four-stroke large bore optical engine (bore 340 mm) operating on natural gas. Unburnt and burnt gas regions in planar cross-sections of the combustion chamber are identified by means of planar laser induced fluorescence (PLIF) from acetone seeded to the fuel. The emerging jets from the pre-chamber, the ignition process and early flame propagation are studied. Measurements reveal the presence of a significant temporal delay between the occurrence of a pressure difference across the pre-chamber holes and the appearance of hot burnt/burning gases at the nozzle exit. Variations in the delay affect the combustion timing and duration. The combustion rate in the pre-chamber does not influence the jet propagation speed, although it still has an effect on the overall combustion duration. PLIF images also show that there is mainly lean unburnt gas, originating from the main-chamber, that exit the pre-chamber in the initial phase of ignition, indicating incomplete mixing of the gases in the pre-chamber prior ignition. These findings are also supported by CFD modeling performed on the Wärtsilä 34SG combustion system. Similar to the air entrainment in a diesel spray, the jets exiting the pre-chamber causes pronounced entrainment of compressed fresh charges into the burning jets, thus promoting heat transfer and subsequent ignition.