Methane-based fuels compared to the traditional liquid fuels display favorable properties such as a low carbon content and a high knock resistance. State-of-the-art, highly turbocharged gas fueled engines reach “diesel-like” overall efficiency along the full load curve. However, at the low load the spark-ignited gas engine is less efficient than the diesel one due to increased pumping losses caused by throttling. Desired de-throttling can be achieved by the combustion of the extremely lean mixture. Simultaneously, the lean burn concept allows a significant decrease of the NOx formation owing to the low temperature combustion. Yet, poor ignitability and the low flame velocity of extremely lean mixture causes high level of cycle-to-cycle variability and possibly an occasional misfiring. The topic of the paper is a combustion system with a fuel-scavenged pre-chamber, commonly used in large-bore engines, full time operated with extremely lean mixture. The authors had to cope with challenges when implementing the scavenged pre-chamber into the light duty truck-size engine with a bore of 102 mm. The engine operation with the stoichiometric mixture at high power was considered. This work outlines the pre-chamber design features. The results of experimental investigation of the pre-chamber ignition on a single cylinder testing engine are introduced. Selected set of simulation results from both 1D and CFD models are also presented. It was concluded: The pre-chamber engine is able to operate at the low load with air excess ratio above 2 and the engine-out NOx emission below the limit of Euro 6. Despite relatively high content of methane in exhaust gas, the indicated efficiency is better compared to the throttled stoichiometric operated conventional SI engine. Naturally aspirated pre-chamber engine is able to operate with the stoichiometric mixture and fully open throttle, featuring similar performances as the conventional SI one, including possible use of a three-way catalyst.