The objective of this study is to enhance the full-load power and the partial-load thermal efficiency of a gasoline spark-ignition engine for large motorcycles. To achieve these goals, it is important to increase the combustion speed and mitigate knocking, so a passive pre-chamber jet combustion system was evaluated. In the specification study, a three-dimensional combustion simulation incorporating detailed chemical kinetics was used to analyze the combustion mechanism, including knocking detection. For full-load conditions, a passive pre-chamber jet combustion system was evaluated. It accelerated combustion by increasing turbulent kinetic energy in the main chamber through jets sprayed from the pre-chamber. By increasing the compression ratio by 2.0, the full-load indicated work increased by 3.6% compared to conventional SI combustion. Under partial-load conditions, the passive pre-chamber jet combustion system faced challenges, such as reduced jet temperature due to increased residual gas in the pre-chamber, heat loss at the holes, and excessive initial jet penetration, which inhibited ignition in the main chamber. To address these issues, the pre-chamber jet 2-plug combustion system was evaluated, where main-chamber side-plug ignition was followed by pre-chamber ignition. The pre-chamber jet 2-plug combustion system enhanced jet ignition through flame interaction in the main chamber, resulting in increased combustion speed. Furthermore, relocating the main-chamber side-plug to a position between the exhaust valves closer to the bore center increased combustion speed and mitigated knocking. As a result, the pre-chamber jet 2-plug combustion system, with the side-plug located between the exhaust valves, enhanced partial-load indicated thermal efficiency by 1.7 percentage points compared to conventional SI combustion.