In order to get better results in the Formula SAE of Japan, it is necessary to develop a small displacement engine with an ideal fuel consumption rate. Therefore, the authors started to improve an existing engine by combining with glow plug heated sub-chamber and lean burn. Lean burn conditions are usually adopted in gasoline engines, having the advantages of high specific heat ratio, low pump loss, and low cooling loss due to requiring a decreased combustion temperature. The combination of these elements can be expected to vastly improve thermal efficiency and fuel consumption. Unfortunately, however, when the mixture becomes lean, the ignition delay increases, and the flame propagation speed reduces. This leads to an increase in combustion fluctuation. The authors intend to solve this problem by installing a glow plug in a newly designed sub-chamber. This type of device would usually be used to heat the sub-chamber of a diesel engine to solve the cold start problem. Experiments were conducted on a modified single-cylinder four-stroke CI engine (YANMAR TF120V) to operate as an SI engine with a higher compression ratio than conventional SI engines- 15.1:1. The engine is operated at a constant speed of 1000 and 1500 rpm, and temperature variation is created by varying excess air ratio and the glow plug’s voltage. The coefficient of variation of BMEP was calculated to ensure the engine’s cycle-by-cycle variation. According to the experimental results of glow plug voltage, when the excess air ratio is between 1.2 and 2.0, stable combustion can be achieved at excess air ratios up to 1.96 - this value is shown as the maximum lean burn limit in this study. Also, when compared to a system with the same conditions except without the glow plug, the brake thermal efficiency has been improved by up to 5.0 points, the brake specific fuel consumption rate also has been reduced by up to 17.4%. In the follow-up experiments on the ignition timing, it can be found that in case of ignition in the sub-chamber, MBT is after top dead center and the variation of ignition time has a limited effect on brake thermal efficiency or brake specific fuel consumption at a fixed glow plug voltage.