Experimental research has been performed on a single cylinder naturally aspirated spark ignition engine that was modified to operate with coal-bed gas fuel to investigate methods of improving the stability of operation and lean burn limit. Various fuel compositions, with methane concentrations from 30% to 100% and CO2 volumetric fractions from 0 to 0.7, were employed to simulate coal-bed methane. Hydrogen was then used to improve the operational stability and lean burn limit. The results show that a stable operating range of the engine was obtained with most of the fuel compositions. In addition to operating beyond the lean burn limit, unstable operation with COVIMEP>10% only occurred at the lean burn limit with a CO2 volumetric fraction of 0.7 at each equivalence ratio. The lean burn limit was significantly reduced from the equivalence ratio of 0.6 to 0.4 by adding hydrogen. Stable operation with COVIMEP<5% at an equivalence ratio of 0.4 was also obtained with some high hydrogen concentration conditions. Hydrogen addition induced the reduction of both CO and THC emissions at all the tested equivalence ratios, especially with equivalence ratios of 0.4 and 0.6. However, a high CO2 volumetric fraction will lead to unstable operation, which worsens CO and THC emissions. Hydrogen addition improves the operation stability and enlarges the lean burn limit of coal-bed gas engines, and this addition could have a significant practical impact on the improvement and application of coal-bed gas engine technology.