This paper presents an analysis of the mass fraction burn (MFB) of an imitated low BTU gasification gas used as fuel in a four-stroke, single cylinder, direct-injection (DI), spark-ignition (SI) gas engine. The composition of the gas is 19% H2, 30% CO, 5% CH4 and balance of N2 with a lower calorific value of 7.67 MJ/Nm3. It is aimed at studying the combustion characteristics of the fuel for possible replacement to fossil fuels in the latest engine technology.The engine is operated at wide open throttle position at lean operation condition. Pressure reading from the engine cylinder by Kistler Piezoelectric pressure transducer is used to analyse the MFB with the help of Rassweiler-Withrow model.The result showed that the MFB function of low BTU gasification gas at ultra-lean operation deviated from the normal S-curve, experiencing a double-Wiebe function with a plateau shape at around 50 to 60% of the MFB for all speed ranges studied. The result was further compared with that of CNG operating at very lean charge and injected late at 90o before top dead centre (BTDC). The low BTU gasification gas was observed to have shorter flame development stage duration while longer rapid burning stage and longer overall combustion durations as compared to CNG.