In view of rising oil prices and concern for the greenhouse gas emissions, the need for greener and efficient engines is increasing. Thus, automobile manufacturers are trying to improve the performance and efficiency of the engine while keeping compliance with the stringent emission norms. CNG, with its high H/C ratio, makes it a clean fuel by significantly reducing the emission of green-house gas carbon-dioxide. CNG, being cheap compared to other conventional fuels, is an added advantage and hence is gaining popularity. Along with improvement in the part load and full load efficiency, Engine manufactures are looking to lower the idle speed for better fuel economy. Lowering the idle speed has to be optimized as, it reduces the combustion stability of the engine which in turn increases the variation of Indicated Mean Effective Pressure (IMEP) resulting in high structural vibration from the engine and to vehicle body.Investigations have been conducted on idle condition of a four cylinder naturally aspirated engine propelled by CNG to find out the effect of ignition timing, idle speed and Air fuel ratio on the combustion stability. Pressure sensors were fitted on each cylinder to measure the cylinder pressure for the calculation of IMEP. Lowest Normalized Value (LNV) and standard deviation of IMEP along with the Hydrocarbon emissions are used to evaluate the combustion stability. Valve overlap is also one of the major contributor for combustion stability at idle, experiments were conducted with different valve overlaps to study the effect of valve overlap on the idle combustion stability.