In developing countries like India, large numbers of portable gensets are used as a power source due to the scarcity of grid power supply. The portable gensets, ranging from 0.5 kW to 5 kW are very popular in the residential areas, for example, small restaurants, and shopping complexes, etc. These gensets are using various fuels like gasoline, diesel, LPG, and kerosene in small internal combustion engines. Such engines are the significant source of air pollution, as these are running in the vicinity of populated areas and higher human exposure to these pollutants.Theses gensets are regulated by exhaust and noise emissions norms, set by statutory bodies like the ministry of environment and forest and central pollution control board of India. The gaseous emissions of carbon monoxide (CO), total hydrocarbon (THC) and nitrogen oxides (NOx) and particulate matter (PM)-diesel only, are regulated, and emission testing is performed as per three mode cycle as specified in the regulation.The production gensets are typically undergoing a break-in running to stabilize engine performance parameters, mainly, engine power output. However, during a break-in running of the engine, exhaust emissions show different individual characteristics and need to be studied, to find an optimal running in period for a particular genset category.In this study, a portable genset consists of a 4-stroke forced air-cooled engine of 256 cc was investigated for a “break in” characteristics of engine output and exhaust emissions of CO, THC, and NOx for a duration of 10 hours and the suitable “break in” period is recommended by considering engine output and emissions.