In developing countries like India, large numbers of portable gesnets are used as power source due to scarcity of grid power supply. The portable gensets, ranging fron 0.5 kW to 5 kw are very popular in the residential, small restaurants and shopping complexes. These gensets are using various fuels like gasoline, diesel, LPG and kerosene for small spark ignited and compression ignition engines and significant source of air pollution as these are running close to populated areas. Theses gensets are regulated by exhaust and noise emissions norms set by statutory bodies like central pollution control board of India. The gaseous emissions of carbon monoxide (CO), total hydrocarbon (THC) and nitrogen oxides (NOx) and particulate matter (PM) are regulated and emission testing is performed as per three mode cycle as per standard test proecdure. The production gesnets are normally undergo a break in running to stabilize engine performance parameters mainly engine power output. However, during break in running of the engine exhaust emissions show different individual characteristics and need to be studied to find a optimal running in period for a specific genset. In this study, a portable genset consists of a 4-stroke force air cooled engine of 256 cc was investigated for 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.