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From a view point of reducing exhaust emission level on a small utility 4-stroke spark-ignition engine, this paper describes the exhaust emission materials analysis and the affect against California Air Resources Board (CARB) specified testing mode on a new prototype while applying very lean mixture burning. The combustion chamber shape and the intake air passage of a production model (Displacement 286 cc, 4-stroke, air cooled, over head valve (OHV), single cylinder) was modified and optimized on the prototype to achieve a stable run up to a lean air-fuel ratio of 20 : 1. When the air-fuel ratio was changed, the highest density of NOx was a little higher than 3,000ppm at an air-fuel ratio of 16 : 1 and the lowest was about 300 ppm at an air-fuel ratio of 19 : 1. The emission level of THC was a little higher than 2,000ppm at an air-fuel ratio of 12 : 1 and less than 1,000 ppm at an air-fuel ratio of 15 : 1 and higher.