In the present study, the possibility and limitation of lean burn in a motorcycle engine were explored, from the aspect of exhaust emissions. The CO and NOx emissions were studied by experimental measurements and combined simulations on a twin-spark motorcycle gasoline engine under wide range of operating conditions. The results illuminated that, the CO emissions drop sharply when air/fuel mixture changed from rich to lean. Particularly, at full load the CO emission decreases by about 4 times when lambda changed from 0.85 to 1.1, and it keeps decreasing to only about one-tenth at lambda = 1.2. Besides, at lambda = 1.1 and full engine load the NO emission is about 2.4 times of that at lambda = 0.85. Not so much variation has been observed between lambda =1.1 and lambda = 1.2. This indicates enlarging the lean burn range will not further deteriorate NOx emission, but benefit from CO emission by a large margin. It is further found the variation in CO emission amount between lambda s is not mainly due to which reaction is in favor, but due to the amount of its precursors. Besides, almost all the NO is produced by the thermal-NO mechanism. (C) 2018 Elsevier Ltd. All rights reserved.