Abstract: Experimental study was made on the different emission of unburned hydrocarbon and carbon monoxide from different cylinders in 492QC S.I. engine under idle condition. The sample of the hydrocarbon and carbon monoxide emission was taken from the exhaust port exit of each different cylinder, which shows that the difference of carbon monoxide emission from cylinder to cylinder is not large; while the difference of hydrocarbon emission is very large; in fact the hydrocarbon emission of No. 1 cylinder is the largest and the hydrocarbon emission of No. 4 cylinder is the least if the hydrocarbon emission of all the cylinders can be decreased to the level of No.4 cylinder, the hydrocarbon emission of the experimental engine will at least decrease to 30% of the volume fraction of HC measured at the general exhaust pipe.