In this study, a laboratory test procedure to mimic the life cycle of the small air-cooled engines in field operation is developed. A characterization of exhaust emissions over the life cycle of the engines is achieved with special focus given to the hydrocarbon emissions. Both Briggs and Stratton four-stroke (four-cycle) overhead-valve and side-valve engines with a nominal power output of 3.7 kW (5 hp) are used in this study. Different levels of emissions are observed for each type of engine configuration, and it is noted that the hydrocarbon emissions changed more than CO or NOx emissions. These data support the idea that combustion chamber deposits (CCD) are a significant cause of deteriorating emissions. Chemical analysis techniques are applied to the CCD, and it is found that the deposits consist primarily of polynuclear aromatic compounds and unsaturated hydrocarbons.