Earlier papers by the principal author in conjunction with others have described the prediction of noise and performance characteristics of two-cycle spark-ignition crankcase compression engines. These calculations are performed on a digital computer and are shown to simulate accurately the unsteady gas flow and thermodynamic processes in such power units. The engines described previously had induction control by the piston or with a disc valve. In this paper the work is extended to engines fitted with reed valves controlling intake air flow and examples illustrating the effectiveness of such calculations are presented. In particular, a single-cylinder industrial engine is employed to show clearly the effects of changing such parameters as reed petal thickness, stop-plate radii and numbers of reed petals on the performance characteristics. The mathematical model is extended also to a twin-cylinder outboard marine engine and comparisons of measured and predicted performance behaviour are presented.