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This paper describes a stratified scavenging system of two–stroke cycle engine, developed to reduce HC (hydrocarbon) emission caused by short–circuiting mixture at scavenging process. The fact, the maximum short–circuiting of fuel–air mixture occurs at the timing soon after the scavenging port open, led us to the idea of stratified scavenging, that is, first stage of scavenging by air without fuel, then second stage by air–fuel mixture. The stratified scavenging system consists of long passages from crankcase to scavenging ports, and the supplemental air intake system directly to the scavenging port. The newly developed stratified scavenging two–stroke cycle engine cuts HC emission to about 1/4 of conventional two–stroke, and can meet the CARB Tier 2 emission regulation.