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Recently, motocross racetrack configurations have been changing from speed-oriented layouts to rider's talented performance-oriented layouts. This has created a demand for engines that can provide both power at high speeds and torque at low speeds as well as having quick response at all speeds. The most efficient way to achieve all round power range in the 2-stroke-racing engine was to increase the area of exhaust port and to vary the exhaust chamber volume at the same rate as the increased engine revolution. The authors developed an automatic exhaust control valve device called “KIPS”. In this paper we will describe how KIPS was developed, its output characteristics, and operation. Figure I shows a 1992 model KX250 motocross racing bike equipped with the newest KIPS device (3-Way KIPS).

Motocross (Motorcycle Cross Country), a sport of motorcycle racing held outdoors in rugged natural terrain, was grown in Europe 1960. Then in the later half of the decade, the sport was introduced in the United States as a off-road motor sport as well for leisure. Bringing this outdoor sport of Motocross into baseball stadium, introduced a new sport of indoor Motocross, which is called stadium cross race. This stadium cross the retains showing unmeasurable growth each year world wide and they commonly call a Supercross race. As for supercross racing machine requires high riding technique, which is constituted by durable and high performance racing machines. The engine which is constituted by 2-stroke single cylinder, must be in balance with high power and quick precise response. The suspension system is also a major importance for supercross racing. Landing stability for large jumps, and handling stability for the washboard portions is essential.

A conventional cast iron cylinder with sleeve or even a porous chrome plated aluminum alloy cylinder is most likely not durable when used on a low pollutant motorcycle engine which is frequently operated with a lean air/fuel mixture. In order to solve this problem, we have developed a new version of aluminum alloy cylinder that has, on its bore, a hard, porous layer of metallic particles which is formed by the explosion of a metal wire in the bore. This new cylinder has a high resistance to seizure and abrasion as well as good productivity when compared to conventional cylinders.