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The original YZ250 of 1974 used an air-cooled 250cc two-stroke engine of 70 mm bore and a 64 mm stroke, which was improved semi-annually. The air-cooled motor was replaced in 1982 with a 249 cc liquid-cooled two-stroke reed-valved engine with a mechanical, rather than servo-driven, YPVS exhaust valve for a wider spread of power. Although other ...
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Yamaha YA-1. YA-1 built August 1954, produced January 1955. The first bike manufactured by Yamaha was actually a copy of the German DKW RT 125; it had an air-cooled, two-stroke, single cylinder 125 cc engine [1]
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It appeared in production on the 1974 Yamaha YZ-250, a model which is still in production, making it Yamaha's longest continuous model and name. Yamaha continued racing throughout the 1960s and 1970s with increasing success in several formats. The decade of the 1970s was capped by the XT500 winning the first Paris-Dakar Rally in 1979. [13]
The 1990 season continued the trend of crashes as riders tried to cope with the harsh power output of the V4 two-strokes. [1] Honda put forth a proposition limiting the top class to 375cc and 3 cylinders, but that never caught on. [1] Still, with 500cc lap times becoming stagnant, it was clear that something needed to be done. [1]
In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two-strokes. [5] Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four-stroke race motocross bike. [6] The Yamaha YZ400F was developed to fit into this category.
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