The History of the Yamaha YZ250 Two-Stroke

The Yamaha YZ250 belonged to the YZ family of motocross motorcycles. It debuted in 1975 and was the first off-road bike with a single rear shock absorber. The innovation led to significant changes in how motorcycle makers manufactured off-road bike suspension systems. Unlike many two-stroke motocross bikes, Yamaha consistently updated the YZ250, which that led first-place wins in such off-road classics as the 2009 American Motorcyclists Association National Hare Scramble.

  1. Background

    • The Yamaha YZ250 traces its roots to the YA1 motorcycle developed by the Yamaha Motor Co. in 1954 as basic road transportation. The YA1 was the first motorcycle to mix the lubricant and fuel systems in two-stroke engines. From the YA1 came the DT1, equipped with a 250 cc two-stroke engine and the first bike built specifically as an off-road vehicle.

    Initial Production

    • Sales of the debut model year 1975 for the YZ250 didn’t come close to the immensely popular YZ125 versions. Yamaha produced 101,620 YZ125 bikes for 1975 compared to just 2,759 YZ250s. The 1975 versions featured a yellow with black and white “C” checkerboard stripes color scheme. A strap held the fuel tank down, but Yamaha replaced it with a bolt-on style in 1976. Production for the YZ250 models in 1976 leaped to 103,405. All YZ versions except the YZ80C received the monoshock system. Yamaha replaced the aluminum fuel tank with a plastic version. By 1977, the motorcycle maker restyled the chassis and engine combination. In 1982, the YZ125 and 250 versions received Yamaha’s first power valve system to increase output. The YZ250 also converted from air-cooled to a liquid-cooled system in 1982.

    2002 Specs

    • The 2002 version of the Yamaha YZ250 featured a 249 cc single cylinder, two-stroke engine with an 8.8-to-1 compression ratio. Its bore was 2.6 inches and the stroke measured 2.8 inches. The front suspension travel was 11.8 inches while the rear cleared 12.4 inches. The YZ250 rode on a 21-inch front wheel and 19-inch rear. Stopping power came from a single front 9.8-inch disc brake and 9.6-inch rear disc. It sat on a 58.5-inch wheelbase, measured 85.9 inches long and 51.3 inches tall, and its frame cleared the ground by 15.4 inches. Its curbside dry weight was 215 pounds.

    Improvements

    • For 2003, the YZ250 underwent a series of improvements. Yamaha modified the frame’s rigidity and swingarm to reduce weight and improve handling. To improve overall performance, Yamaha reshaped the cylinder exhaust ports, made the crankshaft more rigid by increasing its taper and changed the second gear ratio. The bike’s weight was further reduced by redesigning the rear brake caliper while at the same time increasing the piston’s diameter to make it more rigid. Yamaha also shaved 350 grams from the fuel tank.

    2011 YZ250

    • With the continuing weight reduction measures since 2003 and the addition of the lightweight aluminum frame in 2006, the YZ250 didn’t gain any weight despite numerous rigidity improvements to the frame and components. Whenever Yamaha enlarged a component to ensure durability, it compensated by changing another component to keep the bike light. The engine’s compression ratio increased to 8.9-to-1 for better torque. The bike’s dimensions have been virtually unchanged since 2002.

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