The two-stroke that set the tone in the U.S.

Yamaha YDS-2 magazine layout

Yamaha's YDS-2 launched a two-stroke revolution and redefined the 250cc motorcycle in the U.S. Before it, people's idea of a quick 250 was either a Triumph Cub or, (if you could find one) an NSU Max. The YDS-2 was mass produced, but its design features had grown out of Yamaha's program to win domestic races and then to enter European GP racing and U.S. AMA national races. Today, 23 horsepower at 7500 rpm in a 309-lb motorcycle is unimpressive, but 53 years ago it was sensational. The racing derivative, the TD1-A, gave a claimed 32 hp at 9500 rpm.

The YDS-2 was one step along Yamaha's design path leading to the large market share taken by its 1972 to 1984 modular two-stroke twins, which we may call the "RD350 series." The modular engines had 54mm strokes until the coming of the RD400, and that 54mm stroke served for R5, RD 250 and 350, the racing twins TZ 250 and 350, and the mighty TZ750, winner of every Daytona 200 from 1974 to 1982, inclusive.

In the hands of savvy riders, YSD-2s left Honda 305 Superhawks for dead in stoplight drag races. The Yamaha rider stood over the bike, his weight on his feet, ran the revs up, dropped the clutch, and then sat down on a tire already spinning. Gone. This transformed the perception of two-strokes. Before, they were feeble, smoking cost-cutters like BSA's Bantam or Harley's Hummer. Suddenly two-strokes were fast. What next?

Getting there was a learning process. Yamaha and Tohatsu had studied the German Adler before designing their own twins. Both companies initially attached their carburetors in Adler fashion, not to the cylinders but to the crankcase, and they also chose the traditional vertical split construction. Yamaha also adopted the Adler’s crankshaft-mounted clutch. This seemed a good idea because a clutch turning at crankshaft speed could be lighter, having to transmit only engine torque. A gearbox-mounted clutch would have to be bigger to transmit torque multiplied times the primary drive ratio.

Yamaha YDS-2 magazine layout

YDS-2 was a two-stroke parallel twin of 56 x 50mm bore and stroke, with air-cooled cast-iron cylinders and aluminum heads. Each cylinder had a single piston-port intake and a single exhaust, with two transfer ports. Efficient modern pistons had smooth, slightly-domed crowns–no heat-gathering, seizure-inducing deflectors! A five-speed gearbox was shifted by a grooved cam plate lying flat beneath it.

Yamaha had changed many things from Adler’s design either to improve function or to allow efficient production. Every model change revealed a multitude of refinements, large and small.

The awful truth about YDS-2, however, was its crank-mounted clutch, sitting on a slender 20mm diameter crank extension. Just a little out-of-balance in that clutch and you had a force tending to wiggle that 20mm shaft until it broke off. At Watkins Glen, I stared at a TD1-A which had just suffered breakage of that shaft, and its clutch, spinning at some 10,000 rpm, had cleaned off the entire left side of the engine.

Yamaha did the right things–increasing the radius where the clutch shaft joined the left flywheel, then in the following model YDS-3 increasing shaft diameter to 25mm, but in the end they had to admit the design was unworkable, so YDS-5 and 6 had a large gearbox-mounted clutch and all was well.

The YD series inherited a fair amount of Germanic complexity, and as Yamaha engineers gained experience, they put Japanese design simplicity in its place. In the following R-series twins they made the leap from vertical split construction to horizontal, and adopted a modern drum-shifted gearbox.

The two-stroke revolution, with its ever-increasing horsepower, would force the development of new concepts in tires, suspension, and chassis that now carry the improved lightweight four-stroke engines of today.

Yamaha YDS-2 magazine layout