Motorcycle Classics

History of the Yamaha YM1

Hermann Weber’s name rarely shows up in the annals of great motorcycle engineers. Yet he was principally responsible for designing a two-stroke motorcycle engine that was probably copied more than any other, and he was deeply influential in the development of two-stroke motorcycle engines well into the 1960s, that included the Yamaha YM1.

Weber’s contribution to the motorcycle world was the 1939 122cc DKW RT125, an up-to-date two-stroke featuring unit construction and a lightweight aluminum case and cylinder head. With help from drawings “liberated” from DKW’s Zschopau factory, BSA produced the Bantam, Harley-Davidson the Hummer, Russia the Moska, and Yamaha its early line of two-stroke singles and twins.

Enter the Dragon
In 1953, Genichi Kawakami, president of musical instrument maker Nippon Gakki in Japan, decided he would use some of his idle manufacturing capacity to build motorcycles. And perhaps looking for a fresh identity for his new line of motorcycles, he named his motorcycles after one of the company’s founders, Torakusu Yamaha. The company’s first motorcycle was a two-stroke single of 125cc, the YA-1 “Red Dragon,” and its design drew heavily from Weber’s work for DKW.

By 1957, Yamaha had paired the 125 engine to produce a 250, the YD-1. Modified and developed, this formed the basis of Yamaha’s supremely successful line of two-stroke twins. By 1962, the 250 had evolved into the 19hp YD-3 roadster with a pressed-steel backbone frame and electric start, joined the next year by the sporty but kickstart-only 25hp YDS-2.

The big development came in 1964 with the YDS-3, which used Yamaha’s Autolube system. Until then, almost all two-strokes were lubricated by mixing oil with the gasoline fuel, in ratios that varied from 12:1 (the British Villiers) to 50:1 (some rotary-valve Vespas), depending on the engine. Yamaha’s innovation was to carry engine oil in a separate tank and inject it into the engine, with a small oil pump driven by the transmission input shaft injecting oil into the intake in a ratio that was determined by engine speed and throttle opening. (There was a catch: because the pump ran off the input shaft, no oil was pumped when the bike was stationary and in gear. This was presumably to avoid over-oiling at traffic signal stops, but could starve the engine of oil if the engine was revved repeatedly in this state.) In practice, it worked well. So well, in fact, that before long, most larger two-stroke Japanese road bikes used a version of Yamaha’s Autolube system.

Additionally, Yamaha had become involved in racing; Fumio Ito riding the 250cc RD56 might have won the 1963 Isle of Man TT but for a bungled 50-second fuel stop. And when Ito was sidelined by a crash in 1964, Phil Read went on to take the 250cc Grand Prix world title. The RD56 spawned the highly successful TD range of production racers that were still winning in the Seventies and continue in vintage racing to the present.

So if 250cc was good, more must be better, right? In 1965, Yamaha introduced its first big stroker, the 305cc YM-1, which carried the company’s banner for two seasons before being upstaged by Yamaha’s 350cc YR-1. The YM-1 effectively became a footnote in Yamaha’s development, while the YDS-3 and YR-1 ultimately begat the reed-valve RD250, 350 and 400 series, culminating in one of the raciest road-going strokers ever, the liquid-cooled RZ350. Because of its relatively short market presence, and the development of the RD series, the YM-1 was — and is — an oft-ignored model in the Yamaha line. 

Read more about the motorcycles mentioned in this article:
• Restoring a 1965 Yamaha YM1

  • Published on Feb 8, 2011
© Copyright 2022. All Rights Reserved - Ogden Publications, Inc.