Yamaha YZR500 OW35K

1978 Yamaha YZR500 racing motorcycle
By guest writer Richard Fowler of Motorsport Retro. When a young Kenny Roberts first ventured to Europe in 1978 to ride in the 500cc World Championship, no one was expecting him to be a title contender. But he was armed with the brutal Yamaha YZR500 OW35K, and created history by winning the 500 cc crown at his first attempt. He became the first American to take the title, despite having never ridden the European circuits before.

Part of this is obviously due to his machinery. The original OW35 appeared in 1977; it was Yamaha’s first new 500cc factory racer since the OW23 of 1975, which Giacomo Agostini rode to World Championship glory. For Roberts’ title attack in 1978 the factory chose to evolve the OW35, chiefly by adding an aluminum swingarm and the ‘Yamaha Power Valve System’. (By running an oval shaped valve, the YPVS altered the size of the exhaust port at different revs—effectively changing the power delivery and producing optimal power and torque across the entire rev range.)

Even in Roberts’ hands, and despite the YPVS, this Yamaha was a handful. The two stroke, water-cooled inline four punched out an arm-snapping 120hp, and revved like a bastard all the way up to 10,500rpm. And despite weighing a mere 135 kg, it still needed two front discs to slow it down from a 290kph+ top speed.

The 1978 title fight went down to the wire in the end, with Roberts’ third place at the Nürburgring finale proving enough to beat Pommy superstar Barry Sheene to the title by ten points. The rest is history: Roberts famously won the Championship again in 1979 and 1980, giving him three consecutive 500cc World Championships to his name.