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.

  • Brooks Moses

    The Wikipedia description of the YPVS is sadly lacking in providing enlightenment for those of us who don’t already know how the system works and what it looks like, or at least I found it so.

    This page, unfortunately not in English, has a good diagram about how the valve works; it’s the second picture on the page. Once you’ve seen that, this page has some nice photographs of the valves themselves and the ports, and between those and the general idea of “a cylindrical servo-actuated valve” and “an oval-shaped port”, you should be able to see how it works.

  • Tin Man 2

    Great Bike, Those were the days of unlimited race R+D funding. It worked good on the street also in the RZ 350s, until Envo Regs shut it down. Im still waiting for the return of the 2 strokes, With modern Fuel management, decent Fuel Economy and Cleaner Emmissions should be possible.

  • Sportal

    Fantastic machine.

  • Davo

    What year did Yamaha start running the bumblebee yellow and black?

  • Takes me back to my youf! Classic lines. Surely the peak of this era was the ’79 British GP at Silverstone, where the whole of the UK (Sunday afternoon peak viewing on BBC1) saw the finish come down to a tyre width, with Sheene almost on the grass.

    I believe the Yellow & Black was Yamaha US colours, presume Yamaha Co. used yellow for maximum returns in the US market? (as Red & White was used by Ago & Read in the mid Seventies, and later on by Sheene)

  • Squizza

    Long live King Kenny. For my money he was the greatest of all time. He had an unmatched hunger to win.

  • really?

    Part of this wasn’t due to the machine. Although brutal the OW was no where as ridable as the square 4 suzuki RG’s. Kenny, nay anyone who tasted the the OW during 78-79 said you could not open the throttle more than half way below 8000 revs or it would stall. I still can’t imagine how he won on tracks he’d never been on, on a bike so hard to control in his first year.