Classic Racer

THE MOTOR

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The OW60’S stepped-cylinder square-four engine weighing 45kg and employing Yamaha’s traditiona­l 56 x 50.6mm cylinder dimensions is identical in architectu­re to the Suzuki’s, with the rear pair of cylinders mounted higher than the front two, and the combined bank inclined forward by 30° to produce a compact 1400mm wheelbase. Four separate crankshaft­s contrarota­ting in opposed pairs are housed in magnesium crankcases, with each cylinder featuring six transfer ports and a single exhaust with twin supplement­ary ‘moustache’ exhaust ports. The 36mm magnesium-bodied cylindrica­lslide Mikuni carbs are strangely devoid of the powerjetsj usedd on Roberts’rb’ piston-iport bikes,bik and feed mixture to the motor viateflonc­oated steel discs, whileyamah­a retained the electrical­ly-operated guillotine-typeypvs electronic powervalve­s it had previously used on Roberts’ title-winners. With a Hitachi CDI, 156 bhp was delivered at 10,600rpm to the rear wheel from the freshly rebuilt engine on Jacky Germain’s dyno, with significan­t torque for a disc-valver, peaking at 7,800rpm. That’s a dramatic step from the 120 bhp at 11,500 rpm delivered by Roberts’ OW48R title-winner from just two years previously, made even more significan­t by the OW60’S featherwei­ght 121kg. half-dry weight, according to Germain’s scales. Compare that to the 135kg of Roberts’ final title-winning OW48R, and you can see what a bibig improvemen­ti theh rotary-valvel racer was.

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