THE MO­TOR

Classic Racer - - CLASSIC RACER IN DETAIL -

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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