Yamaha struggles to hold its own against some seriously tough competition towards the end of the 70s – VJMC’s Steve Cooper remembers…
1977 saw Yamaha back in full factory action with Venezuelan Johnny Cecotto and American Steve Baker both riding the latest and essentially all-new version of the TZ500.
The OW35 marked a significant direction change for the half-litre four with the model defining reed valves dropped from the design. Despite their known benefits, Yamaha had concluded their presence within the inlet tracts was, ultimately, impacting upon gas flow. Seeking to address this radical deviation from the original concept, bell mouths were fitted to the Mikuni carburettors which also featured Power Jets – something would later appear on road-going strokers from the company.
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Another piece of artifice was the use of tuned inlet tracts to support the revised port timing; Yamaha was seeking all-out power. Another marked change was the revision of the OW35’s bore and stroke. Yamaha had favoured to ‘square’ 54 x 54mm set up for many years but now switched to 56 x 50.6mm as previously used a decade previously on some of the air-cooled 250cc racers. This arrangement favoured top-end power and also, fortuitously, meant smaller flywheels which in turn aided crankshaft reliability.
Another massive change from convention was having the engine running backwards which allowed the motor to maximise incoming fuel/air charge without compromising piston skirt integrity. Needless to say, the backwards running engine came with a weight penalty in that an additional jack shaft was needed to reverse drive momentum before it entered the gearbox but this wasn’t considered to be a major issue. Finally, a vertical drive taken off the jack shaft powered both the cooling water impellor and the new lubrication pump that served the gearbox. With revised the expansion chamber layout and now mandatory end cans on the exhausts, the piston-ported TZ500 Mk2 was ready to take on all comers. The OW35 was a fairly radical step on from the old TZ500s!
Early tests showed the new engine was capable of delivering 110bhp but only between tightly defined rev limits of 10,000 and 11,000rpm. With Agostini back with Yamaha for his final year of GP racing together with Cecotto and Baker, most people thought the firm was in with a good chance of regaining the title but it wasn’t to be. The combination of Barry Sheene on the Suzuki RG500 proved to be unbeatable with 108 points at the end of the season. Steve Baker came a creditable second on 80 but it wasn’t sufficient for Yamaha’s bosses who then sacked him, despite him taking the F750 for them! Cecotto could only manage fourth with 50 points and Agostini bowed out in sixth place with just 37 points and was refused factory machines for the following year! In the top 10 riders of the 1977 season all but three were on Suzuki RG500s – the top people back at base in Iwata, Japan, were not in the best of humours! Something fundamental had gone wrong despite all the time, money and effort invested in the 1977 OW35. If Yamaha’s top three riders couldn’t beat those damn Suzukis from Hamamatsu on the best TZ so far, perhaps the faults lay closer to home? Some soul-searching and critical analysis were to be undertaken and a fair few home truths handed out.
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