Power is almost everything in 125 racing. No other class of machine will get revved as hard – and for so long – as a 125 with a decent rider on board. And, with the launch of the liquid-cooled YZ125H in 1981, Yamaha delivered a 125 that not only churned out the ponies, but could keep delivering the goods as effectively at the end of a race as at the beginning. As the air-cooled competition faded towards the end of a long moto, the little YZ125H just kept churning out the goods. It had to be a winning formula.

In truth, Yamaha had given themselves a hard act to follow with the 1980 YZ125G – the last of the factory’s air-cooled 125s. Just about the only complaint an often critical contemporary press could level at the G model was that the rear suspension was a touch softly sprung in stock trim and that the bike wasn’t the out and out horsepower king of the 125 class. But overall, many of the magazine testers of the day thought the Yam was the best of the bunch – especially when fitted with the optional stiffer rear spring.

But however good the G model was, motocross development waits for no man – and Yamaha were already running water-cooled engines in their 125cc works OW43 and OW47 machines during the 1980 season as the air-cooled production bikes enjoyed their successes in private hands. And, while the works duo of Marc Velkeneers and Tetsumi Mitsuyasu could only finish fifth and fourth respectively in the GP hunt, they did prove the potential of the water-cooled engines. In fairness, Velkeneers could point to the fact that he was running second in the championship – just one point behind leader Harry Everts – when he broke his ankle at the German GP in mid-season.

Even if the GP results were not quite what the factory dreamed of, the technology developed on the works 125s in 1980 was obviously pretty much on the money as the production bikes for the following season (1981) were strikingly similar to the previous year’s works machinery. But, although water-cooling was becoming de rigueur in the motocross world, the Yamaha engineers took a typically single minded approach to applying the concept to the YZ125H.

For a start, the radiator was perched high up above the front mudguard and the frame was part of the plumbing for the entire system, carrying coolant from the radiator, through the lower part of the steering head, to a hose connected to the engine’s water pump. The ‘return’ side of the cooling system ran from the cylinder head, via another hose to the top end of the steering head (which was, of course, separate to the lower end which carried the coolant from the radiator to the pump) and from there back into the alloy radiator. It certainly wasn’t the obvious layout, but it seemed to work.

- End of online sample -

More in the magazine...

End of sample article | Top of page | Buy Classic Dirt Bike