Yamaha YDS-7

In the evolving line of Yamaha two-strokes, the YDS7 of 1971 is notable as the one in which the till-then popular rounded look was replaced by a more angular line, at the same time becoming slimmer, lower and lighter.
Peak power of the YDS7 was set at 7,500rpm which, with top (fifth) gear engaged, came up very easily, the bike being a trifle undergeared, and worked out to a road speed of 90mph.

A small rider, crouched, could extract a further 5mph with no great difficulty. Steering geometry (62.5° castor, 4.2 in trail) to give a fast response made the bike very diverting transport on winding roads; 30bhp, combined with the effect of a short wheelbase (54in) and low weight (300lb), would keep the front wheel in only light contact with the tarmac.

For this model Yamaha introduced an 'excited field' generator for ignition purposes, which worked well and emitted an eerie whistle as the ignition key was first operated and the system set itself up. In this connection, a more obvious step forward for 1971 was the positioning of the ignition key in the top of the steering head instead of under the nose of the tank.

Road Test

Life before the RD?

Yes, there was one!

It may well come as a bit of a shock but there was a perfectly good and happy life to be had before the arrival of the Yamaha RD series of machines all those years back in 1973. The Yamaha 250cc twin cylinder dynasty actually began as far back as the winter of 1964 with the YDS3, with a 350cc version, the YR3, joining this a little over four years later. These very early models were just like over grown 125cc machines with their vertically split casings and lightly finned heads and barrels, performance wise, whilst generally being a match for British designs, they certainly were no competition for some of the hotter machines being made by other oriental producers.

The YDS7 and R5 appeared on the scene at the turn of the seventies, not only to counter the threat from Suzuki, who were also making some pretty impressive small capacity two strokes, but also out of a need to save money for the middleweight race projects. Not that the factory racers needed cheaper parts for their machines, the OW’s that did all of the winning in the hands of the various Grand prix stars that rode for the tuning fork brand were vastly different, it was the production bikes that had to be built to a price. The cheapness of the air-cooled TR and later water-cooled TZ’s was directly attributable to the many similarities found in cycle parts and engine castings of the roadsters.

YDS7It wasn’t only the engine that bore a remarkable resemblance to the race bikes, the chassis too was strikingly similar to the Yamaha racers of the period, hence the new roadsters rating as a fine handling and eminently manoeuvrable piece of kit. Much store is placed in the RD’s family heritage, but as gene pools go, the YDS7 and R5 are far more closely related to the piston port race machines than the later roadsters ever could be.

Within a few short weeks of their launch the road going twins, and in particular the 350 version, were being ranked as giant killers with staggering superiority around the twisty bits and with a top speed in treble figures, at least some of the time, almost unmatchable straight line ability too. The combination of light weight and good reliable horsepower figures giving a sprightly performance in all areas.

Yamaha YDS7They are very stylish machines, with lines so much sleeker and all up weight considerably lighter than their immediate successor, the more bulbous RD A and B series. In actual fact the RD series continued to pile on the pounds with each and every new version right up until the launch of the featherweight LC range in 1980. Despite very similar design features, and an almost identical looking engine unit, the bike actually shares very few components with the later RD version. The five-speed gearbox is not directly interchangeable, despite the early RD’s having the use of only five of the six gears ratios available (another story for another day!), and many other engine parts are not useable between the two types.

Riding any of the piston port twins is a rewarding experience once the idiosyncrasies of the type are mastered. The need to keep the engine buzzing, and with it the engine comfortably up in the power band, can sound to innocent bystanders as reckless, loutish behaviour but its all part of living with an early seventies two stroke. Once up onto the pipe the little Yam, with its lack of reed valve induction, gives a far bigger kick to the seat of the pants than the same capacity RD ever could despite putting out less power at the rear wheel. The sensation remains all the way throughout the arc of the speedo, providing you keep throwing in those gear ratios with sufficient speed the acceleration remains constant and even by today’s standards, impressive. It is crucial not to short shift as once the revs have dropped off the pipe then all the engine does is bellow at you until you clutch it and downshift eventually getting the bike buzzing again.

YDS7It is important to mention that the noise created at tick over is not in any way confidence inspiring, the early air-cooled engines certainly do make some mechanical noise. We have, in the last twenty years or so, grown too cosseted with the water-cooling and body work serving to cocoon the whirling and clattering engine components and effectively damping any engine noise. With the Yams a heady mix of piston slap, small end bearing jangle, exhaust ding and fin rattle combine to create a din, not too dissimilar to a bag of spanners being thrown around an empty industrial unit. Once the engine is given the go ahead however the mechanical noise either diminishes or takes more of a back seat, making way instead for the hollow resonating roar of the tiny metal air filter box and the sharp cackling dissonance of the one piece, cigar shaped silencers.

Corners appear quite quickly and the need to slow the whole plot down a bit soon becomes a matter of urgency. A quick squeeze of the most important lever on the right yields little initial response worth calling a brake as the 180mm diameter, twin leading shoe, drum set up found at the bottom end of the cable needs some waking up. The first thought is always that the brake has failed to operate in some way as the cable gives a fair bit of spongey feel to the lever but, keep hold of it and the initial grab that was so lacking at speed makes itself apparent in a more subdued way as the shoes begin to heat up and consume the kinetic energy. Judicious amounts of the identical diameter, albeit a single leading shoe design, rear drum adds to the stopping experience no end, unlike a more modern disc rear that have little impact upon the trajectory of a speeding bike the Yamaha drum actually does something. Although not as immediately efficient as the later race replica cast iron, twin piston, disc stopper, a well set up Yamaha drum brake can be ridiculously fierce at pedestrian pace and once hot, full of feel at the higher end of the velocity spectrum.

Yamaha YDS7With the speed comfortably scrubbed off the story found with the later RD’s is all too evident with the bike that started it all, you have to get the front working very hard. The forks are not happy when slack, the front wheel has to be relied on fully for the sharp handling that the type is renowned for. On both models a primitive, friction type steering damper, running directly through the headstock, is fitted but in reality its use was never really necessary on the smaller version and only very light settings useful with the 350.

Tyre sizes are small by modern day standards but this adds to the general manoeuvrability both at low and high speeds, these tyre sizes are perfectly adequate for the power of the engine with the 250 machine never giving cause for concern although the R5 can be a bit more of a handful. Of course with such little power on tap you are never going to light the rear up so there are few if any problems to speak of with that end. Even so the small horsepower outputs that are evident do create some problems especially with the standard rear shocks and their total inability to damp and spring at the same time once a few miles had passed underneath the little twin. Fortunately Japanese suspension has moved on, literally in leaps and not so many bounds, from those early days. Those YDS7’s and R5’s that are in use either have replacement units or rebuilt Yam items those, hopefully, work better than the original 1970’s jobs.

YDS7Any capacity Yamaha twin is a simple machine to live with, easy to work on and cheap to run. I feel the YDS and R5 have just that little bit more of an air of mystery and attractiveness, providing a far better insight into the world of the racing two stroke than any of the later RD series. Finding one in original condition could prove difficult as most were modified with after market and later RD parts to create a mismatch of bikes that effectively is worthless as a collectors item but will be a rewarding ride none the less.

Source classic-motorbikes.net