Introduced in 1968, the Yamaha YAS1 defies all motorcycle logic, even by today’s standards of miniaturisation. As a development of the previous Yamaha roadster twin designs, the vertically split, twin-cylinder engine is compact and powerful, this in turn is wrapped in an equally diminutive frame that does its best to hold the wheels inline and weighs next to nothing in the bargain. Looking around the AS1 chassis you cannot help but notice the amount of parts that wouldn’t look out of place on a moped, in fact the swing arm, wheels, brakes, forks and yoke are all identical in dimensions to those found on the FS1E.
The AS1 is so small it surely could never be a viable proposition for an adult to ride on the road, Once on the move and buzzing away in its sharp power band however, the truth emerges, the 125cc two-stroke is more than a match for machinery several times the capacity. This must have come as quite a shock for the British bike owner seeing cloud of blue smoke and the tiny Yam disappearing off into the distance.
It isn’t all rushing off into the sunset with this baby twin however, the piston port motor is peaky and requires judicious use of the clutch at low speeds and when pulling away. The motor soon drops out of its power band too, meaning the slightest incline or headwind needs a good couple of prods on the lever to get the gears working the engine again. Once on the pipe the twin goes up the rev range with enthusiasm, and a fair amount of high pitched commotion, its thirst for revs almost equalled by its hunger for gear ratios and it leaves one wondering if five was ever quite enough to satisfy this need. The engine is peaky but very usable, and will pull out of the hole left by it dropping out of the power band if full throttle is applied and a little patience used. Of course this isn’t how small capacity, piston-ported, strokers are meant to be ridden so, its clutch in, drop a gear or two and give it some to get the engine up and revving again. Somewhat akin to a puppy dog and the offer to fetch a ball, the AS1 is always keen to play this particular game and doesn’t seem to mind how often either, the rush into the power band is just as frenetic as any larger machine it is just the numbers and speeds achieved that differ.
With a top speed in excess of 75mph, the 125cc twin can really get a move on but, great consideration is needed once at those speeds as stopping is another matter. A lot of the cycle parts are no bigger, or more capable, than those fitted to the 50cc FSIE; the small diameter, single leading shoe drum brake up front looks as if it is barely capable of stopping the bike and in use it isn’t. The lever feels soggy as a mix of cable stretch and lack of power combine to slow the AS but never as rapidly or with the authority as one might desire. Once accustomed to the tiny Yamaha, a combination of front and rear brake is used in conjunction with plenty of down shifts and it is only when these three components are mixed into a cocktail of retardation that the bike feels to be hauling up anything like safely enough.
It can come as quite a god send that the front brake isn’t keen however, try braking hard when leant over to any degree and the chassis lets you know, in no uncertain terms, that it isn’t happy, the front weaves around totally separate to the rear end and it feels like a hinge is placed somewhere around where the engine used to sit. Yamaha may have led the world in two stroke technology but trailed behind some what in the handling stakes during the 60’s and early 70’s. Later versions of this design, the RD125DX etc, featured disc brakes, even then however nothing so powerful as to have any real and startling effect on the proceedings, but thankfully never the harsh, twin leading shoe drum, as fitted to the RD200B.
When left to its own devices, and settled into a bend without provocation, the ride is predictable and the handling more than adequate, it is only when something happens mid corners that the frame gets nervous and loses composure. The wheelbase is a mere 1200mm in length, once again moped proportions and this, combined with the frames ability to do the Rumba, things can get exciting, pretty quickly. With this in mind the AS1 is fitted with an effective steering damper, this works by increasing friction between two plate and is operate simply by screwing the large knob on the middle of the handle bars, to the right for a heavier feel and left for a looser one. This primitive design really works and great care has to be taken to prevent the steering locking up completely, this can happen all of a sudden and not helped by the narrow handle bars, these do give a sporty riding position but little leverage and authority over what the front end is doing. This isn’t a problem in normal use as the bike is so light and agile, the smallest of inputs soon has the AS1 doing your bidding it is just when things do not go exactly to plan that you will struggle to regain control.
Yamaha began making motorcycles in 1955, the first a simple single cylinder two stroke design based originally on European thinking, but they quickly developed their own style to create the range of nippy and agile two stroke twins. The first credible machine, certainly from a European standpoint was the YDS1 of 1959, this was quickly nickname “Japans first sports model” and to some extent truly was. The YDS1 set the design brief for the rest of the Yamaha to follow and the basic engine layout remained the company norm until the introduction of the YR3 in 1969 which, in turn led to the more advanced horizontally split casings of the 250 and 350 RD series as well as the legendary TZ race machines. The YDS series evolved steadily from the YDS-1 into the YDS-3 of 1966 and it was this model that achieved great popularity in the USA. As the first 2-cylinder model to adopt the Auto Lube system, it was very reliable, scotching the old guards scoff of regular two stroke blow-ups while also adopting features like a 3 way adjustable shock absorber.
In 1968 Yamaha launched a whole new range of similar specification twin cylinder two stroke motorcycles across a wide range of capacities, the 100cc YL-1, the 125cc AS1, the 250cc DS-5 and the 350cc YR-1. Looking around the AS1 chassis and power plant it is easy to see the bikes origins, it really is a scaled down version of the earlier, larger capacity designs, with the performance being less so.
For the day, the AS1 featured some advanced design work, worm ports for example, Yamaha’s way of describing the way the transfers meander their way up the side of the barrel, fed the domed compression chambers before the gasses escaped into semi expansion chamber type, chrome exhausts. This returned an impressive level of performance that when compared to the established 125cc machines of the day went more like a 250cc one.
The basic design of the AS1 lived on for the next 12 years or so, with the bottom end of the engine remaining largely unchanged, on through the RD125 and electric-start 200 series, while the chassis too differed little from the spindly tube work holding the original machine together.
For such a small machine and engine, the AS1 is incredibly tough, able to handle most tasks with comparative ease and with few failings. Even the dreaded top end seize would be few and far between thanks to the efficient Yamaha auto lube system that, providing it is maintained and kept supplied with oil will look after the reciprocating parts nicely. As is so often mentioned in other areas of this article the only true failing of this machine and the later developments of it like the AS3, RD125 and 200 is the chassis. The top of the design would be adequate if the lower part did its job correctly the main problem is Yamaha chose to save weigh and costs by letting the engine act as a stressed member, completing the lower frame loop and adding rigidity. This is fine in theory and has been a technique used in many designs for some time, however, in practise, the engine bolts and flimsy plates that make up the front mountings are not strong enough to do this effectively and some considerable flex does occur.
Yamaha did produce a full race kit for this and the AS3 engine, enabling it to be turned into a very effective race machine. So effective that examples of them, albeit very highly developed and by now water cooled, actually won two world titles in 1973/74 ridden and largely created by of the late Kent Andersson. This makes the tiny 125 Yamaha the only road based machine to actually take a world title, let alone win a GP. The kit offered a huge increase in power boosting the 15 BHP to around 25, but did nothing to make the frame any more competent. Those who have pushed a road going Yamaha twin to the limit on the road will know of the frames ability to do a great belly dance, so one can only imagine what this was like at even higher speeds and with a good deal more power on tap. At the end of the racers development the figures had become really impressive with a shorter stroke engine now producing in excess of 35bhp and yet, still used the basic AS open loop frame design. For the mere mortal, the best one could hope to achieve on the road would be a set of spannies, of which there were plenty to choose from in the 70’s, although serious improvements in any area other than the volume produced would be doubtful.
1968 Yamaha YAS1 125cc twins timeline
The first of the air-cooled 125cc twins. It featured a very high specification for its day with autolube and a steering damper. The type was an instant hit in the UK.
This was slightly larger, heavier and chunkier version of the YAS1. Styling wise this was a pointer to what was about to come, the reed valve RD range.
Clearly just an update of the YAS3 design with a major engine top end redesign the only noticeable difference. Performance wise there as little to show for the extra parts.
1975 RD125 B
The A grew into the B but with no changes other than the colour it cam in being made to what was a very nice motorcycle
1976 RD125 DX
The 1976 RD125DX marked the first of the disc braked and coffin tanked models once again FS1E parts were used to implment the move to a more modern stopper. The styling mimicked that of th larger RD range. Avalible colours for this year were Brilliant Red, High Sparkle Light Green, Silver Dust, Low Gloss Black and Marine Blue
For 77 the X was dropped form the title and the model codes fell in line with the RD250 and 400 models. That apart the D was little different except for the colour it came in, Crystal Silver, Space Blue, Competition Yellow, Chappy Red were available in this year.
The 125E model. Around this time there was a change over to alloy wheels but in the early months some still sported spoked wheels. Many small and minor changes were made to the gearbox clutch and oil pump while a new brake caliper now sat behind the right hand fork leg, although it was still stolen from the later model FS1E
1979 RD125 F?
This was the last of the RD125 from with alloy wheels although many were registers a good few years after this period so do expect to find the odd 81 and 82 model. No modifications were implemented and in reality this was just a left over from 1978 with the new range of water cooled 125cc single just around the corner there could have been little point in developing this model further.
1968 Yamaha YAS1 Specifications
- Engine – Two-cylinder two-stroke air-cooled
- Capacity – 124cc
- Bore & stroke – 43 x 43mm
- Compression Ratio – 7.0:1
- Carburation – Two 17mm Mikuni VM17SC
- Max Power – 15bhp @ 8500rpm
- Torque – 11ft-lbs @ 7000rpm
- Ignition – Contact breaker ignition system
- Transmission – 5-speed chain final drive
- Frame – steel tube open cradle
- Suspension – 27mm telescopic forks twin shock rear
- Wheels – 2.50 x18 2.75 x18
- Brakes – 110mm single leading shoe front and rear
- Wheelbase – 1200mm
- Weight – 98kgs
- Fuel capacity – 9.5 litres
- Top speed – 75mph
Yamaha YAS1 Road Test Gallery