Yamaha RD125B

Yamaha RD125B Road Test

Yamaha RD125BThe Yamaha RD125 is a strange mix of ages, taking much of its architecture and design from the Yamaha’s of the 60s. With only minor engine mods separating it from the early AS3 range, it is a small and compact machine with a wheelbase more akin to a modern day scooter. The rider doesn’t so much sit on, as perch precariously, elevated high above the proceedings. The bars are high and wide, easily over powering this tiny machine and as such great care has to be taken when inputting commands. Any ham fisted technique is highlighted by this flighty and lively machine, the simple chassis barely able to cope with the stresses and strains of a spirited ride as the lack of any steel work passing under the engine allows a small amount of flex to wreak havoc. Keeping a close eye on the engine mounts does have an impact upon the handling, but even when the bolts are fully tightened there is still considerable movement to be found in the design.

RD125BWhile the RD250 and 350 models received the latest in braking technology, at the front end at least, the smaller models in the range had to make do with drum brakes all round. The twin-leading-shoe drum up front has to be the most unpredictable brake ever invented, certainly at speed. Sometimes it digs in and bites hard from the first squeeze of the lever, while on other occasions it shows complete indifference to any desire the rider has to stop quickly, usually in inversely proportional rates to the actually importance of the job in hand. It is best to have a second grab when this happens as often the brake does get a better understanding of what is happening outside of its cosy warm alloy hub, and gets to work with gusto when a brief respite, and second chance are offered. At low speed the brake is always willing to embarrass the rider, don’t ever think about using the front brake while turning in confined spaces, particularly if anyone is watching you, as, given half a chance, the bike will have you on the floor in seconds.

Yamaha RD125B EngineThis attribute aside however the RD125 is a delightful machine, easily the best of the 125cc bikes from the period and able to embarrass more than a few larger machines in a straight fight. Away from the lights and the RD will give a good showing of itself providing the needle is bouncing off the stop before the next gear is thrown in to the equation.

Styled in keeping with the larger twins in the Yam range, the RD125 does look every bit the part of street racer, a hat that the engine enjoys wearing too. Once again using construction techniques more akin to the previous decade, the 125’s bottom end is a well tried and tested vertically split unit that, although not as strong, or as easy to build as the later horizontally assembled engines of the RD250-400 series, was cheap to produce using castings and techniques that had been around for many a year. The parallel twin may only produce a shade under 14 horses, but it packs them in right at the top end of the engines rev scale at 9500rpm, adding in peak torque at this point too for the maximum amount of punch. The reed-valve induction does little to smooth the introduction into the power band and it actually feels to have a far sharper and stronger kick in the pants between 7 and 8000rpm than the piston port AS3 model that proceeded it. Surprisingly for a machine so closely based upon the racing thinking of the time, the gearbox fails to keep the engine spinning up in the range it needs to be in to keep pulling strongly. This is most noticeable in the gap between first and second, letting the engine rev way past the maximum power still sees the revs drop to 6000rpm once the next ratio is fully home. The engine note drops into that hollow honk that all strokers make when they have dropped off the pipe, and what feels like an eternity passes before the revs rise to the all important 7 digit on the tacho and it all starts again. The next two shifts are relatively close and the revs maintain a healthy position on the dial, but come the fourth to fifth shift and it’s the same old story again with the engine struggling to pull into its top gear with any kind of urgency. Again, once the revs are up into the zone of max torque and power the RD will burble along quite happily in top gear peaking out in the mid to high 70’s depending upon the engines condition and its surroundings.

RD125B YamahaThe slightest incline, or change of wind direction, soon sees the need to drop down a gear as, once the engine is on its back foot, there is little that can be done to save the day. Stamp the lever down a cog and get the engine buzzing again being the only cure, save for slipping the clutch madly which, although strong in normal use wouldn’t take such abuse for long without crying out and heading south. Once used to the engine and chassis characteristics, the ride is nothing less than great fun; it isn’t hard to see why this machine caused a bit of a stir when first introduced, with its mix of lightweight and performance, while getting it right is eminently satisfying. Small, but none the less positive, inputs being the strict order of the day to reduce any upsetting of the delicate ride. Treat it with respect when cornering, allow a bit of extra distance for stopping from speed and above all rev the engine for all its worth.

RD125 History

RD125First seen in 1974 the RD125 used much of the thinking and components from the earlier AS3 series. New for the RD was a reed valve top end that despite, not feeling much different to the older piston-port machines, does give a wider spread of torque high up in the rev range. The model changed little for the B of 1975 but did get a face-lift for the DX model of 1976. A coffin tank and square seat unit, finished off in the familiar Yamaha speed block graphics, saw the RD125 emulate its 250 and 400cc cousins, while a single piston floating caliper disc brake also sharpened up the stopping experience. This style remained until the last of the RD125’s was produce in 1980 although many were still in dealer showrooms and registered far later than this cease in production. The 1978 E model also gained cast wheels while the following year a new design of brake caliper was also introduced this time languishing behind the fork slider rather than on the front face of it.

Yamaha RD125In the summer of 1982, the RD twin was replaced by the single-cylinder water-cooled RD125LC. Many doubted the decision to go for a simpler engine design and feared the new bike wouldn’t out perform the older model but how wrong they were. Available in two versions, the 12bhp restricted model to comply with the new learner laws and the full on 21bhp model. In reality the differences were few and before too long most were liberated full and were capable of speeds near the magical ton. The RD125LC in turn led to the development of the TZR125, first seen during 1987 this model featured a steel “alloy look alike” frame and racer looks.

Yamaha RD125 B Specifications

  • Engine – air-cooled twin-cylinder two-stroke
  • Capacity – 124cc
  • Bore & stroke – 43 x 43mm
  • Carburetion – 18mm Teikei
  • Max Power – 13.8 bhp @ 9500rpm
  • Torque – 7.6 ft-lbs @ 9500rpm
  • Ignition – contact breaker
  • Transmission – 5-speed chain final drive
  • Frame – steel tube engine used as a stressed member
  • Suspension – 28 mm telescopic forks twin rear shock pre-load adjustable
  • Wheels – 2.75 x 18, 3.00 x 18
  • Brakes – 150mm twin leading shoe drum, 130mm single leading shoe drum
  • Wheelbase – 1245mm
  • Weight – 115kgs
  • Fuel capacity – 11.4 litres
  • Top speed – 76mph

Yamaha RD125B Gallery

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