Yamaha – always the innovator – helped make the two-stroke a more flexible powerplant with the likes of powervalves and before that the reed-valve induction system.
Back in the 1960s, Yamaha relied on the use of rotary-valves on their two-stroke engines, as did other Japanese manufacturers of the time but the air-cooled RD350 changed all that.
In 1973 the 350 class was popular in the USA, which was a massive market for the developing Japanese machines of the era and the RD350 was intending to cash-in on the popularity of the class. Looking back, the air-cooled RD350 itself was a development of the YR5, which had an identical frame as well as sharing some running gear and engine components. The motor itself featured a different head from the YR5, as well as new cylinders, seven transfer ports instead of five, 28mm carbs and a 64 x 54mm bore and stroke. The RD pumped-out around 39bhp at 7500rpm and more to the point the motor was punchy and flexible – thanks in part to its reed-valve induction system which was new to motorcycling in 1973.
So, what was a ‘reed-valve’? Reed-valves performed the same task as disc, or rotary-valves but were simpler and cheaper to produce. Yamaha called their system ‘Torque Induction’ which did partially explain the function reed-valves played in the engine but the RD350 also had an impressive top-end power too. ‘Torque Induction’ meant that the part of the rev-range most of us found ourselves in, when riding in an urban area, was also useable and flexible and the motor on the RD wasn’t too ‘peaky.’
This motor, ‘induction’ technology and a light-weight bike of 143 kilos (along with a decent front disc brake) meant the RD was a street howler and a mild-mannered machine: it was both Clark Joseph Kent and Superman combined – all you had to do was twist the throttle to see who got out of the telephone box… It was cheap too at just £455.
Reed-valve induction is basically a one-way valve of flexible petals. The amount of petals used depends on what the motor is being used for, so there can be as many as eight petals down to as few as a single petal ‘valve’.
These valves let air pass one way but not the other and they’re very handy in the intake tract of a two-stroke engine, as when the piston of a stroker goes down, it compresses the fuel mixture in the crankcase and this can blow-back through into the carburettor, but a reed-valve in the carb manifold will let the mixture in, but not out. This means you can use bigger ports and therefore attain a higher state of tune.
The system stayed in place as the RD family transformed into the LC series of liquid-cooled roadsters in the 1980s. The legendary LCs shared much with the previous air-cooled family (air-cooled barrels slot straight on the LC bottom end) and Torque Induction and water-cooling helped give the RD350LC of 1980 a maximum power output of 47bhp @ 8500rpm. That was a lot of fun, back in the day. In the 1980s/1990s of course, we had plenty of ‘home tuners’ who thought that a rat-tail file was the way to more brake horsepower! And then there were ‘racing reeds.’ How many of us fitted Boyesen or Hy-Tech replacement reed valves?
While reed-valves moved on, disc/rotary-valves did too. As to what’s the best induction method, up until the end of two-stroke GP racing, the reed versus rotary valve scrap was fought ‘on every Sunday’ in the 250 and 125cc classes. The general feeling was that the Italian machines (Aprilias and the identical re-branded Gileras) would have the top-end with their disc/rotary valves while the Japanese with their reed-valves would have some hefty mid-range.