Back in the olden days, Yamaha had a reputation for making powerful 125s, writes Alan Dowds. From the (unrestricted) air-cooled RD125DX two-stroke twin of the 1970s, through to the amazing-when-derestricted TZR125 of the 1990s, the tuning fork folks knew how to get the most from an eighth of a litre.
They’ve done it again with the new R125. Amazingly, the new bike has a fully-fledged variable inlet valve system in its little four-valve cylinder head. The single overhead camshaft has two intake cam profiles – one high-speed and one low-speed. The intake valves are operated by a two-piece rocker arm, with one follower on each cam profile. Below 7400rpm, only the low-speed cam profile follower operates the valves, giving great torque and efficiency. Over 7400rpm, an ECU-controlled solenoid moves a pin which locks the two-part rocker arm together, and letting the high-speed follower take over. So the valves are opened for more lift and duration, giving great peak power.
Okay, the total output is still limited to 15bhp; this is still a learner legal bike, of course. But on the road, the little Yam feels by far the best of all the 125s I’ve ridden in recent times.
Read the rest of this feature in the June issue of Motorcycle Sport and Leisure, in the shops NOW!