THE FAMILY TREE

The YAS1 (or AS1 for short) is part of Yamaha's second generation, twin cylinder, two stroke range. The first generation were the early YD and YDS

250 twins that sprouted from the latter half of the 1950s. Initially based heavily on the German MB200/MB250 Adlers, the bikes soon evolved their own character, and by the start of the 1960s they were going very much in a different direction to the high-end, Teutonic, gentleman's motorcycle. As the 1960s ticked along, Yamaha was aware it needed to expand its product portfolio and offer both a wider range of twins and in capacity brackets below the initial quarter litre models that had brought them pretty much instant success. The year of 1965 saw the launch of the commuter/delivery AT90; a 90cc sporty twin with a pair of minuscule Mikuni round slide carburettors that enabled the bike to deliver 8bhp at a then staggering 8000rpm. Although a totally new model to the 250s, the AT still used the same basic architecture

– a vertically split alloy crankcase with four ball race main bearings, helical gear primary drive – but with a four-speed gearbox and, crucially, Autolube oil injection, dispensing with the need for premix. Just one year later, the AT90 became the YL1 with almost 10bhp at 8500rpm. When fitted with the factory tuning kit (chrome-plated alloy barrels, high compression head, magneto ignition and raucous expansion chambers etc.), the YL1 was delivering close to 20bhp or 200 bhp/litre! The year 1967 saw the impending release of the YA S1/AS1 125cc twin. Now equipped with five gears, this one-eighth litre out-and-out sports twin changed the public's perception of the Yamaha brand and almost overnight made every four-stroke 250 single obsolete, such was the performance from the 14bhp motor. At the same time, a 180cc version was launched and sold as the CS1 with electric start and almost 20bhp at the crank. Revised, refined, updated and honed, the 125 and 180 (soon bored out to 200) would remain on Yamaha's sales list as first the AS/CS range, before morphing into the iconic RD125/200. The latter ran until 1980/81, but Yamaha eked a few more years from the design; combining the 125 chassis with smaller wheels and a single carburettored, non-reed valve motor to produce the RS200 commuter. With the last RS200s being punted out cheaply after the 125cc learner law, Yamaha managed to run the same basic design for almost 20 years, remarkable for what had started out as basic, 90cc, twin cylinder urban delivery bike.