Yamaha AS1

Yamaha have long been known for their two-stroke, twin-cylinder machines. Steve Cooper takes a look at a machine that kickstarted the trend

In 1966, Yamaha produced an apparently inconsequential small-capacity machine called the AT90.

With just 90cc, but two cylinders, this tiddler would go on to spawn a raft of similarly constructed motorcycles that would still be bringing in the money some 15 years later.


The AT90 was to be a home-market-only machine but, as soon as Yamaha saw just how popular the bike was, they rapidly increased its capacity to 97cc – thereby delivering the YL1, aka the Yamaha Twin Jet.

This machine, in turn, proved the concept of small-capacity twins built around a vertically split crankcase, and the basic architecture would still be in use when coffin-tanked Yamaha RD200s were being seriously abused by feral teenagers.

In amongst the plethora of machines derived from the AT90 is an iconic Japanese stroker that, even now, has old men going weak at the knees.


It’s a machine that readily embarrassed other learner riders on homespun 250 four-stroke singles and regularly gave the owners of 350s something to worry about. For many a teenager of the late 1960s, Yamaha’s AS1 was the machine of their dreams.

Based around a simple tubular steel frame, the bike differed from many before that had utilised pressed steel chassis.

The frame ran a single downtube to the front engine mount, then used the engine as a stressed member which then bolted into the frame just ahead of the seat post.


Despite being marketed as a sports bike, the AS1’s styling was old-school, with enclosed suspension front and rear, chromed tank panels and rubber knee pads. Little, if anything, of the bike’s styling gave the merest hint that it was a miniature pocket rocket.

Only the two chromed air cleaner housings that spoke of twin carburettors possibly gave the game away, but the cynics normally just sniggered at the two containers that looked like tea caddies.

Read more and view more images in the June 2019 issue of OBM – on sale now!


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