From our Old Bike Archives – Issue 69 – first published in 2017.
Story: Jim Scaysbrook • Photos: Tony Morisset and Jim Scaysbrook
Like its Japanese competitors, Yamaha had a fairly nippy little 100cc single in the mid-sixties, the YG-1. That model was a two stroke fed via rotary valve, like the Bridgestone 100, but Yamaha wanted to be a jump ahead of the rest, and in early 1966 released the model that shot straight to the top of the 100cc class – the YL-1. This was a twin cylinder 97cc model that looked stunning in a miniature way, and went like blazes. To satisfy home market requirements, the YL-1 was produced in 89cc form as the AT-90, but the rest of the world got the bigger version, which was marketed as the Twin Jet 100 in USA.
With its angular styling, wedge shaped fuel tank and twin chrome mufflers, the YL-1 rapidly became the darling of the tiddler set, and the factory soon responded with a genuine racer kit – the GYT (Genuine Yamaha Tuning) kit that included expansion chamber exhausts, barrels, heads and pistons, carburetors and a new ignition system. Heart of the matter was the simply designed but highly effective engine, with bore and stroke of 38mm x 43mm and a four-speed gearbox. Top speed was just shy of 100km/h, at which point the engine was spinning at 8,500 rpm. The crankshaft was seriously robust (and took to subsequent increases in power output without a flinch), with roller bearings on the con-rod journals, and four ball bearings supporting the crank itself.
Yamaha’s “Autolube” direct injection system supplied the oil, which was housed in a tank under the seat. An engine-driven pump injected the oil directly into the inlet ports, and from there, the air/fuel mixture picked up the oil and carried it to the bearings. A mechanism within the pump varied the stroke of the pump, the control for which came from the twistgrip throttle, thus automatically adjusting the oil-feed rate to suit conditions. Of course, the already popular YDS-3 used similar principles, but instead of having the clutch carried on an extension of the crankshaft (Adler-style), the YL-1’s clutch was in the conventional location of the gearbox mainshaft. Because of the relatively high-revving nature of the engine, this allowed the clutch to rotate much slower, creating less inertia and giving smoother operation.
Not surprisingly, the engine was no powerhouse at low revs, but once motivated would spin freely to the maximum rpm, with maximum torque achieved at 7,500 rpm. The very compact engine unit sat in a pressed steel backbone frame, with a pressed-steel swinging arm and a bolted on single front downtube. Conventional (for the time) telescopic front forks and twin rear shocks comprised the suspension, with 17 inch wheels. Initially, the YL-1 used 6 volt electrics with starting by kick only, but in 1968, a 12-volt version appeared, fitted with an electric starter.
Of course, the little twin soon began to put on weight, first as the 125cc AS-1, then to 180cc as the CS-1, and finally 200cc in the form of the CS-3. The AS-1 hit the market in 1967 but the fact that any survive today is remarkable, given that many, many of these superb little 125cc twins became race bikes, and very effective race bikes too. With the addition of a GYT kit or any number of bits from tuners around the world, the 125 was a weapon, and a machine that, in the right hands, could achieve considerable success in the booming 125cc class.
The AS-1 broke from the YL-1’s angular styling with a 9.5 litre slab-sided fuel tank sitting a top a tubular steel frame running on 18 inch wheels. But the lineage was still there – a logical progression from the 100 which was capable of 110km/h in standard form and much more when breathed upon. With the pressed frame and its stocky midriff now gone, the air filters were housed in a pair of vertical, chrome stacks in front of the battery case and oil tank. Also gone was the YL-1’s fully enclosed rear chain case – replaced with a single top cover. Power was officially quoted as 15hp at 8,500 rpm from the 43mm x 43mm (124.8cc) engine, with the whole package tipping the scales at 98kg. The gearbox became a five-speeder with a beefed up clutch.
Local lovely
The YL-1 featured here has a fascinating history, having resided in the Clare area of South Australia all its life. Owner David Dunstan, who runs a long-established electrical business established by his father, has known of the bike since it was sold new. “It was sold by J.O. Denton and Sons, who sold Ford tractors and had a sub-agency for Yamaha from Pitmans in Adelaide. They were in a little town called Farrell Flat, about 15km from here and when they closed down it just about wrecked the town. They eventually got their own Yamaha dealership and sold heaps of farm bikes – sometimes they would even fly the bikes to the outback properties. This bike was purchased new by Lynton Stevenson, who worked for my father for 32 years. He was a big bloke – about six foot six – so he hung over the back of the seat and used to rev blazes out of it. Lynton passed away a few years back and I bought it from his widow, and had Don Tonkin restore it for me. He does brilliant work and the whole job only took about four months. I would have loved Lynton to see the bike back to its as-new condition. The Yamaha had lain around for a few years and the kids had put water in the tank, but it was mostly intact and only had a small dent in the petrol tank. Don put on new rims and stainless spokes and a few other parts that he sourced from overseas. Ian Denton, who is one of Joe Denton’s two sons who took over the business, kept all the sales records and still has quite a few old stock Yamaha parts in his shed. I was able to get new pistons from him. Funnily enough, Ian’s wife also worked for our family company – it’s a small world out here in Clare!”
It is no over statement when David describes Don Tomkins’ restoration as ‘brilliant’. Don’s attention to detail is legendary down in McLaren Vale, where many of his former recreations regularly circulate in the hands of local owners. The YL-1 is finished in the original candy red (blue was an option), with the distinctive two-tone black seat with white sides. It certainly is a substantial saddle, and would have been appreciated by the likes of Lynton Stevenson. Considering its diet of water and glug when stored, the petrol tank has come up fine, and the chisel shaped chrome-sided item is just so ‘sixties. The last items to be reclaimed and fitted were the mufflers, and fortunately they were internally self-preserved by a healthy build up of lubricant over the years. The plating on the mufflers, like the rest of the bright work is by A Class Metal Finishers in Lonsdale, SA.
With the YL-1 now complete, David Dunstan is hankering for another project and has his heart set on an RD350 Yamaha of around 1972 vintage. If a reader can assist, David can be contacted during business hours on (08) 8842 2144.
Last of the line
The ultimate refinement of the 100/125 Yamaha twins was the AS-3, produced from 1971 to 1978. Gone were the old cast iron cylinders, replaced by aluminium alloy versions with revised porting for more low-down power. Styling mirrored the 250cc DS-7, with a rounded fuel tank and twin speedo and tacho. Weight had crept up the 105kg and at the front, a purposeful looking twin leading shoe brake, which looked identical to that used on the factory TA-125 racers, hauled the show down. The final versions of the AS-3 gained reed valve induction. In this form, power was up to 17hp at 9,200 rpm. In some markets, the AS-3 was sold as the RD 125 or RD 125 DX. In some European countries, the 125 continued after 1978 as the RDX125, with a front disc brake and cast alloy wheels.
The motorcycle featured here belongs to Lindsay McKay, a man who knows his way around two stroke twins, having successfully raced a TD3 for many years. “I started racing on a Yamaha DS-7, so I’ve always had an interest in them. I saw this AS-3 for sale on-line for $2,500 fully restored and I thought, ‘that’s such a good deal, I couldn’t restore one for that’, so I bought it in 2012. I’ve always wanted a TA125 race bike to go with my TD3 but that never happened so I bought another AS-3 to build up as a road bike. That one is still sitting in boxes, waiting. I also thought it would be nice to have a range of these little twins, so I got hold of an LS-2, which is a 1972 model, the same as the AS-3. It is totally complete and original but a bit rusty, so I will probably leave it like that. So now I have the full family – 100, 125 and 250”.
instruments like the 250.
Lindsay has ridden his AS-3 in several classic rallies, but is now contemplating selling it, and the LS-2 as well, in preparation for a house move in the future.
Specifications: 1966 Yamaha YL-1
Engine: Parallel twin two stroke, piston port induction.
Bore x stroke: 38mm x 43mm
Capacity: 97cc
Compression ratio: 7.1:1
Carburation: 2 x Mikuni VM16SC
Power: 9.7 hp at 8,500 rpm
Torque: 5.8 ft.lbs @ 8,000 rpm
Lubrication: Yamaha Autolube oil injection.
Transmission: 4 speed.
Frame and swing arm: pressed steel
Wheelbase: 1145 mm
Weight: 82 kg
Fuel capacity: 7.3 litres
Wheels/tyres: Front: 2.75 x 17 Rear: 3.00 x 17
Top speed: 63 mph (101 km/h)
