Yamaha has a great history of air-cooled 250 and 350 parallel twins going back to the 1950’s. Their “prototypes” were German two-strokes. A 1957 production license for the German Adler twin, the typewriter company, gave Yamaha an engineering launch point for their own original YD 250 twins. Amazingly by 1958, five high pipe racing versions the YDS1, the YD-1, were sent with Fumio Ito and other Japanese riders to race the Catalina Grand Prix off the coast of California. Ito brought his high-revving twin home in sixth place.
This YDS7 model is the last of the non-reed valve piston-port induction machines, just before the RD250 and RD350 with reed valves came on the scene. In this era the street bikes took engineering design from the best of the Yamaha production road racers of the time. The racers were sold pretty cheaply and with a bit of tuning/timing setting were ready for the track. Likewise, street bikes like the YDS7 and RD’s could be stripped to the essentials, have low bars and race tires installed and be ready to production road race. On a tight road course, or in a drag race, they were giant killers to be sure.
The 250cc twin two-stroke made 30 horsepower at 7,500 rpm. Top speed if you really tucked in was 93 mph. Magazine testers reported a 1/4 mile in 14.8 seconds. The 12 volt battery ignitions were good, but flakey ignition timing or seal leaks could cause a piston seizure. The primary is helical gears, the transmission is a constant mesh five-speed. These bikes were all “oil injected” meaning there was no need to premix the gas and oil.
- Engine: Parallel Twin, Air-cooled
- Type: Piston Port
- Bore & Stroke: 54mm x 54mm
- Displacement: 247cc’s
- Carburetors: Two 26mm Mikunis
- Ignition: Points & Coil
- Lubrication: Pressure Fed
- Horsepower: 30HP@7500RPM
- Primary: Gear Driven
- Clutch: Wet, Multi-Plate
- Transmission: 5-Speed
- Final Drive: Chain
- Frame: Double Cradle Steel Tubing
- Suspension: Telescopic Fork / Twin Shocks, Swingarm
- Brakes: 180mm Dbl. Leading Shoe / 180mm Single Leading Shoe
- Wheels: 3.00 x 18 / 3.25 x 18
- Wheelbase: 51 Inches
- Weight: 315 Pounds
- Top Speed: 93MPH