Yamaha YZ250

The Yamaha YZ250 is a two-stroke motocross race bike made by Yamaha. The model was launched in 1974, and has been regularly updated and is still in production with new releases every year.

Engine
The original YZ250 of 1974 used an air-cooled 250cc two-stroke engine of 70 mm bore and a 64 mm stroke, which was improved semi-annually. The air-cooled motor was replaced in 1982 with a 249 cc liquid-cooled two-stroke reed-valved engine with a mechanical, rather than servo-driven, YPVS exhaust valve for a wider spread of power. Although other makes of contemporaneous motocross engines featured theoretically superior crankcase reed-valves, the YZ250 still has the reed-block integral to the cylinder.

The bore on the water-cooled engine was reduced to 68 mm and the stroke was increased to 68 mm producing a displacement of 247 cc. In 1999, the bore was further reduced to 66.4 mm and the stroke lengthened to 72 mm producing a displacement of 249 cc. The longer stroke engine resulted in a lower redline, slightly less top end power and greatly improved torque at lower RPM. The engine produces a peak 59 hp at 8,800 rpm and 30.6 ft.lbf of torque at 7,500 rpm, with a 9,000 rpm redline.

A large range of after-market tuning parts are available, owing to the bike's design being fundamentally unchanged since 1999 and the sheer numbers sold over the last fifty years.

According to US Dirt Bike magazine, it is the last 250 two-stroke from Japan.

Chassis
On its introduction in 1974, the YZ250 used a single backbone steel frame with a twin-shock swingarm. The following year, reflecting the factory racer Hakan Andersson's factory racer, the swingarm became a monoshock unit, initially made of steel, then aluminum from 1978. The swingarm rear suspension system featured more than 12" of travel, while the telescopic inverted front forks had 11.8 in of travel. In the 2005 model year, the YZ250 gained a new frame made from aluminum that reduced the dry weight to approximately 212 lb. All YZ250s feature front and rear hydraulic disc brakes. The 2014 US model had wet weight of 227 lbs.

Racing

 * 5 AMA National Motocross Titles
 * 9 AMA National Supercross Titles
 * AMA National Supercross 2004 championship, by Chad Reed.
 * 6-time AMA National Offroad Championships by Jason Raines