Contents • • • • • • • • • • Introduction [ ] For many years, the motocross world almost exclusively used two-stroke engines. Racing classes had two classes: 125 cc and 250 cc two-strokes, with no provision for four strokes. Most riders considered four-stroke engine technology to be largely utilitarian and uncompetitive, relegated solely for trail riding. In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two-strokes. Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four-stroke race motocross bike.
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The Yamaha YZ400F was developed to fit into this category. It solved the power dilemma by borrowing superbike technology and giving the YZ a five-valve head, liquid cooling and a 12.5-1 compression ratio. In 1997, Yamaha rocked the motocross world with the introduction of the YZ400M prototype, a concept motorcycle which borrowed much technology from road racing. The YZM was far ahead of all competition among four-stroke motocross bikes. Piloted the YZ400M to its first victory in 1997 at the Las Vegas Supercross. This was the first time any four-stroke had won an AMA event. The YZ400M was the predecessor of the production YZ400F, which was released the next year.
Samoprezentaciya v stihah na konkurs uchenik goda. First Generation YZ400F Production 1998-2000 Predecessor 2-stroke Successor YZ426F Five-valve, 400 cc, compression ratio 12.5:1 5-speed Weight 250 lb (110 kg) () First generation: YZ400F 1998–2000 [ ] Yamaha introduced the YZ400F in production in 1998 after a successful AMA season in 1997. It was 'the first modern production four-stroke motocrosser that was directly competitive against two-strokes.' Initially, Yamaha targeted a dry weight of 233 pounds (106 kg) (on par with the 250 two-strokes of the time), but by production, the bike weighed 250 pounds.
The bike had an 11,600 rpm redline [ ] power and torque close to its 250 cc two-stroke rivals. It benefited from engine compression braking, which allowed the engine to slow the bike down during deceleration, giving the brakes a rest. The 1998 was the first bike to come stock with a FCR. In 1998, won the AMA National Motocross Championship aboard the YZF, becoming the first rider to win a championship on a four-stroke motorcycle.
This victory is considered by some to be the major turning point in the motocross world—for the first time, four strokes were considered a competitive racing machine. Second generation YZ426F Production 2000-2003 Predecessor YZ400F Successor YZ450F Titanium five-valve, 426 cc, 95mm x 60.1mm stroke, compression ratio 12.5:1 5-speed Kayaba Disks 58.7 in Fuel capacity 2.1 gallons (7.95 L) Second generation: YZ426F 2001–2002 [ ] In 2000, Yamaha updated the YZ400F, increasing the displacement to 426 cc for greater power and throttle response. Aveyond free full version. In addition, the carburetor and jets were updated to ease the YZ400F's starting woes. The next year, in 2001, Yamaha replaced the previous steel valves with titanium ones. The new valves were more than forty percent lighter than the previous valves, allowing for lighter and softer valve springs, in turn allowing a quicker revving engine, improved throttle response, higher rev ceilings, and more power. The crankshaft was also reshaped and the whole assembly was redesigned for quicker throttle response and, Yamaha claimed, 'less high-end horsepower loss.'
In addition to motor modifications, a few changes were made to the transmission to help contain the power and ensure longevity. The suspension also received a bit of an overhaul with the goal being reduced weight and smoother action throughout the stroke. Yamaha also designed a new exhaust pipe design so that exhaust header removal would no longer be required when replacing the oil filter. Carburetor tuning was modified to deal with starting and off-idle take off difficulties. Also in 2001 the subframe was changed from a steel to a blue painted aluminum style. In 2002 the blue painting was stopped and left to a bare aluminum look. Motorcycle.com says that 'The gas tank is reasonably thin and allows good forward and backward movement while providing something nicely shaped to hold onto with your knees.
In fact, the entire ergonomic package on this YZF is well thought out. The handlebars have a nice bend to them and are well-placed for good rider control and legroom. The footpegs are well-made units with a wide platform and sit in a position that keeps them from dragging in ruts without cramping a rider's legs.'