Yamaha YZ 426 F

The AMA rules had changed where the gigantic 500cc liquid-cooled engines were no longer able to race. Most riders considered four-stroke engine technology to be largely utilitarian and uncompetitive, relegated solely for trail riding.

The AMA introduced a displacement exemption for one season allowing any displacement four stroke 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.

. 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.

After a successful AMA season

Yamaha updated the YZ400F, increasing the displacement to 426 cc for greater power and throttle response. In addition, the carburetor and jets were updated to ease the YZ400F's starting woes.

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 subframe was changed from a steel to a blue painted aluminum style.

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.

The YZF received the biggest update in its history. First, the engine displacement was increased with 449 cc in compliance to the new AMA Motocross maximum displacement rule, the engine now produced over 52 hp (39 kW).