Yamaha first started with the frame by adding a three-box tank rail section instead of its original two-box design for increased rigidity. The frame design also includes a new swingarm pivot point and new engine brackets for more predictable handling.
Housed within the new frame design is a re-worked engine. Yamaha opted for a lighter piston, piston pin and wire circlip. Yamaha claims the new piston gives better performance by reducing overall mass and friction, providing more power from 8000-12,000 rpm. Yamaha engineers also took into account vibration on the 250F, revising the crankshaft balancer to reduce the vibes felt at several key touchpoints including the handlebars, seat and footpegs. In addition, the balancer shaft is now optimized for the lighter piston, which further reduces vibration.
Biggest news for the 2012 model is what�s not changed, as the YZ remains the lone 250 to shun electronic fuel injection. Yamaha did tweak the carburetor, however, boring it out another 2mm, and a high-flow air filter cage has been fitted inside the 250F to enhance top-end performance. CDI settings have also been reformatted to give the bike better throttle response in the mid to high-speed range, and a new silencer with a smaller outlet is designed to meet AMA Pro Racing sound regulations.
Up front the KYB Speed Sensitive System fork is revalved and is now offset by 22mm. The outer tube has also been enhanced for 50% more rigidity, and the steering stem was made longer. The spring rate on the fork is now 0.46kg. The 250F�s rear shock sees new rebound damping and a new swingarm adds more strength to the axle block. Yamaha touts that these changes add more refined damping character and improve bump absorption, allowing for more aggressive sessions on the track.
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