With only four parts returning from the previous YZs, it’s safe to say that Yamaha made a complete overhaul to the big bikes in its YZ lineup for 2014. The only returning parts on the 2014 YZs are the brakes, front number plate, front fender and rear wheel.
 
The 2014 YZ250F probably benefits most from the updates, as it is finally gets the fuel-injected DOHC engine Yamaha introduced on the YZ450F three years ago. This engine features a forward-positioned cylinder head and rear-inclined cylinder configuration that reverses the intake and exhaust ports and places the throttle body and the airbox in front of the engine. This helps the motor produce power more easily due to the symmetrical intake ports that give the intake charge the straightest possible shot into the combustion chamber.

The rear-inclined cylinder not only centralizes the engine’s mass more effectively, it’s also offset slightly so that the connecting rod is vertical at the moment of greatest combustion force. This minimizes piston-induced drag against the cylinder wall and frees up even more power while also improving throttle response. The YZ250F engine loses one intake valve, going from five valves to four larger titanium ones to shave additional weight.

Although the 2014 YZ250F features the same 77.0 x 53.6mm bore and stroke and 13.5:1 compression ratio found on the previous ’13 model, Yamaha claims the new engine makes more power, specifically in the midrange and top end. The new YZ250F is outfitted with a larger 44mm throttle body and 12-hole injector that replaces the outgoing 39mm carburetor found on the 2013 model.

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Yamaha also outfitted the YZ250F’s new engine with a lightweight, forged two-ring piston that is that is oil cooled to reduce heat, new cam profiles, a new ECU, a new carburized crankshaft designed for better durability, a newly designed transmission and stronger clutch.

Yamaha YZ450F
The YZ450F also receives many upgrades for 2014. Although the engine isn’t all-new for the 450F, improvements to the powerplant headline that list, as does a new aluminum bilateral beam frame design shared on the 250F and the 450F (the only difference on each model being where the engine is mounted). Yamaha decided to forgo the 450F’s dry sump oiling system and external tank in favor of a more compact wet sump system that allowed engineers to reduce the size of the 450’s crankcase so the big engine could be shoehorned into the extremely compact chassis.

Performance-improving revisions to the 2014 YZ450F include a revised piston, intake and exhaust valves and ports, valve mechanism, ignition timing and fuel-injection mapping—all designed to improve combustion efficiency and make more power while the ignition and fuel mapping changes are also intended to create a smoother powerband. Yamaha also outfits the YZ450F with a new ECU, which features timing and mapping specs designed to contribute to a stronger feeling of power from third gear up. The 450F’s transmission also gets new gear ratios to give the 450F broader powerband feel in second and third gear.

The 2014 YZF models will feature the same new aluminum bilateral beam frame, which is 14mm narrower than the previous chassis at its widest point and places the steering head 10mm closer to the rider. Although the new exhaust is 7.3 inches longer than the previous pipe design, the new piece wraps the head pipe around the cylinder rather than curving behind it as on the 2013 YZ450F to allow for a more compact fit of the pipe inside the chassis while retaining optimal power. The pipe uses three different diameter sections throughout its length to better tailor the power character on the respective engines.

The 2014 YZs feature more conventional forks compared to the separate function (single coil spring) forks or air spring forks on the outgoing YZs. KYB’s fully adjustable inverted fork retains the Speed-Sensitive System valving system that was found on the 2013 YZ450F, but the 2014 YZ250F and YZ450F get new-spec inner and outer tubes. The aluminum front axle diameter has also been beefed up to 22m, so you won’t be able to swap that aftermarket front hub/wheel from your 2013 YZ to the 2014. Out back, both new models get a new rear shock that uses a vertical reservoir instead of the horizontal unit found on the 2010-2013 YZ450F, mainly just to better negotiate the new pipe placement. Suspension travel is 12.2” up front and 124” in the back.

Another major changes on the 2014s relates to the placement of the gas caps. Due to an all-new layout for the seat, airbox and fuel tank, the unique two-piece seat features a removable front section under which the gas cap is accessed. This new design lowers the height of the fuel tank to improve handling by centering the weight mass even further.

Even with all of the upgrades and changes, the YZ250F will have an MSRP of $7490, which is only $200 more than the 2013 model. The 2014 YZ450F will be offered for the same price as the 2013 model at $8490.

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