There are so many new items on the 2014 Yamaha YZ250F and 450F that it’s easier to just get the carryover items out of the way first. The only pieces that Yamaha has deemed worthy of its new four-stroke motocrossers are the brakes, rear wheel, the front number plate and the front fender. That should give you a good idea of the lengths Yamaha has gone to revamp its YZ boomers. The factory has left no stone unturned in an attempt to make the YZ250F and 450F its most-powerful and best-handling motocrossers ever.
The YZ250F needed the most help, and it gets it in the form of a new, fuel-injected DOHC engine with the same-style forward-positioned cylinder head and rear-inclined cylinder configuration that Yamaha introduced on the YZ450F three years ago. The design reverses the intake and exhaust ports and places the YZ’s new EFI throttle body and airbox in front of the engine, and symmetrical intake ports give the intake charge the straightest possible shot into the combustion chamber for easier power production. The cylinder is also inclined toward the rear of the chassis, which not only centralizes the engine’s mass more effectively but also offsets the connecting rod so that it is more 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. As part of the redesign, the YZ250F the new head also loses one intake valve, going from five valves to four larger ones—all made of titanium—shaving weight and reducing complexity at the same time.
While the YZ250F uses the same 77.0 x 53.6mm bore and stroke and 13.5:1 compression ratio as the ’13, Yamaha claims that the new engine makes a lot more power, particularly in the mid-to-high rpm range. Additional improvements include a lightweight, forged two-ring piston that is cooled by an oil spray jet for better combustion heat management, new cam profiles, a new ECU, a new carburized crankshaft designed for better durability, a newly designed transmission and stronger clutch.
The fuel-injection system alone is a welcome change. A massive 44mm throttle body and 12-hole injector replace the 39mm carburetor found on the 2013 250F. It’s fed by a battery-less high-pressure electric pump that relies on 3D-mapping from the ECU to atomize fuel more efficiently over a wider variety of atmospheric, temperature and track conditions than a carburetor. The goal is to improve throttle response and control compared to the 2013 model.
The YZ450F isn’t left out in the cold with regard to engine improvements. These changes are meant not only to increase power and centralize mass but also to allow Yamaha to share a new aluminum bilateral beam frame design between the 250F and the 450F—the only difference being the engine mounting points for the respective engines. In order to shoehorn the massive 450 motor into the space normally occupied by a 250cc engine, Yamaha engineers eliminated the 450F’s dry sump oiling system and external tank in favor of a more compact wet sump system that shrinks the size of the 450’s crankcase.
Performance-improving revisions to the 2014 YZ450F include a revised piston, intake and exhaust valves, revised porting, and a new valve mechanism, ignition timing and fuel-injection mapping. All are said to improve combustion efficiency and make more power, while the ignition and fuel mapping changes are also intended to create smoother power delivery from idle to redline. A new ECU also helps, with timing and mapping specs designed to contribute to a stronger feeling of power from third gear through fifth gear. The 450F gets new gear ratios devised to give the 450F broader powerband feel in second and third gear.
Up top, the 450F’s enlarged titanium Intake valves are actuated by an intake cam with a new profile designed to enhance engine performance. In transmission department, the YZ450F also receives a redesigned transmission and shift mechanisms intended to provide quicker, smoother shifting under power with less effort.
Both 2014 YZF models 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. A new exhaust pipe design wraps the head pipe around the cylinder rather than curving behind it as on the 2013 YZ450F. The design allows for a more compact fit of the pipe inside the chassis while retaining the optimum overall length for power, and it also allows the silencer to be positioned 186mm further forward behind the side panel and lower and closer to the middle of the chassis for better mass centralization for more responsive handling. The pipe also uses three different diameter sections throughout its length to better tailor the power character on both the 250F and 450F.
While its competition, such as Honda and Kawasaki, has begun to push the envelope with more radical technology in the form of separate function (single coil spring) forks or air spring forks, it’s nice to see that the 2014 YZs feature decidedly more conventional forks. 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. One other item of note on the front end is that the aluminum front axle diameter has been beefed up to 22mm, so previous YZ owners were hoping to swap their trick aftermarket front hub/wheel from your 2013 YZ to the 2014 won’t be able to. 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” out back.
Finally, take a good look at the fuel tank on the 2014 YZs and you might notice that something is missing—as in, the gas cap. That’s because both models feature an all-new layout for the seat, airbox and fuel tank. The new models sport a unique two-piece seat features a removable front section under which the gas cap is accessed. The design lowers the height of the fuel tank all in the name of, you you guessed it, mass centralization with the goal of better handling. It certainly contributes to the sleek new look of both machines.
But perhaps the best news for prospective buyers is that all is that with all of this trickery the YZ250F’s MSRP of $7490 is only $200 more than the 2013 model, while the 2014 YZ450F’s is unchanged at $8490. Time will tell if the new technology pans out, the podium could be clear blue.
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