2014 Yamaha YZ250F and YZ450F

The 2014 Yamaha YZ250F and YZ450F four-stroke machines have broken cover, with the 250 now getting the same engine configuration as the 450 -- one which places the throttle body and airbox in front of the engine, with a straight intake into the combustion chamber. The rear-inclined engine also helps to centralise mass.

Yamaha introduced this design on the YZ450F three years ago, so it’s taken the YZ250F a while to catch up. Now that it has, Yamaha is claiming more power in the mid to high rpm bracket, with a lot of that also down to the adoption of fuel injection. The bike has a 44mm throttle body to replace the 39kmm carburetor. Bore and stroke are 77.0mm x 53.6mm, and the compression ratio is 13.5:1. Other engine changes include a forged piston, a new ECU, an updated transmission, strengthened clutch, and the exhaust is now closer to the bike’s centre of gravity.

Yamaha hasn’t gone to such great lengths to alter the fabric of the Z450F, but it still has a new aluminium bilateral beam frame -- also shared by the YZ250F. The engine changes include a revise piston, intake and exhausts valves and ports, a new ECU, and the fuel injection mapping. The end result is improved combustion and improved performance across the whole powerband. The transmission has also been redesigned, and includes revised gear ratios.

Both bikes share KYB suspension, with new inner and outer tubes at the front and a new shock absorber. The seat on the pair is a two-piece design, with a removable front tab to access the fuel tank. Hence, there’s no visible fuel tank cap.

Yamaha Motor Australia has confirmed that the YZ250F will arrive here in August, with the YZ450F following in November. Pricing is yet to be announced.