Yamaha’s 2014 four-stroke Motocross range has broken cover in the US with an array of updates to both the flagship YZ450F and quarter-litre YZ250F.

The YZ450F features a completely redesigned engine. Engineers created this all-new powerhouse to produce more power while remaining controllable.

The engine features a new the piston which improves combustion and increases torque in the low to mid range of the powerband. A two-ring design and updated surface coating also reduces friction in the cylinder and improves throttle response.

The intake system has been redesigned and now uses a traditional-style filter housed within a larger box. The volume of the air boot has been increased and reduces the air intake noise in its new location.

All-new fuel mapping and engine timing settings have been applied to suit the increased airflow that the redesigned intake system provides.

The all-new 2014 YZ450F.

The diameter of the intake and exhaust ports have been increased (1mm on the intake valve and 0.5mm on the exhaust valve) and the shapes of both have been changed.

Within the five-speed gearbox, the gap between second and third gear is closer and the gap between third and fourth is wider. A new shift lever and larger shift shaft diameter allow the rider to click through the gears with less resistance.

To handle the increased power output, the 2014 YZ450F clutch has been built 20 percent stronger than the previous model. The pressure of the clutch springs have been increased and the friction plate material has been upgraded.

A wrap-around exhaust system has been used change on the 2014 YZ450F. This concept was used by the JGRMX/Toyota/Yamaha team and FMF Racing at the recommendation of the Japanese OEM during the 2013 season.

The silencer has three different diameters on the inside for the best overall power. It also concentrates more weight to the front end so for improved handling and a ligher more responsive feel.

Aside from the all-new look, the 2014 YZ450F features updates throughout.

The engine’s lubrication system has changed from dry sump to wet sump and the engine cases have been compact. This allows the oil capacity to be reduced to 0.95 litres.

Centralizing the mass was the focus of Yamaha engineers and they have designed an all-new chassis for the 2014 YZ450F, coupled with completely new body work aside from the front fender.

A two-piece seat and the fuel tank’s placement allows the rider to move far forward without interference. Regular maintenance is a much simpler task, thanks to the revised airbox and fuel tank; both come apart and move out of the way with fewer tools.

The total number of body pieces has been reduced to eight due to changes on the shrouds, side panels, and rear fender. The graphics are now embedded into the radiator shroud plastic.

New steering tube, tension arm, and engine cradle, and the thickness of the tank rails has been changed on the frame. The material of the engine brackets depends on their position – the top brackets are steel while the bottom are aluminum.

The all-new 2014 Yamaha YZ250F.

Suspension wise, there is a change to the rigidity of the outer fork tube and the surface finish of the inner tube, the layout of the rear shock to accommodate the fuel tank and exhaust, and both have revised settings to absorb impacts.

A four position handlebar mount is now rubber mounted into the clamp and a more rigid top clamp holds the forks in place. The diameter of the front axle has also been increased from 20mm to 22mm.

The new subframe and fuel tank drastically reduce the amount weight spread over the surface of the bike. The tank’s capacity has also been increased by a full litre and will now hold 7.5 litres.

Much the same as the 450, the 2014 YZ250F receives a range of updates and a completely new look. The biggest change being the introduction of electronic fuel injection and reverse engine design.

Featuring the innovative rearward-leaning cylinder that features the intake in front and exhaust in back, the 2014 Yamaha YZ250F has become a slightly smaller version of its big brother.

Internally, the new YZ250F uses the same bore and stroke dimensions (77.0 x 53.6mm) as in previous years, but its new twin-cam head has four titanium valves in place of five.

The 2014 YZ250F features an all-new look.

Much the same as the YZ450F, the exhaust of the YZ250F wraps around the engine from behind, optimizing mass centralization and helping achieve an ideal header length.

The all-new fuel-injection system features a 44mm throttle body with a high-pressure battery-less electric pump. The new underseat tank design now allows 7.5 litres of fuel, an increase of around 1.5 litres compared to the previous model.

An all-new bilateral-beam frame has been added to the YZ250F, while KYB’s air-oil separated, speed-sensitive fork and KYB shock have been used.

The YZ250F features the same air-filter design as the YZ450F, making access to the foam element much easier, while its forward location keeps it hidden away from dust and roost on track.

Both models will be available in either blue or white colour schemes. Stay tuned for further information on an Australian release date and pricing when available.