Off road racing and Yamaha have been – and always will be – inextricably linked and their bikes are in great demand in the Arab world where off road racing in the desert over the weekends is a passion for many. Yamaha riders have underscored the technical supremacy of the company’s ‘reverse head’ YZ-F models by winning the MXGP World Championship, as well as the prestigious MXGP Manufacturer’s Championship in the recent past. An innovative and forward thinking approach has made Yamaha a dynamic and successful brand in the off road category and for 2017, the comprehensive range of motocross and endurance bikes are now within the reach of every off road rider.
For the 2017 season, Yamaha Motor has introduce the new off road competition line-up with the higher performance 2017 YZ250F, and detail changes to the other YZ-F, YZ and WR-F models. The stand out model is the YZ250F featuring an innovative reverse cylinder head engine. This groundbreaking bike has demonstrated it has got what it takes to outperform the toughest opposition. The motocross bike powered Jeremy Martin to consecutive AMA 250 Motocross Championships in 2014 and 2015, while YZ250F rider Cooper Webb has also taken consecutive AMA 250SX West titles in 2015 and 2016.
For the 2017 model year the YZ250F benefits from a range of important technical enhancements including an all-new reverse cylinder head – that are aimed at delivering increased performance and enhanced ride-ability. Other key improvements include an increased pulling power in the mid to high rpm range, improved acceleration, slicker gear shifting and smoother clutch feeling, refined handling and suspension giving enhanced stability and control Improved braking performance. With major enhancements to the intake and exhaust efficiency, together with a slicker shifting transmission and remapped ECU, the 2017-specification engine delivers a significant increase in race- winning performance.
With its front facing inlet and rear exiting exhaust, the YZ250F’s innovative reverse cylinder head offers many advantages over a conventional design. These include increased intake and exhaust efficiency, as well as enhanced mass centralization thanks to the use of a top-mounted air filter and under-seat fuel tank. Yamaha’s engineers have taken this cutting-edge technology to the next level, and developed a new reverse cylinder head that gives the YZ250F rider even stronger mid to high rpm pulling power.
For an increased flow of fresh air into the 4-valve head, the intake port’s position is raised, and the intake valve diameter is increased. New higher lift intake and exhaust camshafts give increased valve lift for a stronger feeling in the mid to high rpm range – while for enhanced low to mid-range performance the intake camshaft’s working angle has been narrowed. Stronger intake and exhaust valve springs are fitted to correspond with the use of new higher lift camshafts – and to accommodate the increased engine performance, a newly designed nickel- chromium-molybdenum steel con rod is used for the 2017 model.
Further significant power increases in the mid to high-rpm range have been achieved by shortening the intake funnel to achieve reduced pumping losses – and the use of a larger throttle joint between the injector and intake port gives an increased cross-sectional area of around 5% – while its new internal design promotes better fuel vaporization.
To achieve an optimal intake/exhaust balance, the twin exhaust ports have been redesigned, and the snaking exhaust header pipe is equipped with a new larger diameter front section. As well as featuring a bigger bore pipe, this front section also benefits from larger radius bends, giving a greater internal cross sectional area that creates a much stronger exhaust pulse effect.
To match the increased engine performance produced by the redesigned cylinder head and high efficiency intake and exhaust systems, the 2017 YZ250F is equipped with a newly developed ECU with revised ignition timing and fuel injector mapping. The new ECU cuts in at higher rpm, and features a rev limiter that gives an improved over-rev feeling.
Riders of all abilities can also use the optional Yamaha Power Tuner to adjust the YZ250F’s power characteristics. This compact device plugs straight into the bike without the need for a PC, and enables the rider or mechanic to fine-tune the engine performance in seconds to suit the track conditions.
In order to handle the uprated engine’s increased performance, the YZ250F is equipped with modified high-strength crankcases. While their exterior shape and construction is identical to the 2016 model, the aluminum die-cast cases on the 2017 model have been subjected to a heat treatment process that increases strength by around 5% for a high level of reliability and durability.
To match the higher-powered engine’s increased driving force, the 2017 YZ250F is equipped with a new transmission featuring wider teeth on the 2nd and 3rd drive gears. The shape of the dogs on 1st, 3rd and 5th gears has also been modified, and these changes – together with a redesigned shift cam and modifications to the clutch push lever and axle design – give smoother and quicker gear shifting. Finally, a more rigid gear shift lever gives a more positive shift feeling.
Made possible by the use of the reverse head engine, the YZ250F’s top-mounted air-box and under-seat fuel tank help to centralize mass for agile and responsive handling. The chassis changes introduced for 2017 include a revised balance of frame rigidity, together with a lower center of gravity and refined suspension.
