- The design goals for the fuel injected YZ250F are a balance of easy cornering, lightweight and compact feeling thanks to excellent mass centralization. The fuel injected engine features a wide, controllable power band that sets the standard in the 250 class.
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Revised, "reversed" liquid-cooled, 250cc, DOHC, 4-valve (titanium), 4-stroke, single with fuel injection. The engine has been tuned for excellent "ride-ability and controllability" and delivers a broad, easy to use power band with excellent power characteristics.
- Unique engine design features a rearward slanting cylinder that is "reversed" compared to traditional designs. The FI throttle body and air box are front mounted and provide a straight intake path for maximum power output, while the exhaust exits from the rear of the cylinder. The front positioned intake means engine heat will have less "heating effect" on the incoming fuel charge (cooler fuel helps increase power). The benefits of the rearward slanted engine include the straight shot intake, mass centralization which improves both handling and suspension performance and a light weight feeling.
- Compact 4-valve cylinder head features a 13.5:1 compression ratio. The titanium intake valves are 31mm in diameter while the titanium exhaust valves are 25mm, with a 23.75 degree valve angle for a compact combustion chamber design. The 4-valve design offers optimum flow, a higher rpm limit and improved power.
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Lightweight titanium valves (intake & exhaust) and load-reduced oval valve springs reduce reciprocating mass for less friction, faster-revving and smoother-hitting power delivery across the rev range.
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Automatic cam chain tensioner reduces maintenance while special cam chain "sliders" reduce internal frictional losses.
- High performance, light weight, hollow intake and exhaust camshafts enhance this unique engine design and offer broad power especially in the mid to high rpm range.
- The lightweight aluminum cylinder features a ceramic composite coating applied directly to the aluminum block, eliminating the need for a liner or sleeve. The ceramic coating insures faster, more uniform heat dissipation, reduced friction and a longer service life.
- Special cylinder design features cutaways that allow the air "trapped" under the piston to enter a side chamber (or cutaway) to reduce horsepower-robbing "pumping losses."
- The crankcase also utilizes cutaways to reduce "pumping losses". The crankcase cutaways match the cutaways in the cylinder for maximum efficiency.
- Revised crankshaft features a new heat treating process and new molybdenum coated thrust washers on either side of the rod to reduce friction. The balance ratio of the crank has been revised to reduce vibration and improve torque feeling.
- New, lightweight, short-skirt, forged aluminum piston features a single compression ring and a special designed oil control ring. The new, flat top piston features lateral ribs on both sides of the piston pin area. Benefits include a 7% reduction in piston weight for faster engine response. The 2-ring design decreases sliding resistance, reduces weight and improves horsepower.
- Revised lighter weight piston pin features a diamond like coating (DLC) to reduce friction.
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Piston oil cooler utilizes a revised nozzle that directs oil to the desired portion of the cylinder and underside of the new piston for great durability. The revised nozzle reduces piston temperatures by up to 10 C.
- Revised, gear driven balancer shaft utilizes a new balance ratio for reduced vibration and improved torque feeling. Less vibration means less rider fatigue during a long moto.
- The crankcase features a tri-axis layout for the crank, main and drive axles. This tri-axis design keeps the crankcase more compact while also centralizing mass for excellent handling.
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Revised, 5-speed transmission features new engagement dog shape on selected gears for improved shifting. The transmission ratios are designed to match engine output to reduce "driving force" losses.
- The YZ250F features a 2-bar shift guide system with a 14mm shift shaft for excellent durability. Revisions to the shift stopper lever include changes to the roller shape, spring load and support design for smoother, more accurate shifting.
- Revised, multi-plate wet clutch features a new surface finishing process that increases the flatness of the clutch boss contact area for improved engagement feeling.
- Quick-access, lightweight magnesium outer clutch cover for fast, easy clutch maintenance.
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44mm Keihin throttle body fuel injection system is a battery-less design. The 10-hole injector and 16 degree spray pattern maximizes fuel atomization for more complete combustion and excellent power delivery. The benefits of fuel injection include constant ongoing adjustments to insure the optimum fuel - air ratio regardless of outside air temperature and altitude. Other benefits include improved starting and improved throttle response. There is a cold start knob which also doubles as an idle adjuster too.
- The injector spray pattern, spray direction and the distance to the intake valves have been optimized, while the specially designed straight intake tract helps to maximize the benefits of the FI system and overall power output.
- Optional, compact, hand held "YZ Power Tuner" allows tailoring of both fuel injection duration and ignition mapping. Nine (9) different setting points can be accessed for both the FI map and the ignition map. Maps can be stored and shared with other Power Tuners if desired.
- Forward mounted air box features a flat style, washable, dual-stage, foam air filter which is easily accessible without removing the tank or seat. Quick release, no tools required, fasteners on the air box cover for faster servicing.
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Wet sump lubrication system reduces weight, allows for more compact engine cases and eases maintenance.
- Oil volume in the engine has been optimized to reduce weight and oil drag related power losses.
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Maintenance-free transistor-controlled digital ignition (TCI) ensures fast starts and great performance at all rpms.
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High capacity ACM (alternating current magneto) is used to eliminate the need for a battery.
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Automatic decompression system ensures easy starts in any gear.
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Wrap-around exhaust system exits from the rear of the engine and circles around the cylinder body before turning to the rear of the bike. This design ensures the optimum tuned header length, maximizes the exhaust "pulse effect" and centralizes mass by moving weight closer to the centre of the bike.
- Revised muffler bracket reduces weight. The re-packable lightweight aluminum / stainless steel muffler (aka silencer) meets the current 94dB AMA and CMRC sound regulations.
- Compact, lightweight, single core, dual radiators provide outstanding cooling efficiency.
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New ECU settings for both ignition timing and the FI system settings.
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Special "G" sensor system will shut down the fuel injection system if the bike lies on its side for more than 10 seconds, to help prevent costly engine damage. There is no special procedure to re-start the bike... just kick it and go.
- Works-style race-spec "barrel adjuster" for on-the-fly clutch lever free play adjustments.
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Easy access element style oil filter.
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