2010 Yamaha YZ250F Revealed

New bike season is officially in full swing with the release of the 2010 Yamaha YZ250F. We're actually testing the new Yamaha YZ250F as you read this so check back for our first riding impression later today.

While you patiently wait for the story on how the new Yamaha YZ250F runs and works on the track, take a look at what it's made of with Dirt Rider's first look.

Let's look at the new Yamaha YZ250F motor first.

While we were expecting (hoping) for fuel injection, the 2010 YZ250F comes with a carb. Not much to complain about since the Yamaha YZ250F has been one of, if not the best carbureted bike in this class. Instead of electronics, this motor gets better breathing, lighter valve-train components, more efficient combustion and a new exhaust system starting at the cylinder head porting.

Bottom end improvements range from new gear ratios and strengthening as well as clutch leverage and spring rate changes.

Interestingly, the power improvements across the board are designed, and this is a direct quote, to, "Improve power while reducing noise."

Here's the new Yamaha YZ250F motor in detail.

-New Airbox shape
This is designed to straighten and smooth the intact tract from air filter to carb. The smoother and straighter the path from air to fire, the better-especially for fuel injection (which we expect to see next year on the YZ250F. The entire air filter is shifted to the left 10mm.

-New Carburetor with new Bore Ventruri Shape and new settings
Continuing the straight-shot air management, Yamaha designed an all-new carb body. This eliminates the step in the old intake bell. The accelerator pump cam is changed for quicker fuel delivery. Also, the jetting is changed to the richer side of things. Yamaha is going for a good "torquey" feel at low RPM while delivery the best response possible under quick openings.

2009 Yamaha YZ250F Jetting specs:
Main: 178
Pilot: 42
Needle: NHKR-4
Leak: 70
Fuel Screw (turns out): 2

2010 Yamaha YZ250F Jetting Specs:
Main: 180
Pilot: 45
Needle: NDJR-4
Leak: 70
Fuel Screw (turns out): 2.25

-New Cam Profile
The YZ250F's intake cam profile is changed for better low end to mid-range power and response. The Yamaha YZ250F took a little beating in the past from the press for being a bit sleepy on the bottom. Maybe this will silent that. The cam now has 7.7mm of lift versus the 7.6mm from last year while maintaining the same cam timing. That means the valves are opening a bit more but opening at the same time-basically. There is a hard surface treatment applied to the very top of the valve stems, too. Yamaha's proven YZ250F valve durability should see an improvement with the fancy Chromium Nitride coating.

-Valve Spring Retainer Changed From Steel to Aluminum
Lighter is better and with the reduced load on the spring thanks to the Aluminum retainer, the spring rate can also be lightened. The intake spring load is reduced 12% and the Exhaust spring load is reduced 9%. Reduced friction and better low-end to mid-range power and response are the goals.

-Exhaust Port Shape Is Changed and Volume is Reduced
Yamaha wants its exhaust gases to have more velocity so it shrunk the exhaust port into a D-shape. Guess why. Yep, for better throttle response.

-New Exhaust System
Yamaha went back to (after going away from) a two-step mid-pipe for 2010. It steps up from 45 mm to 50.8 mm right after it attaches to the frame. Further back the muffler grows 50mm and gets a reduced core diameter. Inside, the "pipe perforation pitch" (meaning the amount of holes, really) changed from 4-5mm. That means the less holes and they are farther apart. The muffler mount changed from rubber to solid metal-on-metal. All this to reduce sound without reducing power from the new Yamaha YZ250F.

-Clutch Mods
Internally, the 2010 Yamaha YZ250F clutch basket center boss gets hardened material for more durability and the clutch arm ratio and clutch spring load are both increased. The result? Easier lever pull (3% easier) with stronger engagement and wider engagement area feeling. That means less herky-jerky clutch on-off action.

-Transmission updates
Gear strength is increased in 3rd, 4th and 5th gears with 3rd and 4th getting a slightly taller ratio. That's like a half tooth difference on a rear sprocket. There is also some revision to the drive dog shape. Stronger, better engagement is what Yamaha is looking for here.

-Ignition
Ignition mapping is changed to match all the intake and exhaust specifications for the 2010 Yamaha YZ250F.

-New Compact Oil Tank
While total oil capacity in the 2010 Yamaha YZ250F is reduced to 1200ccs (down 100 from last year), the biggest change you'll see is with the smaller oil tank. Keen eyes will also catch the new position of the oil level sight glass that jumped from the crankcase to the ignition cover. The oil now drains from the back of the engine instead of the side, too.

-New Radiators and mounting/hose routing
The 2010 Yamaha YZ250F radiators mount 15mm lower and 10mm farther back than 2009. Also, the hose routing changes and the mounting points increase from two to three. This makes stronger radiators without the external bracing we've seen in the past.

That's the 2010 Yamaha YZ250F motor in full detail. Here's what's new with the 2010 chassis.

-All new 2010 Yamaha YZ250f Frame
Yamaha stays unique with a "Bilateral Beam Frame". Unlike everyone except Europe's twin-spar full perimeter frame, Yamaha tucks their beams (spars in) to follow more closely in line with their previous offerings. The frame uses a 20-piece combination of forged and extruded components. Lateral and torsional rigidity is increased to transfer dieways shock into longitudinal shock smoothly. The idea is to take twisting and bending forces coming from side to side to flow into the straight-line or vertical line of the chassis so the suspension can deal with it. After all, that's the suspension's job.

-Frame Head Pipe Position
The frame head pipe is 12mm lower and 7mm further back than 2009, changing the caster and trail dimensions. The 2010 Yamaha YZ250F has a more compact chassis putting the rider closer to the front wheel for lightweight handling and quick direction change. The YZ four strokes don't win turning competitions in our shootouts, but maybe now they will.

Caster:
2009: 27.2 mm
2010: 27.5 mm

Trail
2009: 117mm
2010: 120mm

Continue to page 2 for more on the 2010 YZ 250...