First Test: 2005 Yamaha YZ250F

The changes to the inner carburetor... 

   

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The changes to the inner carburetor boot allow the motor to run without throttle hesitation.

It has been just four years since Yamaha stunned the world with the debut of the now-legendary YZ250F. In that time, the Yamaha crew has gradually fine-tuned the little thumper, but with the release of the new, jaw-dropping '05 YZ125 and 250 two-strokes, many wondered if the tuning-fork engineers had made any kind of effort with their four-stroke program. The care the 250F received may not be obvious, but don't be fooled: The YZ250F's competition has left Yamaha no choice but to up the ante.

LOTS OF MEAT

For '05, "the little engine that could" received changes to the cylinder-head intake port and combustion chamber, plus a new piston shape. This translates into usable power that really connects to the rear tire and grabs traction. The bottom-end requires a bit of clutch to get the revs up, but the midrange hits hard and carries over to strong top-end.

Overall, throttle response was nearly perfect as the Yamie ran cleanly even when bouncing off the rev-limiter. The initial throttle response is cleaner compared with that of the '04, especially in critical timing or rhythm sections. The secret is a new inner carburetor boot using a built-in velocity stack. The boot still maintains two separate air cavities that force the flow into the air jets in the back of the carburetor.

Both Perris Raceway and Staben Ranch's supercross track highlighted the quick and clean throttle response. We ran the stock jetting as we were testing near sea level. All the gears pulled long—the strong point of a four-stroke engine—and the clutch took abuse well.

WHAT BUMPS?

Confounding expectations, there is no aluminum frame this year. The frame on the 250F is basically the same as the '04 model's with the major improvements coming in the suspension department. The 250F received a much needed all-new Kayaba fork. It resembles a factory Yamaha works fork both externally and internally; the Air/Oil Separate System provided superior damping control by eliminating any air bubbles that could cause the oil to foam or permit cavitation. This system also allows the fork to be more speed-sensitive throughout the stroke. Initially, the stroke feels a bit softer, especially in tight corners, where the front end really settles. Much more progressive, the fork has major bottoming resistance, which was missing last year. Even when running stock settings, the fork worked flawlessly through braking and acceleration bumps. During testing at Staben Ranch, we experienced a few problems with the front tire bouncing off the ground under hard landings. We went in two clicks on the rebound to slow it up. Just that slight bit of adjustment made a huge difference, and with an otherwise-stock setting, the YZ-F took tons of abuse.

Tester Matt Armstrong flew... 

   

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Tester Matt Armstrong flew around Perris Raceway till the sun went down.

The shock received very few updates, with the main focus on the top-out of the rebound stroke. Its great feel is super-progressive and complements the fork well. The shock soaked up big, square-edged bumps and responded to minor clicker adjustments. At Perris, the stock shock settings suited all of our riders. On our second day of testing, we went in three clicks on the low-speed compression to help soak up the bigger supercross hits. We did experience a bit of front-end push on some dry, flat, hard-packed corners, so we ran the sag at around 98mm but with poor results, finally finding we were most comfortable with the bike set at 104 to 108mm of sag. To improve overall shock performance, the Yamaha crew made some major changes to the swingarm, shedding almost a full pound off the '04's. This hydro-formed swingarm now uses a double taper system for weight savings and strength.

TOP OF THE PODIUM

All of our test riders felt at home on the bike after just a few laps. The ergonomics feel the same as on the 2004 bike, only enhanced by the new Renthal handlebar. The brakes work great, with the front displaying strong stopping power and no sign of fading. Overall, the entire package is built with quality hardware, and the only shortfall is the plastic. Yamaha claims the new plastic doesn't get the white creases as easily when it hits the ground, but after a light crash, we noticed a big white mark on the front fender—not huge but still annoying. As for the MIA aluminum frame on the '05 YZ250F, we're not too bothered because the fork is as important an ingredient to this package as any frame material. Maybe it will show up next year, maybe not. But don't bet too heavily against the bike that changed small-bore motocross!

Opinions

The new Kayaba suspension... 

   

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The new Kayaba suspension setup works great, offering a supple yet progressive feel.

All I can say is, "Fork it over!" Man, what a difference the new front end makes. Even though the 250F has been my favorite YZ since its introduction, I've never been a big fan of its suspension. I held Yamaha test rider Doug Dubach personally responsible for the suspension; his goofy yet productive riding style seemed unaffected by the initially stiff and nonprogressive nature of the setup. And the bottoming was too much for my style (read: bad jump timing), especially in the front. Now the front works as well as that of any bike out there, giving me trust in the front wheel everywhere, especially in chop going in and coming out of turns. The plusher initial stroke is felt through both ends, giving me more traction and less of a beater ride. As for the motor, though still new and tight when I rode it, there was some meat to this 250cc four-stroke, more so than with any of the others I've ridden. It has an exciting feel that gets better with abundant traction.
Jimmy Lewis/5'10"/180 lb/Vet Pro