Yamaha Yz400f
YAMAHA YZ400F
CYCLE WORLD TEST
A Thumper like no other
PUT POLITELY, YAMAHA’S NEW-FOR-’98 YZ400F motocrosser has dropped a bomb, opened a can of worms, stirred the pot. Put impolitely, it has grabbed its competitors’ trousers, yanked them down to the floor and kicked some bare-naked motocross butt. And it’s done so in a way that we are only just beginning to understand.
The YZ400F project began in the summer of 1996. With one eye on forthcoming off-road emissions constraints, and the other on competing head-to-head with 250cc motocrossers, Yamaha’s racing division hatched the idea of a four-stroke 400cc motocrosser. Eighteen months later, we’re riding the result.
»Though the YZ400F was R&D’d in parallel with the YZM400 factory racebike, the two were developed to a different set of guidelines. Most significant difference was that the factory bike was fermented in typical spare-no-expense fashion, with a goal of beating the world’s best motocrossers. It did, too, winning a few MX GP motos and a U.S. Supercross. For sure, the YZM’s success piqued the public’s interest, which came to a head with the surprise introduction of the YZ400F as a 1998 production bike.
More-traditional camdrive and exhaust-port arrangements, coupled with the ever-present weight concerns, however, have prompted skeptics to question the “Average Joe’s” version of the 400. Predictably, weight climbed as the bike progressed from pre-production to production form, and delayed press introductions brought further questions. But a peek inside the YZ400F’s motor shows just how serious Yamaha is about its Thumper. The slipper-type piston is barely tall enough to hold its rings in place. The hemispherical five-valve head is yards ahead of anything found on rival four-stroke dirtbikes. The five-speed transmission features involute-splined gear shafts and pinned shift forks (and three of those, instead of the usual two). Electronics tie the three-dimensionally mapped ignition to the carburetor, with a throttle-position sensor helping to determine spark advance in accordance with engine load. All this is crammed into a motor that is remarkably close in size to the two-stroke YZ250. The aftermarket will pull out plenty of hair looking for ways to improve this Thumper. Suspension-wise, the YZ400 is as well set-up as the highly praised YZ125. The burly spring rates (especially in the rear) are the result not only of the four-stroke’s extra weight, but also of the massive amounts of torque the engine generates. But don’t fear, the YZF’s ride is plusher than any of the twostroke YZs. Overall, the standard settings should keep riders of almost all weights and abilities happy. Only our fastest testers chose the optional stiffer, .46-kg. fork springs. Even more impressive, the YZF’s suspension works well over a wide range of settings, and it isn’t hyper-sensitive like some other motocrossers. For example, going to a jump-infested > supercross track from a natural-terrain outdoor MX course only called for a few extra clicks of compression on the fork. moto-ing before the top end needs opening. Oil changes are a simple 10-minute job, but accessing the filter requires loosening the header pipe. We ran Yamaha 4R, a 10w-50 synthetic/petroleum blend, and changed it every couple of rides. And exotic fuels need not apply: The YZ400 loves 92octane premium pump gas.
The YZ400’s chassis mirrors the YZ250’s as closely as possible, considering the requisite differences in carburetor placement and the necessary plumbing for the four-stroke’s dry-sump lubrication system, which employs the frame as an oil tank. Chassis geometry, though, is identical, as is the bar-seat-peg relationship. If anything, the 400 feels slimmer, with room to move farther forward on the seat. Front and rear suspension are the latest KYB components used on the YZ250, differing only in spring rate and valving. The front fork has the new elastomer bottoming device, a .45-kg. spring, and compression and rebound damping adjustment on each leg. The shock comes standard with a stout, 5.4-kg. spring, and is adjustable for spring preload, compression and rebound damping.
Starting the engine is cut and dry: Find top dead center, use the manual compression release to squeak past TDC, and then give the kickstarter a healthy boot with no throttle. As the YZF has no automatic decompression device (such as those used on Honda XRs), impatient riders will fumble. But most will find the YZF a first-kick starter, hot or cold. Its Keihin 39mm pumper carb is jetted spot-on, the slightest twist of the throttle producing instantaneous revs and an authoritative bark through the muffler. The sound alone lets you know how fiery and quick-revving the 400 is.
Even idling out onto the MX track, it’s apparent that the YZF possesses characteristic four-stroke torque. Crack the throttle and the bike lurches forward like a twostroke-except that the rear tire doesn’t break traction. Even on slippery, sun-baked Carlsbad adobe, the YZF feels as though it’s clawing through tacky Unadilla loam. And that’s just the beginning: Roll into the throttle and the YZF pulls harder than most 600cc four-strokes. Let it rev out and you’d better hang on, ’cause until you hit the 11,000-rpm rev limiter, you’re in for the ride of your life! Bang another gear and the engine pitch barely changes, there’s so much power on tap. Above half-throttle, you can barely shift quickly enough-but then, you really don’t need to. The spread of power over the entire rev range is so good that gear selection is the least of your concerns. And when shifting does matter, like when you’re dragracing down the start straight, full-throttle shifts are easy. This is one of the bestshifting, most solid-feeling transmissions ever.
And what about the chassis? The original target of 233 pounds dry has gone out the window, and the bike now sits at 250 pounds dry, or 262 pounds with a full tank, ready to moto. But in actuality, the weight isn’t a hindrance unless the track is extremely slippery. The mass is well-centralized; even when jumping, the 400 feels no more massive than a 250. The bike’s weight is most noticeable when you’re lifting it back onto a stand after a moto.
Supercross? Yes. Flying overjumps and skimming across whoops may be the arena in which this Thumper shines the most. The engine’s controllability makes it super-easy to time double-jumps; even our Novice testers routinely cleared doubles that they wouldn’t have attempted on their own bikes.
As expected, the 400 is a bit different on takeoff, due to the four-stroke engine’s inherent compression braking. As a result, you need to keep the throttle on a bit longer, or risk having the front end plummet in mid-air. Even so, it took no more than a handful of laps for most of our testers to figure this out.
In the turning department, the YZF holds an unfair advantage. Again with just a bit of familiarization, riders were able to use the compression braking to set themselves up for turns. It gives the rider more time to think: Instead of constantly dogging the rear brake, he’s free to get set up for the upcoming tum. And if you can’t come to terms with the compression braking, just turn the idle up slightly.
It all boils down to this: The YZ400 is easy to ride. Moreover, it is easy to ride at a level that most of our testers would be hard-pressed to duplicate on a 250cc two-stroke. Not that anyone was instantly faster on the Thumper, just that most were amazed at how much control they had over the machine. And at how much more potential there was hinging on a further twist of the wrist.
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Our testers also unanimously noted that they felt slow aboard the YZF-one even laid so’ after rünning in the top
three of a local 250cc Pro moto! Is the YZF less tiring to ride? Initially, no. The majority of our riders’ arms pumped-up during the first day of testing on the 400. We’re not sure why-blame it on unfamiliarity and the necessary adaptation to a slightly different technique-but once they were used to the YZF, they found long motos less tiring. For example, our Expert-level tester’s hands usually sport signs of blisters following a long, 45-minute moto at ultra-rough Carlsbad, and the bike more often than not suffers bent rims. But with the YZ400F, neither transpired.
On four different tracks, from natural-terrain to supercross/ arenacross, deep loam to baked hardpack, sea level to 4000 feet, there wasn’t an area where the YZ400F faltered. In fact, if you pushed us to find complaints, we’d find very few. You can get the front end to push a bit in hard, dry turns if you’re not careful, but then the condition of the front tire is usually the culprit. The YZF is going to boost tire sales, as it quickly scrubs knobs, particularly up front. And if you stall it in a crash, you can’t jump right up and kick it; you must go through the entire starting procedure, or it’s kick ’n’ pant time.
Given the YZF’s ultra-high-performance, there’s got to be a tradeoff in durability, right? Nope. It is, after all, a stock fourstroke. If you keep up on oil and filter changes, the most likely parts to need replacement will be the clutch plates and piston rings. Even the wildly conservative owner’s manual claims five races (or 600 miles) for the rings. Translated to the real world, we’d estimate a half-season of every-weekend >
If you’re more than a little shocked that a “Thump-acrosser” can be so good, get in line-you’re far from alone. And if you’re even more surprised that it was Yamaha who built it, then let us remind you of monoshocks, boost bottles, powervalves, brake-activated suspension systems and liquid-cooling-all Yamaha firsts on production dirtbikes.
The YZ400F is in a class of its own; it relegates pretty much every other off-road Thumper to farm-implement status. The YZF’s level of refinement makes even the trickest Euro-Thumpers feel like unfinished kitbikes. And all this at a price that’s about half what it would cost to make a Honda XR400 motocross competitive.
Yamaha’s YZ400F has to be considered a resounding success. The company gambled on an unproven concept, rushed the result into production, and in the process opened a new chapter in motocross history. Or perhaps re-opened a long-forgotten chapter.
We used to be scared of a four-stroke motocross future. Not anymore. □
YAMAHA YZ400F
$5899
