Yamaha Yz400f

YAMAHA YZ400F

Wonder Thumper on sale soon

DON’T LOOK NOW, BUT THUMPERS ARE WHERE IT' S at off-road. Yamaha’s got one and it’s as serious as a 90-foot triple-jump. It’s the YZ400F,4 available in dealerships this December.

A full-blown 400cc four-stroke racebike may be shocking news, especially from a conservative Japanese company, but the hint was there in the hand-built form of Doug Henry’s factory YZM400. An experiment triggered by the AMA’s one-year exemption of its production-bike-only rule, the YZM gave Yamaha a shot at testing four-stroke technology in

national-caliber competition-not to mention providing an excellent publicity stunt for the new production YZ-F ’crosser.

It all started about a year ago when Henry was called over to do some testing of a new racebike that Yamaha Japan had up its corporate sleeve. Initially, he was not all that happy about the assignment. “I was just coming back (from a bro-

ken back) and I didn’t need another disadvantage,” says Henry of the experiment. In December, Yamaha went public with the racebike, with little info other than that Henry would race it in the AMA 250cc Motocross Series (which allows four-strokes up to 550cc), and that two other riders, Andrea Bartoloni and Peter Johansson, would compete in the 500cc World Championships. Results came quickly, with wins in Europe in warm-up rounds and then two victories early in the GP season. Stateside, Henry was still learning, but put in the shock ride of the season when he ran away with the final round of the AMA Supercross series in Las Vegas, marking the first time ever that a Thumper had beaten the two-strokes in SX competition.

Now comes news that Yamaha will release a production version. How close will the YZ400F be to Henry’s racebike?

Yamaha’s race team manager, Keith McCarty, keeper of many secrets about the YZM400, says, “There are lots of similarities.” In fact, the F-model shown above is the third prototype, and there still may be some small changes before production. As it sits, the bike is claimed to weigh 240 pounds without fluids. The target weight is 233 pounds, and engineers are looking into adding magnesium sidecovers and eliminating the castings for an automatic-compression release that will not be used. The radiators are longer than a standard YZ250’s to handle the extra heat produced by the Thumper, and the gas tank is a bit smaller as a consequence.

Enduro riders, take note: That 1.8-gallon fuel tank is proof that the new YZ400F is a motocross bike from the get-go. There may be a WR version in the future, but Yamaha is intent on proving this bike on the MX track. Compared to a YZ250 motor, the 400 is the same width and only a bit taller. Its five-valve, dual-cam head was developed with help from the same guys who worked on the YZF sportbikes, and is an extremely compact package. The 95.0 x 60.1mm motor uses a dry-sump oiling system, with most of its 1.5 quarts held inside the frame’s backbone. One change from the works Thumper is the use of a chain to drive the cams-Henry’s bike uses a hybrid gear/chain setup similar to that in Suzuki’s TL1000S sport-Twin. Also, the M’s dual-header pipe arrangement gives way to a singleport pipe on the F.

The transmission is a close-ratio five-speed, and (dualpurpose riders take note) there is a small counterbalancer, too. A two-piece clutch cover makes for easy internal access. Carburetion is via a 35mm Keihin flatslide pumper with a throttle-position sensor hooked into the ignition to

help smooth low-end response. There is a handlebar-mounted manual compression release to ease starting.

The chassis is strictly YZ-no cost-cutting or skimping here. Suspension is straight off the ’98 motocrossers (see Roundup, page 26), as is the front brake and wheel. The rear wheel and swingarm are off the ’97 YZ; due to the YZF’s larger muffler, the ’98 models’ oversized rear brake would hit it ^ on full-bump. There’s even works-like aluminum case protectors and mastercylinder guards.

So, this thing is going to cost a fortune, right? Think again. The suggested retail has been set at $5799-you can thank better dollar-to-yen exchange rates-just $200 more than the ’98-model YZ250.

How will it work? Speculation and the claims of Yamaha test riders are all that we have to go on, but all indicators are positive. When Cycle World jokingly asked Doug Henry if he is now a “four-stroke guy,” he responded, “I’m not sure, but I know I’m a four-stroke believer.”

And so is Yamaha.

Jimmy Lewis