Roost Revival
ROOST REVIVAL
THUMPERS COME BACK BIG
JIMMY LEWIS
IT’S TIME TO TAKE FOUR-STROKE MOTOCROSSERS SERIOUSLY. CASE IN point: Yamaha is racing and winning with its works YZM Thumper this year, here and around the world. Other people are following the same train of thought. White Brothers and Kawasaki’s Team Green, for example, are dead-serious about Thumping around a motocross track.
Cycle World recently sampled both outfits’ bikes, special-built racers meant for the AMA Western Four-Stroke Motocross Series. Both have the same goal: to make a serious MXer out of a less-than-600cc trailbike.
According to White Brothers’ main R&D man, Gary Jones, Honda’s XR400 is a better base to start from than the XR600. Its lighter weight and more-rigid chassis overshadow the extra power of the big XR. The WB formula is simple: Throw its catalog at a project bike and fit everything that sticks-then make more parts as needed and add those to the catalog.
Starting with the motor, the stroke is lengthened 6mm and mated to a JE piston that pumps up compression to 11:1. Total displacement is 452cc. Of course, a more aggressive cam is part of the power-up kit, and to make 10,000-rpm music the stock ignition is replaced with a FlashPoint Rev Kit, basically a new black box with a slightly different ignition curve. More power means more problems for the already-weak XR400 clutch. An upsized Hinson Racing billet clutch basket is the cure. A WB stainless-steel header and prototype oval silencer muffle the 400.
Chassis-wise, everything attached to the reinforced stock frame is beefed up or altered. The ever-changing suspension is currently set up with CR250 inverted forks and a WP bleed-adjust rear shock. Rider Spud Walters wanted more brakes, so Braking rotors are used front and rear with WB pads. The hydraulic lines are by Fastline. Wheels are made up from trick-looking Talion hubs, stainless-steel spokes and Excel rims, a combination way stronger than stock. A Regina chain tugs on Renthal sprockets. Walters runs Bridgestone tires, usually an M39 front and an M40 rear. In the looks department, the WB bike really stands out with UFO CR-style fenders, color-matched frame, and Ceet graphics kit and seat cover. To finish off the package, a WB titanium handlebar (in a CR type-1 bend) with Renthal grips
was fitted, as well as wide-boy IMS footpegs.
Over in the Kawasaki four-stroke camp, Team Green motocross techni-
cian Spencer Bloomer took a KLX300 and, along with Chuck Warren of Metaltek in San Diego, basically grafted KX250 parts onto the KLX. They handformed an aluminum fuel tank and utilized a KX subframe, and moved the footpegs up and back 20mm to keep them off of the ground. Better suspension was a priority: A lengthened KLX shock runs through a Stroker linkage to retain the KX250’s leverage ratio; the swingarm was lengthened 25mm and a KX-type chain guide and adjusters are used for more strength; and KX500 forks, revalved by Acme Suspension, are held in place with Terrycable tripleclamps. The KLX main frame is very similar to the KX’s, only needing minor reinforcement to handle team rider Ty Davis’ skyshot landings. Motorwork was handled by Stroker Speed Equipment and NASCAR engine specialist Billy Tally of California Precision Racing. The displacement is currently at 365cc via bore and stroke increases, and the top end breathes through a 35mm Keihin FCR pumper carb. The stock automatic compression release was tossed in favor of a Stroker manual unit. The header, also by Stroker, flows into an FMF MegaMax muffler. Clutch worries are cured with KX125 plates and KX250 springs.
Trickery continues with a 10.6-inch works front brake rotor and Dogtown brake mods and lines. Stock KX hubs are laced to black Excel rims (a 19-inch in the rear), and Dunlop tires are used. Also added: an LR Racing billet rear brake reservoir, Pro Circuit pegs, Renthal handlebar and sprockets and Acerbis “Flo-green” bodywork, KX-style of course.
Riding either bike can be summed up in one explosive word: Rippin 7 Performance compared to their stock trailbike predecessors is exponentially better, though each bike gets the job done in a different way.
The WB 400 shares very few traits with a standard XR. First, it has to be the easiest-starting Thumper ever, a far cry from most hopped-up fourstrokes. Abundant torque and snorty horsepower are evident right off idle and all the way through the power spread. So much so, in fact, that you generally shift way too early—it feels like there’s no way there could be any more steam left. But there is. The Flashpoint ignition lets the bike rev to 10,000 rpm and, believe me, it pulls hard the whole way.
Handling-wise, the XR is more stout than stock, with way more front-end precision. And while it’s plenty stiff landing off big jumps, the 400 remains surprisingly plush over the little stuff. Heat has been the WB bike’s biggest bugaboo, hence the addition of a second oil cooler.
On to the KLX: This is one wild ride-light and maneuverable, but absolutely solid-handling. The stock KLX’s good points are improved in spectacular fashion. Let the clutch out and it’s surprising how much torque is on tap. Power builds forever, and when the KLX gets to the top of its pull, just before the 10,500-rpm rev limiter cuts in, it sounds more like an IndyCar than an off-road Single. Overall, the KLX feels just a bit slower than the White Brothers’ XR, but by the slimmest of margins. About 90cc worth. By comparison, it’s about as fast as a strong-running 250cc two-stroke. None too shabby.
As you’d guess, handling is very KXlike; it’s just a little less stable-blame the shorter wheelbase. Davis likes his springing stiff, the advantage being that the bike stays up in its suspension stroke and out of trouble going up the faces of jumps or through ruts.
VAU Most of the quirks with this bike are related directly to carburetion. Though jetting was spot-on, the idle was super-finicky, tending to gain rpm when the throttle was shut, negating compression braking. This, in turn,
puts more emphasis on the brakes, which, fortunately, are excellent. Starting also can be an unwanted adventure with the Kawi, especially if it stalls when hot.
This year, if you wanted to go four-stroke MX racing, your choices were limited. Buy a KTM, buy a Husaberg, or make a racer out of a trailbike. Plenty of people took the
_ latter route. In fact, suppli-
ers have been having a hard time keeping hop-up parts on their shelves. With bikes as good as the White Brothers Honda and Team Green’s KLX leading the attack, and a production version of Yamaha’s YZM400 just over the horizon, Thumper fans never had it so good. □
