With all of the hi-performance four-strokes scorching the tracks and trails, you?d think that the Kawasaki KLX300 would go the way of the dinosaurs, but you?d be wrong. Hop-up shops are doing more KLXs now than ever before, and the comparatively inexpensive Kawasaki has a lot of things going for it. The first water-cooled Japanese thumper to hit our shores, the KLX has modern suspension and a perimeter frame that?s great for smaller riders. Back when the KLX was first introduced, Gary Hazel was working at Team Green, so he knows the machine inside and out. Those first 250s were hard to start and had cam-chain problems. Kawasaki fixed weak points and punched displacement to 300cc for the 1997 model year. Now in its eighth year, the KLX has gained a following like the XR250R, and the aftermarket knows how to make it rip.

From The July Issue Of Dirt Bike Magazine


Just as the KLX benefitted from a jump from 250cc to 300cc, more displacement equals more bark. Thumper Racing bored the Nikasil cylinder to accept a sleeve and an 84mm piston, bumping displacement to 340cc and dropping compression to 10.5:1 for the kit. Pistons with compression of 11:1 (stock) and even 12:1 were tried (YZFs are 12.5:1), but the intake valves would cup and fail with the added pressures and temperatures. Hazel felt the lower compression and extra torque of the larger piston would give better low-end without having to pour thousands of dollars into the engine.
Next, more top-end was on the list, and that meant uncorking the intake and exhaust. Kawasaki uses a very dense airfilter foam and an airbox lid to meet government noise requirements. The lid was tossed, and the stock filter was replaced with a freer-flowing Moose airfilter. Big Gun was chosen for the exhaust for several reasons. The larger headpipe flows more for more top-end power, and the whole system is ceramic-coated to dissipate heat. Most importantly, the Big Gun off-road pipe has a quiet-core muffler, since less sound equals more ground on public and private land alike.

Hazel tried a few carburetors, including a pumper FCR Keihin, and saw gains, but fitment was a problem, so the stock carb was rejetted for the mods. More for durability than performance, the project 340 also got a Hinson clutch, heavy-duty clutch springs and Thumper serviceable oil filter. Lastly, Hazel felt the KLX was geared too low, especially with the added boost, so he raised overall gearing by going from a 50 to a 47.

Low-end is noticeably better with the 340 kit, but the KLX still retains its electric-appliance personality. It?s easier to loft the front end for obstacles and snake the machine through the twisties, but there?s no hit to throw you off-balance. The KLX simply starts pulling earlier and harder, and it revs out with more authority. The freer-flowing intake and exhaust improve upper-end power, too. In the tightest woods sections on the McKool Ranch, the mighty 340 proves to be faster and easier to ride than stock, with very little done to the engine. It?ll even pull a taller gear than usual, but its best to still ride the 340 like a two-stroke 125 or 200?pin it and row the gearbox.

Also, the kitted KLX is easy to start and keep running. Lump got first-kick starts every time, while Dick Burleson usually took three to light her up, and we never stalled it on the trail.

When Hazel moved from Team Green to Thumper, he still had green in his blood, so he rode a KLX250 punched to 331cc and fitted with a KX fork. Wanting to keep the costs down for customers, Hazel and George from ESP planned on updating the KLX fork to give it rebound adjusters. George made the plan to update the now-dated suspension a reality. With bonafide rebound adjusters, reworked valving and 13mm more travel at each end, the KLX340 takes a huge leap in performance. A half-inch might not sound like much, but the stock KLX is so mushy and wide at the footpegs that it drags in ruts before any other full-sized dirt bike. To further raise the bike and improve the ride, the stock 0.38Kg/mm fork and 5.4Kg/mm shock springs were replaced with Eibach 0.41Kg/mm and 6.0Kg/mm coils.
We put the skidplate to good use in Texas, as the wide KLX engine cradle has things dragging in ruts and rocks, even with the taller suspension. The bike does ride noticeably higher in its stroke with the ESP mods. The fork is much stiffer than stock and still does a great job of smoothing out trail trash. We went in five clicks on compression to resist bottoming in the undulating creekbeds and MX track sections. The rebound adjuster is also handy ?we cranked it in for hardpack and improved the ride further. Balance with the shock is also excellent. Both ends work well and way better than stock, but we could still make the front end push fairly easily in corners. Hey, aftermarket, where are the KX-style seat and tank kits?

Even with the extra ground clearance, the KLX still needs better engine protection. Moose?s KLX skidplate has wings to protect the water pump and plumbing. Moose also got the nod for the CR-Hi-bend aluminum bars, adjust-on-the-fly clutch perch, and X-ring chain and sprockets. The footpegs are vulnerable, as they?re very low on the frame, so the stockers were replaced with Thumper Burly pegs. And, do you remember how bad KX brakes were eight years ago? Well, the KLX hasn?t been updated with the stuff James Stewart uses, or even Destry Abbott. Hazel upgraded the front with a Braking oversized rotor and caliper carrier and the rear with Moose pads and high-temp brake fluid. Enduro Engineering supplied some new, lighter handguards with more clearance for the AOF clutch perch, and Thumper?s Pro Series textured seat cover helps the rider stay on the more-potent machine.
Upgrading the brakes is critical with the KLX, especially with the stiffer and taller suspension. The new-found braking prowess lets you snake the 340 through the woods much easier and with minimal effort. Every detail is improved on the Thumper KLX340, but it isn?t transformed into a beast. The 340 simply does everything better and faster with less effort from the rider. Where the WR450F is monstrous, the KLX340 is sweet. And its constant-velocity carb makes it less susceptible to jetting woes with changes in altitude and weather, making it a great mountain goat-trail bike. Colorado, here we come!


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