Kawasaki rethinks the winningest 125 of our time

By the staff of Dirt Bike

Face it, we motocrossers have a very short memory. When we talk about the Kawasaki KX125, do we talk about how it gave Ricky Carmichael his first wins? No. Do we talk about what it did for James Stewart?s career? No. Do we talk about how many number one plates it has accumulated? Not even. For the last two years or so, the only thing people say about the KX125 is that it?s kinda slow. Yeah, Nadia Comaneci might have been something back in the day, but now she?s just another Romanian with big thighs.

Kawasaki knew that even the KX?s unreal string of race wins wasn?t enough to carry it another year. Not now, not with the YZ250F pillaging the 125 class and not with KTM finding even more horsepower in the 125SX. So Kawasaki got back into the race in a big way. The model year 2003 marks a complete top-to-bottom redesign of the KX125. The frame is different, the engine is different, the suspension is different. So does that mean that Kawasaki is starting from scratch? Or will the new bike start off a notch higher than the rest? We thrashed the new bike on six different tracks to find out.


Let?s just say everything is new and leave it at that. But it?s clear that the new KX was designed by the same people as the old KX, because the big-picture designs and goals are the same. The bike still has a steel perimeter frame with KYB suspension. It still has a case-reed motor with the same bore and stroke. It still is a six-speed. The most interesting change in philosophy is that Kawasaki has abandoned the Electrofussion cylinder coating process that it pioneered back in the ?70s. The new coating, now called T-treatment, is a chrome composite similar to Nikasil. The power valve is different (lighter), although it still uses the same basic philosophy of opening auxiliary exhaust ports that Kawasaki also originated.

You won?t find anything startling in the powerplant. There?s just a part-by-part accumulation of changes that add up to a new motor. The cases were designed to increase the crank volume, but then the crank has plastic suffers to [decrease] that volume. The cylinder is mounted to those cases at a more reclined angle and the head gasket is gone, replaced by O-rings. A six-pedal carbon-fiber reed replaces the four-pedal. A new Mikuni TMX38 carb replaces the old one. The piston is new, the clutch hubs are new, the ignition rotor is lighter and everything is just a little different. It?s a very rare clean-sweep of old engine parts.

Chassis-wise, Kawasaki had vague goals for the new bike. Mostly, it had to be lighter. You need to remember that the old KX was only being blasted for its weak-suck motor. In general, it was still doing well in the handling department. So once again, the KX had to be new, but not too new. The KX125 and 250 actually share the same frame, with only differences in the engine cradle. The main spars of the frame were configured to lay in a near-straight line from the swingarm pivot to the steering head. The steering head itself was moved up and forward to get it more over the front wheel, but all the numbers have been juggled somewhat. The rake is more relaxed (from 26 degrees to 27.5) and the offset between the fork and the steering stem has been increased, resulting in less trail. The net goal of all that? Beats us. Ask us about the pretty new bodywork–it looks cool.

More fiddling in the suspension department: The fork is a larger version of the KYB that Yamaha has been using. There?s no more bladder around the cartridge, but there is an anti-bottoming bumper. The shock lost its high-speed compression adjuster. We won?t miss it. It got new linkage, new valving and a lighter spring.

We could go on for pages with detail changes. The airbox, the swingarm, the rear axle, the rear hub; all different. Like we said, this is a legitimately new motorcycle. Get used to it.


Frankly, all that new stuff wouldn?t mean much if the bike wasn?t any faster. Take a deep breath and relax; the new KX125 is a runner. It makes decent power everywhere with no real sore points. The bottom is good, there?s a gutsy mid-range hit and the over-rev is good. If you had to compare it to anything on the track right now, it would be the 2002 Suzuki RM125. That?s good company. The broad powerband makes it easy to ride, so novices and beginners can actually keep it on the pipe. It?s still not the fastest thing in the class. The Yamaha and KTM 125s are still duking it out for top two-stroke honors somewhere behind the Yamaha YZ250F. But the KX is back [in] the pack, rather than in back [of] the pack. We have a feeling that pros will be willing to trade in a little of the KX?s new found low-rpm power for more boost on top. That?s okay; now the bike has material to trade.

Like most other good-running two-strokes, the KX is jetted right on the brink. Stock jetting is good for most conditions, but if its a little warm or a little cold, expect to fiddle around some to clean things up. For example, it was around 80 degrees when we first rode the bike–everything was perfect. When temps went up into the 90s, the bike ran a little fat on the bottom. We dropped the pilot from a 40 to a 37.5 and dropped the needle from position number 3 to position number two. We were rewarded with slight detonation on top, so we fattened the main from 420 to a 430. Now the bike runs well in the heat, but not absolutely perfect. Learn to live with it. The days when you could have perfect carburetion year-round are gone.


With just the motor improvement, we would have been happy. If Kawasaki had stuffed the new powerplant into the old chassis, it would have been enough to lift the KX125 out of the cellar of the 125 class and everyone could go home smiling. But here?s something interesting; the KX has made just as much progress in handling as in power. Digest that for a second. The old KX already handled great!

The 2003 KX125 is perhaps the most stable, secure handling 125 in the class. It absolutely sticks to its line no matter where you point it, and track conditions don?t seem to matter. It never shakes its head, never darts off in an unexpected direction and never does anything that you didn?t ask for. And for bonus points, it still turns great; at least as well as a Yamaha. The class cornering champion has always been the Suzuki, and that hasn?t changed. But for such a stable machine, the KX is amazing in turns.

That?s even more surprising considering that the bike has typical KX suspension. Soft, in other words. There?s nothing wrong with that–it?s perfectly okay for a manufacturer to aim the bike at the masses, who want and need soft suspension. The new KX gets away with soft rates much better than before, though. The frame is so stable that the fork diving and mushiness associated with soft suspension aren?t any big deal. All the same, all but the lightest riders will want to add a few clicks of compression damping to the fork. The stock setting is 14 clicks and most of our staff liked it cranked in to about six clicks. The fork doesn?t really get harsh; in fact it?s still pretty cushy. But you do hear an occasional clank when the fork bottoms. It?s more of a sound than an impact that you feel in your wrists–the result of a good anti-bottoming system. But if your track has big jumps or if you weigh more than 140 pounds, you should consider stiffer fork springs.

The rear end is much more versatile. Even without the high-speed compression adjuster, you can get the back to work well for anyone under 190 pounds. Again, most of our guys turned the adjuster in a few clicks from stock. The rear end already sits a little low, and with the shock set up on the soft side, it can become difficult to keep the front end down, whether you?re on a start, in the air or going into a turn. Low-ride rear ends aren?t good for any of that.


So is it risky to buy a bike that has so many new parts? Probably not. Kawasaki has been working on prototypes of the new KX for a long time, and nothing broke during our test. The only snivels are the soft seat foam (you?re riding on the seat base from day one) and a pipe that looks like a snake trying to digest a grapefruit. Kawasaki says that later bikes will have stiffer seat foam, but the pipe is going to melt your right boot?s shin guard. Deal with it.

Otherwise, the ergos are good. It?s kind of a big 125, with a long reach to the bars. That?s fine–sitting farther forward never hurt anyone?s riding style. But all the same, the bike seems better suited to big riders. The clutch has a light pull but still is a little grabby when it?s cold. That?s a KX trait from way back. The brakes are good, the new Bridgestone 602 and 601 tires are very good on hardpack, and the shifting is average.

Overall, though, we dig the new KX. We expected the engine to have more power; Kawasaki wouldn?t have bothered if it didn?t. But we didn?t expect this much out of the new chassis. Who would have figured that a 250 frame would work so well with a 125 motor? We know that 125s live and die by horsepower numbers, and that even with its new-found power, the KX is going to have its hands full against Yamahas and KTMs. But it?s good to know that along the way, someone remembered that handling still matters.

* Super stable handling
* More power than ?02 version
* No more boot-catching plastic
* Good, spread-out riding position

* Still not a fast as a Yamaha or KTM
* Soft suspension
* Fussy jetting
* Obtrusive pipe
* Steel bars

Temperature…90 degrees F
Altitude…1500? msl
Fuel…Mobil 91, HP-2 40:1
Jetting…430* main, JCHJ11-82, #2*, 37.5 pilot*, 1 1/2 turns
Fork…6c*, 13r
Shock…10c*, 12r
* non-standard settings


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