Performance Tech KLX300 Modifications

Bringing Out The Beast

Dec. 01, 2001 By Mike Hobbs

So, you just bought a new Kawasaki KLX 300... Disappointed with the lack of power?  Or, do you even know that there?s a lot more that your KLX 300 has to offer?

Dirt bike manufacturers are under extreme pressure to make their bikes environmentally friendly.  This is fine and dandy, but when you shell out five G's on a brand new cycle, you want the darn thing to be able to get out of its own way.  In the case of the KLX 300 that we got a hold of, it was a bog dog, hardly what Kawasaki intended us to own.

As delivered, it falls on its face. At idle, a slow twist of the throttle causes hesitation that?s so severe, it had us thinking there was a defect with the Green machine. Not hardly! Blame it on the Eco-Nazis. They?re not only stealing the only land left for us to ride on, but they?re tapping into our mounts and causing a collapse in the power band.

Because of this, it turns out Kawasaki elected to use a  (Keihin CVK 34) constant velocity carburetor instead of the commonly used cable actuated slide carb.  The Keihin CVK 34 carb uses vacuum to pull the slide up, which is unlike most carburetors where the cable performs the duties. 


This novel vacuum approach to carburetion must have seemed like a great idea in the engineers mind, but in the real off-road world, it just doesn't work.




Hesitation while attempting to accelerate could cause major problems.  Imagine wicking the throttle as you head up the face of a jump, and by the time the motor creates enough vacuum to pull the slide up, you're already in the air. Endo city is the result!


Don't let the fact that the Eco-carburetor sucks keep you from buying a KLX 300. It?s a great bike - light, nimble, and an overall competitive 4-stroke - after some tinkering.


Before we get into how to turn the timid KLX 300 into a worthy off road mount, lets take a look at reasons a rider would choose this bike:


  1. Shorter riders will appreciate the low 34.6-inch seat height.
  2. It's a light 4-stroke, weighing in at a claimed 234.5 pounds dry (no fluids in the machine)
  3. Handles well and turns nicely with adequate suspension.
  4. In California, a green sticker is required for year-round off road operation.  The KLX passed the California Air Resources Board (Eco-Nazi invoked) emission standards and receives a green sticker, unlike most high performance 4-strokes, which receive a red sticker.
  5. Last but not least, it?s got an MSRP of $4699.00, and some dealers are selling the 2002 KLX 300 for just about that price out the door.

The first thing that needs to be done in making the KLX run properly is take the exhaust system off and throw it in the trash.  If you don't have the bucks for a new complete aftermarket system, at least remove the restrictor plate from the stock exhaust tip. 

Bob Munden, who's KLX 300 we are using for this project, opted to bolt up a Big Gun exhaust system with a spark arrestor and quiet core.  Big Gun's Quiet series pipe is lighter, quiet and increases power everywhere. The big problem now is getting enough air into the machine to take full advantage of the high performance exhaust system.

Comments? Questions? Drop us a Line!


Here are the three steps we took to rectify the choked down KLX (These tips will also work with the stock exhaust system without the restrictor plate.)

  1. Vent crank case correctly (Remove stock hose and replace with some serious hose.)
  2. Remove air box lid (Trash it. You don?t need it.)
  3. Re-jet the carb. In our case, a jet wasn?t handy, so we drilled our own (in steps) to 1/16 .062"diameter. This equals a #160 main jet size. Then, we changed the pilot jet to a #45. This, like ours, should get it going good enough to actually break the bike in.

After these simple modifications, the bike runs pretty good and will make most novice riders happy, although, there's still a small hesitation at the mid-point of the throttle opening.  This commonly indicates a lean condition, usually cured by moving the needle on the carb to richen the fuel mixture. But, no such luck with the Eco-carb. The needle on the Keihin CVK 34 carburetor cannot be adjusted!

For those that want to make the KLX really rip and have some money to spend, you'll need to dispose of the CVK carb.  This is a pricey upgrade at 500 bucks but well worth it!  Larry Roeseler's Stroker Performance offers a 35mm Keihin F.C.R. (Pumper) carb that really wakes the small bore KLX up!  In fact, it made such a snappy power difference we weren?t sure it was the same bike!   It?s also recommended that a hot start be added in conjunction to the F.C.R. carburetor - the F.C.R. has no choke mechanism.  At first we were weirded out by not having a choke on a 4-stroke.  Rest assured, we?ve accidentally had the Kawi upside down more then once and it?s fired back up within a few kicks.

With the above modifications you have a 4-stroke race bike that would make R.C. proud! 

Mike Hobbs Newsletter
Join our Weekly Newsletter to get the latest off-road news, reviews, events, and alerts!