The venerable Kawasaki KLX300R has retained its popularity because it combines light weight and low seat height with an excellent potential for power. This liquid-cooled, four-stroke dirt bike has a special appeal for vertically challenged riders. Its lighter weight and lower seat height makes it that much more manageable than its competition. It also meets California's strict emissions requirements for a Green Sticker, allowing an easy conversion to a dual-sport and registration for street use (prior to January 31, 2004). With a frame and suspension suitable for motocross, it's potential for high performance has been tapped by 10-time Baja 1000 winner, Larry Roeseler.
In order to get the bike to pass the stringent emissions tests required by California's Air Resource Board (CARB), Kawasaki had to seriously compromise the performance of the KLX by installing a restrictive intake system. Its Keihin CVK 34 constant-velocity carburetor uses a vacuum to actuate the slide instead of a cable. While a CV-carb may be acceptable on the street, it just doesn't cut it for serious off-road work because of its tendency to hesitate when accelerating. Added to this inherent disability is the lean stock jetting. Coupled with a similarly restrictive exhaust system, the stock KLX's performance is rather disappointing.
That's the bad news. The good news is that Stage 1 modifications can be performed fairly inexpensively, and they will boost the KLX's power considerably. Here are the steps involved in the Stage 1 modification:
Remove the restrictor plate from the end of the stock muffler, and re-install the two allen-head bolts so the guts of the silencer remain where they are supposed to. You may want to install a larger diameter header pipe at this time. The stock silencer is adequate with the baffle removed, but more power can be gained with a larger header. If you replace the silencer with an after-market unit, then you will certainly want the larger header to go with it.
Remove the top of the air box, along with the snorkel, and throw it away.
Replace the stock air filter with a Maxima, Twin-Air, UNI or No-Toil filter.
Vent the crankcase correctly by removing the stock breather hose and replacing it with a larger diameter hose. Restrictive breather hoses can rob horsepower by building up excessive pressure in the crankcase. Stroker sells a breather kit for the KLX300R.
Re-jet the carburetor:
N1TC Needle-jet (Kawasaki P/N 16009-1912, $19.95)
Needle clip (Kawasaki P/N 92037-1401, $2.95)
Needle seat (Kawasaki P/N 92143-1667, $3.49)
Keihin #160 main jet for break-in
Keihin #45 pilot jet for break-in
Choose the appropriate groove on the needle for your elevation. After the motor has been broken-in, re-jet the carb for your elevation. The KLX's performance should now be acceptable for spirited trail-riding. Go forth and enjoy!
To increase reliablity of the KLX300R, you may want to replace the stock idler gear, which is brittle and has excessive play on the output shaft. Serious motor damage could result if its teeth break off. Stroker sells an idler gear that is less brittle, and is tighter on the output shaft, so the gear won't wobble from side to side while kick-starting the motor.
After accumulating some miles, the KLX300R tends to develop a problem popping out of gear and missing shifts. Stroker sells a modified shift star and a preload device for the detent spring which keeps the detent arm in the proper location on the shift star. This simple modification keeps the transmission in gear. The shift kit is installed through the access panel by the shifter.
Racers may want to go all-out with a competition exhaust system and a 35 mm. Keihin F.C.R. pumper carb. It's also recommended that a hot-start system be used with the F.C.R. carburetor. There are several other approaches to hopping-up the KLX, which are detailed at these web sites:
Stroker Four-Stroke Speed Equipment
Stroker's KLX300 Stage 1 Mods
Stage 1 Tuning of the Kawasaki KLX300
This article is based, in part, on information provided by Mike Hobbs, Jerry Wiener and Larry Roeseler.