This article covers checking your carb needle to find out which one you have, and doing some basic mods to your KLX300 for quick and cheap performance gains.
Before I say anything else, I have to tell you that according to the Owner's manual for the 2006 KLX300, it is a federal offense to remove or modify any part of the intake system, carburetion system, or exhaust system. It is also a federal offense to operate any vehicle that has been modified in this manner.
Not only that, but making these modifications will void your warranty. Your dealer may charge you for repairs related to these systems if you modify them.
Having said all of that, everyone does it, and the dealers even tell people to do it. I've never heard of anyone getting busted anywhere but California for moding their bike.
Here's the deal:
In models previous to 2006, Kawasaki shipped the bike to 49 of the states with one type of needle in it. The California models got bikes with a different needle in them (and a bunch of other smog equipment). I don't know what the CA guys do to make their bikes run better.
The rest of the world, however, made some very easy mods to greatly improve the bike's performance:
a. Remove the airbox lid.
b. Replace the air filter
c. Remove the restrictor plate from the muffler.
d. Replace the crankcase breather hose.
Those changes, by themselves, made a huge difference.
However, starting with 2006, Kawasaki is shipping all U.S. bikes with the needle that they used to send only in the California bikes. Now, the Canada bikes get the good needle and everyone else gets the "bad" needle. With the bad needle in there, removing the airbox lid causes the bike to run like crap. So, now you need to replace that needle AND do all of the things mentioned above.
Here it is in more detail:
1. Identify your needle.
The first thing you need to do is find out what needle you have in there. To get to it, first remove the seat and gas tank (make sure you shut the gas off first). From there, you'll be able to get into the top of the carb, where the needle is. Here's an exploded diagram of the carburetor:
Make sure that when you take it apart, you carefully keep track of each component and put it back the way you found it.
The first thing to do is remove the diaphragm cover. That's #14041 in the picture. When it comes off, the spring under it will try to escape. Don't let it.
Get yourself a tupperware container and put the cover, screws, and spring in it.
Once you have the cover and spring out, stick your finger down the hole and gently remove the slide. Make sure the rubber diaphragm doesn't get stuck on anything and tear.
Once you have the slide out, put your hand over the hole and turn it upside down. The "seat-spring valve" (white plastic thing with legs, #16007) will come out, along with the needle possibly some other tiny bits like a collar (#92143) and spring clip (#92037) - but the clip, if it exists, should be firmly attached to the needle.
Compare your needle to these two pictures:
If it looks like Needle2.jpg, it's the "bad" needle.
If it looks like Needle3.jpg, it's the "good" needle.
The needles also have little letters and numbers stamped into them near the fat end. If it says "N5AF", it's the "bad" needle. If it says "N1TC" it's the "good" needle.
If it is the good needle, put your bike back together and skip to step #2. Make sure you get that white plastic thing with leg in right. Don't cover the hole in the bottom; make sure it seats all of the way. Your bike will tell you if you get it wrong.
If it is the bad needle, call your dealer and order these parts:
16009-1912 - Needle-Jet,N1TC
92037-1401 - Clamp, Jet Needle
92143-1667 - Collar
The new needle will cost you about $20. Not sure about the clamp and collar, but they'll be cheap.
2. Remove the airbox lid.
This step is frighteningly easy. Just pull off the seat, unscrew the four bolts holding the lid on, pull it off (and store it safely somewhere), and put the screws back in. I recommend this so that you don't lose the screws, and so that your screw holes don't get muddy, corroded, or whatever.
The airbox lid looks like this:
See how small the intake holes are in that lid? See how much more air it can get if you remove it?
Oh, and by the way, see how much more dust, grit, sand, and water can get into the airbox without the lid? Clean and lube your air filter VERY often after you've taken that lid off. Get some air filter oil and remover that is easy to clean off and re-apply. I recommend "No-Toil" filter oil and cleaner. No-Toil also sells filters, but the popular opinion says to buy a "UNI" filter instead. It has a rough outer filter and fine inner filter. It lets more air in while still filtering out the bad stuff. They cost about $20. The No-Toil costs about $7.95 each for the oil and cleaner.
3. Remove the restrictor plate.
This is also very easy. The plate at the back of the muffler has two screws holding it on. Remove them, remove the plate, store the plate with the airbox lid, and put the screws back in.
The restrictor plate looks like this:
The Kawasaki manual calls this piece the spark arrester. I think they do that to try and scare you. The parts diagram on the Kawasaki website calls it the "Baffle-pipe, muffler". I asked the dealer if the spark arrester still stopped sparks with the cover off and he said it did (shrug).
This mod will make the bike a lot louder, so your neighbors might not like it if you run the bike at home a lot after making this mod. It also leaves a gaping hole in the back of the can, so be careful when hosing down your bike. Don't get any water in there if you can avoid it.
4. Replace the crankcase filter.
This mod isn't as popular, but is supposed to help significantly as well. The deal is that the stock crankcase breather hose has a brass couple in it that has a really small hole in it that restricts airflow. A bigger hose without a coupler is supposed to help. You run the hose up into the airbox and stick the little filter on the end.
You can get all of the crankcase breather stuff from your local auto parts store.