The following assumes riding from 0-4000' above sea level. Mods may work for Kawasaki KLX400SR and other models.
The DRZ400E and the DRZ400E/Kick come standard with a Keihin FCR 39 flatside carburetor. It's a great carburetor. It's the same carb as the YZ400, WR400 and the KTM. When it comes from Suzuki it is jetted pretty lean though, and needs to be rejetted. Note: The pilot jet is sometimes referred to as the slow jet and the pilot air jet is sometimes referred to as the slow air jet.
Basic - (For those that would rather be out riding than messing with the carburetor.)
First thing you need to do is open up the airbox. Take off your seat and pull the airbox restrictor.
Now that you have more air coming into your engine it is even leaner, so you need to put in a bigger main jet. The simple solution which yields good results is to put in a 165 main jet. You will also want to increase the size of your pilot fuel jet from a 45 to a 48, and turn your fuel screw out 2.5 turns from closed. Your fuel screw is located at the bottom of the carb. Some DRZ's have an off idle bog, if this jetting does not eliminate your off idle bog proceed to the "Advanced Jetting" below.
Advanced Jetting - (For those who love getting every ounce of performance.)
First remove the airbox restrictor as noted above. Then you will need to remove the coast enrichener. The coast enrichener richens the fuel mixture on deceleration to prevent backfiring. For this advanced jetting to work the coast enrichener must be removed because it makes the mixture too rich. This jetting works really well for eliminating the off idle bog that many DRZ's have.
Coast Enrichener On
Coast Enrichener Removed
Here are some instructions on how to remove it from "Hollywood:" Remove the tank On the left side of the bike (carb side) look for the two nuts, diagonal from each other, holding the body of the coast enrichener on and remove them. Note: they are long Allen head bolts that may need to be held on the other end to keep from spinning. Pull coast enrichener off. It'll now be held on by 3 vacuum tubes. Buy a package of diff. size vacuum plugs from your auto parts store. Save the little clips that held the vacuum tubes onto the nipples at the intake manifold, carb intake bell, and air jet. Pick the right size vacuum plug, put it on over the nipple and put on the clips (use some WD40 to slide the clip on, it'll be tight). Chuck the coast enrichener thingy in the parts bin. Put in a 70 pilot air jet, or 65, depending on which direction you want to go. Just be sure to put in the corresponding pilot jet.
Now that you have the airbox opened up and the coast enrichener removed you will need to put in an EMM needle, on the 4th clip position (4th down from the flat end). Put in a 38 pilot jet (PJ), a 70 pilot air jet (PAJ), and a 158 or 160 main jet (MJ).
I have personally seen the 38/73, and the 38/70 combination work well, but the other close combinations should also work.
You might want to also consider purchasing an Pilot Air Screw (PAS) to replace your pilot air jet. With a PAS you determine the size of the jet by turning the PAS a certain number of turns out from seated (lightly closed). These cost around $20 and can be purchased from Sudco or Carb Parts Wharehouse. Here is the PAS Turn Chart:
The stock exhaust is very quiet, around 92dB. This is a good thing, one of the biggest reasons our riding areas are being closed is because of noise pollution, I highly urge you to leave the stock exhaust on, and to not modify it. If you live in California, a new law takes affect on 1/1/2003 that calls for a 96dB maximum for all enduro bikes built since 1986, that includes the DRZ. So if you want to ride in OHV parks or on BLM land you should leave your exhaust stock.
But, if you don't want to listen to my politics, here's how to modify it for a little more power and a lot more noise:
Use a 1 3/8" bimetal hole saw to cut out the restrictor in the tip of the exhaust endcap. Once that is removed the DRZ will have a deeper sound to it. Start it up and see if that is loud enough for you. If that still isn't loud enough for you, unbolt the entire endcap on the exhaust, and temporarily remove it. Now you should have full access to your restrictive exhaust baffles. Find a drill long enough that can drill through both exhaust baffles, 3/8" or 1/2" drills work well. Drill holes until you are happy with how loud your exhaust is. You do not have to worry about having enough back pressure as this is not a 2 stroke. The stock Suzuki exhaust weighs 11 pounds. You can save some weight and get a little added performance with an aftermarket exhaust.
Credits: Article written by Matt Nelson and edited by 4Strokes.com