4Strokes.com Suzuki Technical: Suzuki DRZ400S Upgrades By Matt Nelson
Page Jump Links: Carburetor and Airbox | Carburetor Jetting | Exhaust Systems | Engine Cams | Engine Compression | Suspension | Gearing/Sprockets | Armor/Guards | Junk Removal | Tires | Taillight Mods | Leaky Water Pump Fix
Note: These Suzuki DRZ modifications may also work on the (2003) Kawasaki KLX400SR as well as other model dirt bikes.
The Suzuki DRZ400S is currently the best dual purpose bike on the market. Stock I would say the DRZ400S is suited for 75% dirt, 25% street. My goal was to make it 95% dirt, 5% street. Fortunately, Suzuki has made this conversion fairly simple. I said simple, I didn't say cheap.
The reason this is all necessary is because in California the DRZ400E/Kick is classified as a red sticker bike. That means it is extremely hard, if not impossible, to make it street legal. Your only option is to buy a bike that is already street legal and improve on it. If you don't live in California (or other "Green" state) you would be better off buying a DRZ400E and putting a dual sport kit on it for under $500.
Carburetor and Airbox (For ridding from 0-4000ft above sea level)
Modified Air-BoxThe DRZ400S comes with a Mikuni BSR36 constant velocity carburetor. It comes with this carb for one reason and one reason only, because it can be tuned lean enough to be Environmental Protection Agency (EPA) standards. The only thing you should like about this carb is that it has the ability to adjust to changes in altitude better than the Keihin carbs.
First thing to do, regardless of what jetting/carb setup you choose, you need to open up the airbox so the engine can breath. Remove your airbox snorkel under your seat, and enlarge the airbox hole so it is a 3"x3" square hole, then use zip ties to secure your CDI (black box) in front of the airbox hole. When cutting the hole make sure not to cut too deep or you'll cut a hole in your rear fender! If you enlarge you airbox hole more air will be coming into your engine, causing it to run leaner. To compensate for the more air you will need to rejet with one of the options below to add more fuel. More air with the right amount of fuel equals more power! Do not open up your airbox hole if you do not plan on re-jetting.
Carburetor Jetting Options
Purchase a DynoJet Stage One jetting kit. These kits are designed for an airbox that is not enlarged, so the main jets that they come with are not large enough. You need to modify the kit, and enlarge the airbox. For a stock, or a modified stock exhaust put in a 140 main jet. If you have an aftermarket exhaust use the stock 142.5 main jet. Then put in a 25 pilot (2.5 - 3 turns fuel screw) or the stock 22.5 pilot (3-3.5 turns fuel screw), you should be able to find both the main jet and the pilot jet locally. The main value of this kit is the needle and spring, they both work much better than stock. Put the needle on the 4th clip position. This kit cost about $60 and can be purchased many places.
The DRZ400S comes stock with a 142.5 main jet, 22.5 pilot fuel jet, change that to a 150 main jet, 27.5 pilot. The jets can be purchased from many motorcycle shops. This option will cost under $15. This jetting option is better than stock, but not even close to as good as jetting option one listed above. I highly recommend jetting option one.
There is a reason Yamaha put a Keihin FCR carb on their YZ400, YZ426, YZ450, WR400, WR426, and WR450. Yamaha did it for the same reason KTM uses it on their bikes, and the same reason Suzuki used it on their DRZ400E, and DRZ400 kick. The reason is because it is a great carb! For about $1,200 you can purchase this same Keihin FCR39mm carb from Suzuki. You will also have to change your throttle cables, intake and outlet boots, and velocity stack.
Suzuki Part Numbers
Don't worry about the Throttle Position Sensor (TPS), the DRZ400E/kick TPS that comes with the carb will hook up perfectly with your DRZ400S digital ignition. This option will end up costing $1,200-$1,400.
The final option is to order a Keihin FCR39 carburetor from a retailer. This is the same carburetor as described above, but it does not have a TPS, or even a place to mount a TPS, a choke, or a coast enrichener. Testing done by Dirt Bike Magazine, Carb Parts Warehouse, and myself shown no noticeable benefit of having a TPS on the DRZ400S/E/kick. I know this seems shocking, but if you don't believe me do a few experiments yourself with your current carb and TPS. Tell your buddy go alone into the garage and either disconnect the TPS or leave it connected, but not tell you which he had done. Then ride your bike and try and guess if it is connected or not. Try this a few times. I think you won't be able to tell. I also find that not having a choke is not a problem either, a few twists of throttle before starting the bike makes the accelerator pump effectively work as a choke. The coast enrichener might be needed if you ride in very cold areas, but usually all they do is add a bunch of extra tubes to the carb. Most people that buy an DRZ400E or kick remove their coast enrichener, so I feel that not having one isn't a problem. With this carb you can use your existing throttle cables, intake and outlet boots, and velocity stack. Everything needed is included in the kit.
Hole Saw to Drill Out DRZ Exhaust BaffleThe exhaust provides good power stock, and is very quiet. The stock exhaust pipe on the DRZ400S produces a very quiet 92dB exhaust note. 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. I recommend you leave your exhaust stock, but if you would like a little more noise, and a little more power you can use a bimetal hole saw to cut out the restrictor at the end of the exhaust. In case you were wondering, the DRZ400S exhaust system is 11.04 pounds.
The DRZ400S has different cams than the DRZ400E/Kick. The intake cam is the biggest difference.
The DRZ400S has an 11.3:1 compression ratio, the DRZ400E/Kick has a 12.2:1 compression ratio. Suzuki lowered the compression on the DRZ400S so that it can run 87 octane fuel. If you want more power spend $40 on the DRZ400E/Kick base gasket and put it in you DRZ400S. This will give you a 12.2 compression ratio. If you make this change you will have to run 92 octane gas or better.
The suspension on the 2000-2001 DRZ400S is very soft, and even though it looks like the DRZ400E/Kick's suspension it is very different. If you weight over 180 pounds fully geared up you will need suspension work done. For about $250 you can have Race Tech install stiffer springs and something they call emulators in your forks. For another $250 they will do the rear shock, although it is not as bad as the forks. The 2000-2001 forks do not allow you to adjust rebound. The suspension on the 2002-2003 DRZ400S is the same as the DRZ400E and the DRZ400E/Kick. This is a really good suspension, and I compliment Suzuki for making this change. If you still want it better, Race Tech also makes improvements for this fork.
The DRZ400S comes with a 15 tooth sprocket in the front, and a 44 tooth in the rear. The DRZ400E comes with a 14 in the front, 47 in the rear. The front sprocket on the S has a rubber bushing around it, this is to keep noise down, and does not reduce vibration to the transmission. Feel free to put on different front sprockets, you do not need the rubber bushing. I spend most of my time off road so I use the 14/47 combo. I can still do 70mph on the road no problem. This cheap mod will make a big difference.
If you ride off road you need a good skid plate. Having a rock take a chunk out of your DRZ wouldn't be any fun at all. If you don't want that to happen I recommend the Baja Designs skid plate (shown here) for $56 which mounts using brackets that clamp around your frame (kind of a pain to take on and off), or my first choice, a White Brothers heavy duty skid plate. It offers the same great protection as the Baja Designs skid plate, but mounts using existing skid plate bolt holes. This makes installation a breeze, and makes it easy to take on and off for cleaning. Mud loves to get trapped in them, and can add several pounds, so you'll want to be able to remove it for cleaning often. The White Brothers skid plate retails for $75. The part number is 31-939X.
The radiators on the DRZ400S are extremely fragile. Many people have accidentally destroyed them only to find out that Suzuki charges around $300 for each radiator. Many companies make heavy duty aluminum guards designed to prevent this from happening. I would rather pay the $70 for the guards than $300 for a radiator.
Junk to Remove
The stock DRZ400S tires are a death trap in the dirt. They should be the first thing you replace if you are going to go in the dirt. Since the DRZ400S is a dual sport bike we will need to find some Department of Transportation (DOT) approved knobbies.
I don't know what the Suzuki engineers were thinking when they put that big ugly 2.76 pound taillight on the DRZ400S. UFO makes a universal taillight (shown below) that can be easily adapted to the DRZ. It costs about $60, and is probably legal enough to keep you out of trouble. The turn signals will flash too fast with the stock UFO light bulbs, put in some 20W bulbs, and the blink rate will slow to a normal level. Some people are using GE 921's they found at Wal-Mart. If you use the GE 921's you will need to drill out the front of the hole were the rubber gets locked into the lens with a 5/8 drill bit. The natural taper of the lens will hold it in place. Or you can go to Radio Shack and purchase the halogen lamp for automotive use, part #272-1177 for $1.99. You need to bend the pins into a "U" shape so they will plug in. These bulbs are easy to break when bending the pins. To keep this from happening hold the pin with some needle nose pliers as close to the bulb as possible and then bend the pin into a "U" shape.
Taillight Update by Chris O'Hara, 2/26/05
DRZ Taillight SocketSince the light bulb almost ALWAYS breaks when you try to bend the pins, I needed to find a better solution. What I do is just leave the pins as they are and instead pull the negative and positive terminals out of the rubber plug. I used a pair of needle nose pliers and did one side at a time. You donít have to pull hard, as there is little resistance and they just pop right out. Now itís easy to slip one side of the bulb at a time onto the terminals and the bulbs' pins slide perfectly into the rubber plug. Then you just slide the rubber plug back up over the terminals and the base of the bulb and it looks just like it came from the factory!
If you can't afford the UFO taillight, you can modify the stock taillight (shown below). The spacers that extend the turn signals out on each side can also be removed. However, some states require a certain length apart. Modify these lights at your own risk! These are safety items, and I'm sure they are ugly because some safety engineer/lawyer says it makes them safer.
RTV on Water Pump Shaft More info on the DRZ taillight mod can be found in our Suzuki Electrical forum: DRZ400S Tail Lamp & Signal Mod
Leaky Water Pump Fix
Many people have had a problem with a leaky water pump on their DRZ400. Replacing parts almost always fails to solve the problem. The problem is a design flaw, not a blown seal. The water pump leak can be fixed with a little RTV. RTV stands for Room Temperature Vulcanizing; it's a generic term for a silicone adhesive/sealant. Permatex is one company that makes RTV. Any brand or color RTV is fine. All you have to do is put a tiny bit of RTV at the base of the (rubber part) seal between the impeller and seal where it meets the shaft. In other words apply RTV to this seal to make it fit the impeller shaft tighter. Make sure to let the RTV dry for at least 24 hours before adding coolant.
Credits: Article written by Matt Nelson, unless otherwise noted on individual sections, and edited by 4Strokes.com
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