In my humble opinion, the Honda XR400R is one of the best four-stroke trail bikes ever made, given the limitations of an air-cooled engine and the challenges of riding a tall, heavy bike on tight single-track, especially for someone with a short inseam. Since no bike can be all things to all people, modifications to meet one's particular needs are usually in order. In addition to installing a dual-sport kit, I wanted to improve my bike's performance in the areas of power and handling.
The stock XR400 from 1998 to 2000 is definitely asthmatic, thanks to EPA noise and emissions regulations. If it is a California import, the CARB green-sticker requirements make things even worse. So, to improve its performance, you'll have to defy those warnings about "tampering" and start opening up its airways. Disclaimer: I'm not advocating that you break any laws. This article assumes that you'll want to ride your bike in "closed course competition." Anything else you do with it is up to you.
First off, literally yank the snorkel out of the top of the airbox and replace it with one from a '96-97 model (P/N 17240-KCY-670). Alternatively, you can simply cut a piece of reticulated foam to fit the opening, which will keep the larger particles out. Replace your stock air filter with a UNI Multi-Stage air filter (NU-4074ST) and you'll practically hear the motor sigh with relief!
Depending upon the altitude and air temperature, your motor will probably be running lean when it gets more air, so you'll need to rejet. If you have the California model Keihin (PD K1E) carburetor, you'll also need to change the jet needle to a richer A16A (P/N 16012-NKK-000), with the clip in the third groove from the top. If you're between sea level and about 3,000', start by swapping the stock #142 main jet for a #155, and the stock #52 slow (pilot) jet for a #55. Turn the pilot air screw two full turns out from lightly seated, and see how it does. Read your spark plug tip with a magnifying glass and make the necessary adjustments from there.
Be aware that the Keihin's choke plate has a tendency to break, causing serious damage to your motor. If you keep the Keihin, install an aftermarket solid aluminum choke plate. A better, although more expensive way to improve carburation is to install a Mikuni 36 mm. accelerator pump carburetor (TM 36-50). This will improve acceleration and throttle response, and will make your carburetion much less sensitve to changes in altitude and temperature. Living in Big Bear, riding to nearly 9,000' and then going down to the Mojave desert, can create jetting headaches with most carburetors. The pumper carb seems to take these changes in stride, and starting the bike, hot or cold, typically becomes a one-kick affair. Keihin also makes a pumper carb that can be adapted to the XR400.
Some people remove the California crankcase emission control system, and replace the breather separator and associated plumbing with a large, filtered vent tube. Restrictive crankcase breathers can reduce power by building up excessive pressure in the motor. Aside from a slight weight-savings, I'm not sure if there's any performance difference, but there's the psychological satisfaction of knowing that my XR's crankcase is now breathing as well as its bretheren in the other 49 states. Be sure to plug the opening that's left in the airbox, if you make this change.
The stock exhaust headers flow well enough, and except for competition, I'd leave them alone. If you have to take them off for other work, you can clean up the welds inside the pipes, but that's the most I would do. Oversize headers are only recommended if you've made very extensive motor modifications. The stock muffler is fairly decent and not all that much heavier than an aftermarket exhaust, so unless you've just got to have a shiny chrome muffler, I'd stick with the stock muffler and spend the money on other things.
Aftermarket exhausts do give you a lttle more power, but lot more noise. Considering that the legal OHV noise limits on State and Federal lands have been lowered, expensive competition exhausts might have to be removed for trail riding. If you really have your heart set on an aftermarket exhaust, make sure it comes with a spark arrestor, has a "quiet" insert that can be removed for competition, and is easy to service and maintain. Some aftermarket exhausts require frequent repacking to keep them performing properly, and with some, you'll have to drill out the pop rivets for disassembly. Personally, I don't need this chore.
It's caveat emptor when it comes to manufacturers' power and noise claims. Most aftermarket exhausts don't get sound-tested according to the SAE standards that Ranger Rick is required to employ. As far as published dyno test results are concerned, "your own mileage may vary considerably." Let's face it. The stock XR400 exhaust system is really a great piece of work. Lots of engineering and testing went into making it perform well and still meet the EPA noise limits. Too bad it doesn't sound more boss!
The most cost-effective fix for the stocker's soft-spoken ways is to install a Vortip tuned exhaust tip in a stock '96-'97 screen-type spark arrester (P/N HM-KCY-A1). I dyno-tested the Vortip against the stock noise-suppressor insert (also called an exhaust tip baffle), and there was no significant difference in the amount of torque or horsepower produced throughout the motor's RPM range, although both curves were smoother with the Vortip. However, where the stock exhaust purred, the Vortip growled. I like that, but your neighbors won't, so keep your revs below 3,000 until you hit the dirt. For about a hundred bucks, the bike sounded studly, but not deafening. Equipped with a full-on competition exhaust and tested on the same Dynojet 150, an obnoxiously loud exhaust note was accompanied by only one-and-a-half more horsepower. Recommendation: Stick with the Vortip and stock muffler.
View Dynojet Exhaust Test Results
If you're going for closed-course competition, you can keep the stock muffler and remove the spark arrester's noise-suppressor insert. It just unbolts, if you have a '96-'97 model. You won't be able to remove the insert on a '98-'00 model without a cutting torch, and then you'll never be able to put it back in when you want some stealth action. The solution is to keep the stock spark arrester (P/N HM-KCY-A2) and also buy one made for the '96-'97 model (P/N HM-KCY-A1). It comes without the noise-suppressor insert, and the exhaust sound it produces will be very loud, so be sure to wear ear plugs. Change your jetting to a #158-162 main and a #58 pilot, with the pilot air screw turned out two to two-and-a-half turns.
Handling is the other aspect of motorcycle performance. For some strange reason, the XR400 comes from the factory with fork springs more suited to a 150 lb. rider, while the shock spring is good for about 180 lbs. Needless to say, the ride is alarmingly unbalanced. The solution is to get new fork springs, suitable for your weight, terrain and riding style. If you're an experienced rider and can tell good suspension from bad, do this right away. You won't regret the expense.
While you're at it, you may want the forks revalved, although some riders prefer to change only one suspension element at a time. If your forks are bottoming, try raising the oil level and cranking up the compression damping first. This is something to discuss with your Suspension Meister. There's no need to buy Gold Valves or other "endorsed by champions" suspension products. Pick a performance shop that you can trust to do just what's needed, and not rip you off with unnecessary work. Then, cooperate with them to get your suspension dialed-in. Good shops will make basic changes to the spring rates, valving and oil height at no extra charge, until you are satisfied. The key is to communicate clearly what you like and don't like about the way your suspension is set up.
One other item that will make a noticable improvement in the way your bike handles, especially on rutted fire-roads and desert sand-washes, is a Scott's Steering Stabilizer. Without ging into detail, let me say that proper installation can range from tricky to downright difficult. If mounted incorrectly, the unit can be damaged and the operation of your motorcycle seriously compromised. Have the unit installed by a performance shop that has had lots of experience with this product. I suggest you talk with people who have already gone down that road, so they can steer you (pun intended) in the right direction.
If you decide to ante-up for this little $400 gem, you'll need to swap your stock handlebars for something with more height to the crossbar (at least 40 mm.), so it will clear the stabilizer unit. A lot of people prefer the feel of Renthal's "CR High-bend" bar (P/N 72201) over the stock XR400 bar, and it will work just fine with the Scott's. You may want to purchase an aluminum alloy bar before you even decide on a Scott's, just so you'll have a quick replacement when (not if) you bend the stock steel handlebar.
There are a number of other things that affect a bike's handling, one way or another. The height of the forks in the triple clamps, the distance between the rear axle and the swingarm pivot, your tires' tread pattern, width and air pressure, etc. These factors all have an important influence upon turning, acceleration and braking. Playing with the compression and rebound adjusters and the shock spring's preload (sag) will also affect the bike's handling on rough terrain.
This information is based on my personal experience with a brand-new Y2K XR400R, plus the friendly advice of my riding buddies and the professional opinions of expert Honda mechanics. It is certainly not the last word on the subject, and your own experiences may differ. Many hours and lots of money can be expended in the trial and error process of finding what works best for you. I wish you much luck in your pursuit of the ultimate XR400R, and urge you to share your discoveries with others.