The KLR250 has a nice compression ratio (11:1), valve train layout, and a generous rev limit, even if it seems like it is in no hurry to get there. All the counterbalancers that make it such a civilized engine contribute in small part to this, but by far the major culprit is the non-generous external intake and exhaust systems supplied with the bike. Well really it was just the carburetor they picked, after that there was no need for an exhaust system that breathed more freely than the carb. Not that it is a bad carb, it is just that CV (constant velocity) carbs are selected primarily for emissions reasons and not for their performance capabilities, or lack thereof.
Strangely, there are not a great many performance enhancing parts available for the KLR250 considering how long it has been in production. At least that is what I used to think, until a did a little digging around and discovered a distant relative of the KLR250, one that still scoots around on all fours. The KSF250 "Mojave". Thanks to this fine machine there is hope for those whose search for more GO will not be satisfied by any number of mere bolt-on power aids. Thanks to the quad craze in the U.S. and shared top-end components your KLR Jr. can go from mild to wild!
Let's start at the beginning.
First, make sure you are getting the most of what you got; a tune-up can restore lost vim and vigor, and make starting the bike easier. Clean or replace the air filter, change and/or gap the spark plug, and check the valve clearances. Checking valve clearances are vital to the suvival of your KLR, as valve train failure seems to be the number one killer of KLRs. Instructions on how to perform this task can be found on the "TOOLKIT" page.
Second is to consider a gearing change. What does that have to do with making power? Well nothing really, but it can make a big difference in the perception of power delivery. Think about the riding you do, do you do little or no highway speed riding? When you trail ride do you ever make it in to sixth or even fifth gear? And if so, at what RPM; the engine was probably turning pretty slowly compared to how much RPM range it has. Then you should consider lowering your gearing (technically raising the ratio) to take advantage of the greater power and better throttle response that generally comes with a little more RPM. Try dropping one tooth on the countershaft sprocket (stock is 15). That is about the same as changing to a 47 tooth rear (stock is 44), or fine tune it with something between 44 and 47 in the rear, but changing the front sprocket is easier and cheaper. (Also, you are, of course, supposed to change both sprockets and the chain all at once, or else the old parts will accelerate the wear of the new parts, but before you drop all that money why not experiment with the countershaft change first so you know you will be happy.)
The next step is bolting on an aftermarket performance exhaust. There isn't the plethora of choices out there for the KLR Jr. that there is for other bikes, but there are a number to chose from. Understand that there is one concession you make in doing this, that is, that many aftermarket exhaust systems are louder than stock, some quite a bit louder (the standard Cobra, for one-now out of production). It is, however, a fairly straightforward procedure and can make a noticeable improvement in power. As an added bonus this usually sheds several pounds from the bike as well.
The second half of this step is to re-jet the carb to take full advantage
of the flow capabilities of the new exhaust. This isn't always necessary, but
you may notice that your new pipe makes your bike pull harder at
some rev ranges, but that it has developed lazy, or flat spots at other rev ranges.
Turns out the carb just cant feed the motor as much as it wants now, if it could
the motor would make even more power for us. So do it! Jets are reasonably
priced (which is good as you may end up trying more than one). KLR250's have
come with a number of different sized main jets over the years so the amount
of change needed will vary (also depends on the exhaust you picked), but a
few jets in the mid to upper 120's should get you in the ballpark, with 127.5 being
most often recommended (better off going a bit rich, that is larger, than lean).
The main jet is on duty from about half throttle on up, so if you still have a
flat spot just before the main comes on line you will want to try shimming up the
needle in the slide (it is not adjustable like most carbs) with a very thin
washer in order to richen up the mid range. It might also be possible to use the
needle from the KLX650R/300R which is the normal adjustable type (the 300 uses the same
carb); does anyone know if it fits? You may also want to adjust (usually richen, by unscrewing)
the idle mixture screw, which is located up inside a cylindrical metal protrusion on the bottom,
engine side of the carb. KLR650 owners rave about the just off-idle improvement in throttle response
this little adjustment can make. To gain access to the idle mixture screw you will have to drill out
the anti-tamper tin cap (see photo for location), but be careful that you don't chew up the screw in
the process. A KLR650 rider has passed on another secret; you
can improve throttle response by enlarging the carb slide vacuum port, but can expect
to see a loss in MPG. Better do it a little at a time and let us know what size drill
worked for you. Others have said they achieved better throttle response by cutting off
up to four coils from the carb slide spring, reducing the amount of preload on the slide.
I could never fully explain how to jet a carb here. There are web sites dedicated to the science and art of jetting a carb, and many books available on the subject if you really want to get your brain around the process. When you do get it running right, tell me what you ended up using.
Or take the easy route and get a K&N or Dynojet Research Stage 1 Jet Kit for the Mojave; be sure to get the one without the air filter as they are different. [If you are considering using the Dynojet kit it has been reported that the KLR250 did not like the slide needle that comes with it, and had a nasty misfire (lean?) so the owner just shimmed the stock needle-don't know what exhaust was in use.] [Another rider has reported that the K&N kit, in conjunction with a FMF exhaust worked quite well.] Even without a new pipe K&N promises better throttle response and up to 5% more HP.
For many people, that will produce enough results to live happily with their KLR Jr.
The final step involving bolt-on performance mods will probably make the most difference, cost the most, and be the most difficult (unless you have a shop do it, then it just adds to the second). That is, changing carburetors. This will make the biggest change because the CV carb is what gives the KLR250 such soft power delivery, that is, when you snap the throttle open the bike does not respond very quickly (although this phenomena probably makes the bike easier for beginners to control). As long as you are spending the money and time on a new carb to improve throttle response, you might consider skipping the regular round-slide type and going straight to a flat-slide, or give some serious thought to getting a "pumper" carb. The latter is one that has an accelerator pump which gives a little squirt of gas when you snap the throttle open allowing the engine to respond quicker than it could on the changing manifold vacuum alone. (One downside is that no carb is going to give you the same great mileage that you are getting from the stock CV carb.) What can make this changeover so difficult is that no one makes a bolt-on kit yet. You will be changing the throttle linkage setup and will probably find that the external diameter of the inlet/outlet and the length of the new carb may not be the same as the stock one. Which means you may have to modify the airbox to carb hose and/or the intake manifold, etc. At this point, depending on how much water you encounter and how much you are going to operate the motor at higher RPM's, you may want to consider removing the snorkel and or adding some of the drill and pop-in vents to the airbox to reduce what is now the most restrictive component. After you have the carb installed you may then need to re-jet the new carb to take full advantage of it. Once the pain has passed, most people who have switched, especially to a pumper, swear it was worth it.
Your once docile KLR Jr. should now have more kick when you goose the throttle, and pull noticeably stronger across the entire RPM range.
At this point, if you still thirst for more power you will have to operate on the engine to get it. You
will also find at this stage that short of putting in a big bore kit and nothing else, you may actually start
to loose some power at lower RPM's while gaining most in the mid to upper RPM range. Additionally, some of
this work you cannot do at home, unless you own a machine shop, and even swapping camshafts requires a fair
degree of mechanical ability/accuracy to be successful.
A big bore kit is a sweet idea; more displacement translates to more power across the entire range with no sacrifice in ease of rideability.
Camshafts are available for the Mojave (and so the KLR) in a variety of different valve lift and duration combinations (grinds) and most, but not all, of them can be installed without any other cylinder head modification. Usually they increase the power in part of the RPM range while giving up a little in another, although depending on what cam came in your KLR (they differed by country) they may actually increase the power across the board. Be sure to ask if the cam will work well with the other mods (or no other mods) that you are making; it is hard to tell what brand cam will suit you best, without actually using them, or riding someone's bike that has the exact same mods you intend to make.
You may need or want better valve springs and retainers at this time too, ask the cam maker.
A higher compression piston can really bring an engine alive, particularly in conjunction with the right cam. You might find the bike a little harder to start, and you will have to always use premium, race gas, or aviation fuel. Please note that combining higher compression pistons (taller) with higher lift cams can be deadly to your motor if not done correctly as the valves can meet-and-greet the piston in a most unwholesome manner.
By now your KLR Jr. should be faster than its brakes and suspension.
Stiffer rear springs should be available from Progressive Suspension and Pro Action Suspension. Don't forget to try the different rebound damping setting on the rear shock! Works Performance has made a replacement for the rear shock. Progressive Suspension makes springs for the forks (see the "LINKS" page), and you can also experiment with different weight oil to find the right ride quality/dampening that you want. Stock oil is 10W20 at 190mm from the top with the springs removed and the forks fully compressed. If you bottom out your forks a lot, you may add another 10mm of oil. For smoother action, particularly at lower speeds, consider using synthetic fork oil. The manual also says that you can run from zero to 36 psi of air in the forks (DON'T be tempted to run more air) and that should make quite a difference.
Several manufacturers make pads for the disc brake including DP Brakes, SBS (Scandinavian Brake Systems), Parts Unlimited, Vesrah, EBC, and BraKing. Some give you more than one choice of friction material to provide more stopping power and/or fade resistance than the original pads. The front pads on the KLR250 are the same as the rear pads on the KLR650. If you can find one, or have one made, a stainless steel braded brake line for the front disc usually improves the feel and stopping power during heavy braking.
Let me know what products/setup did or didn't work for you.