The Kawasaki KLX250s is a versatile combination of playbike and full-on trail burner. Right out of the box, the KLX is a willing companion, both on the dirt and on ice! It is an un-intimidating ride with a well-balanced chassis and handling. But, like many bikes, a few accessories and upgrades go a long way to improve an already good machine.
Most of the parts I installed are available from Twisted Throttle. If you decide to buy any of the items I mention in this article, please buy them from Twisted by clicking on the links on this page. That way, I get a small affiliate payment that helps me continue to write and share helpful articles like this. Thanks.
Falling down is often a part of off-road riding, so it makes sense to protect the bike from damage. It’s bad enough to bend a lever or handlebar, but it’s worse if the damage strands you out in the Boonies.
Here is a part that is vulnerable to damage and expen$ive to repalce. Kawasaki thinks that the plastic fins are good enough to thwart an attack by a roosted rock or an errant stick or keep a radiator from being smashed in a fall, but they aren’t…trust me. I toasted a radiator on a KDX200 many years ago during a routine tipover and learned my lesson.
SW-MOTECH also made the skidplate that I installed to replace the OEM plate. The SW plate is made from a 4mm-thick aluminum base plate and 3mm-thick side plates. I’ve whacked some seriously solid rocks with this plate with no consequences whatsoever (see photo).
Handguards should be at the top of the list for things to install. Not only do they protect your vulnerable metacarpals, but they also keep your levers from bending or breaking when you fall or rap a tree.
The Barkbusters are the original handguards and come with various shaped plastics. The VPS plastics with upper wings provide good wind and brush protection without being too large.
Yes, the KLX comes with a handlebar. But, it’s a P-O-S. It will bend the first time you tipover…guaranteed. The ProTaper bar I installed has taken two significant hits and they are still like new. There are many bends to choose from, but the Henry/Reed bend is just right for me.
Dual Sport Grips
Dual sport grips provide a balance between comfort and grip for all-day control. These grips lack the ridges that are found on most off-road grips (including the stockers). Even though they aren’t as secure, I like the comfort these provide (See photo and link).
The stock mirrors are easily damaged in a tipover, so you may as well put them away now and buy an inexpensive folding mirror that can be tucked away when you don’t need it and extended when you have to do some pavement riding to get to the next trailhead.
My tires of choice for both off and on-road are Pirelli MT-21s (front) and the Dunlop 606 (rear). They aren’t the best off-road tires but are full knobbies. They aren’t really made for much pavement riding, but offer a great (90% -off road/10%- on road) balance.
The Ice Tires I use are Fredette Canadians. These are the best combination of grip and slip so you actually learn about traction management…as opposed to the Marcel tires which grip so hard that you can throw the bike in without a care. (aka, Cheater tires)
Big Bore Kit
OK. As great as the stock KLX is, it became apparent that it could use more power when it struggled to ascend a particularly steep and rocky trail at Hatfield McCoy. The power challenge became even more of a hindrance when I started riding on ice where the vast expanse of frozen water begged for maximum drive. While I had a boatload of fun on the rock hard ponds and lakes with the stock motor churning out every bit of its 17hp, I decided to install a 351cc big bore kit from Blue Bill.
The reasonably priced $535.00 kit includes a new re-bored cylinder, piston, rings, wrist pin, and gaskets. Of course, I could have sold the KLX and bought a bike with more displacement, but that would have certainly cost a lot more money. Besides, I like the KLX’s character, so I went for it.
Horsepower and torque increase is modest at somewhere in the 5hp and 6 ft lb range, but that is a 30% and 40% increase from stock (I suck at math, but that’s close enough). The result was noticeable once I went through a short break-in period and was able to open it up. I can now whack the throttle open and the bike responds briskly.
Installing a big bore kit isn’t very difficult, but it takes some mechanical inclination that consists of more than remembering “lefty loosey, righty tighty”. I’ve done my share of moto-surgery, so I knew what I was getting into. Still, it’s unnerving to remove the head, camshafts, cam chain, cylinder and piston ,and then get it all back together again without any important parts leftover.
I disassembled the bike following the Kawi shop manual and all went well. I removed all the leftover gasket goop from the case with an oiled Scotchbright pad and was ready for my friends Adam and Jay to show up and give me re-assembly help. After 4 hours or so in a cold garage, the motor fired up and sounded good. I was scheduled to ride the next day on the ice in New Hampshire, so I would soon know if we did things right.
Jetting and Exhaust
Some jetting was needed to manage the extra displacement. I added two washers underneath the needle clip to raise it and replaced the 118 main jet with a 125 and changed the slow jet from a 35 to a 38. The air screw is turned out 2.5 turns (you have to drill out the EPA plug the get jet access). That combination worked great right out of the box using the stock exhaust.
That’s right, I’m sticking with the stock exhaust for now. I value a very quiet bike, especially when riding off road. Yes, the heavy stock can and header pipe is certainly holding back the power potential, but I’m okay with that for now. I’m told the hot setup that is also reasonably quiet is the FMF power bomb header pipe in combination with the FMF Q4 muffler.
I made sure to warm the bike thoroughly in the 10F temperatures before taking the bike for its maiden voyage on Hoit Pond. It felt good, but the instructions were to keep it at below half throttle for 100 miles before going WFO. Using conservative throttle, I immediately felt the increase in torque, but not much in the hp department. That would come later.
I managed 4 sessions (about 40 miles) at no more than half throttle before I slowly began opening it up to break it in the way racers do: hard. It ran great and pulled strong. I was pleased. The additional power meant I could use different techniques for getting the bike turned; namely whacking the throttle mid corner. This would hook the bike up nicely to finish the corners. The old motor couldn’t manage this feat. Cool beans.
Why a KLX?
My previous off-road bike was a 2000 DRz-400e. The DRz was a great bike; it had lots of rear tire spinning, wheelie-inducing power, but was a beast, especially in tighter trails. I now know the benefits of a more docile bike with a lower seat height and civilized manners. The KLX is a great platform that just needs a bit of love.
The bike protection goodies are a must-do if you don’t want to damage vulnerable parts and possibly become stranded in the middle of the woods. And the Barkbusters are critical to protect your hands and levers. The stock handlebar will likely bend if you look at it wrong and the mirrors will break if you scream too loud. So, get those things taken care of ASAP.
As far as the power upgrade goes, I think it is a worthwhile way to spend some money. The stock motor is reliable, but barely adequate when the going gets tough. It’s a great motor for any around town riding and level off roading. If you are a fast off road rider you won’t be looking at a KLX, so no need to compare this bike with a KTM or CRF-X.
No, this is for the person who rides both on and off road and wants a bike that is easy to ride and instills confidence. This bike is still too tall for a lot of riders, but is perfect for most middle to light weight folks of average height.
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7 Replies to “The Best KLX250s Modifications”
Hello. I hope you are still answering comments. I jut upgraded my 2009 KLX to the 351. The stock main jet was 118. Following Bill Blue’s suggestion, I went up to a 120, and then tried a 122. The 120 worked best, but the bike only performed when the original snorkel was in place. It runs great that way, but I know choking down the intake is not the answer. When I read your article and several others, it seems that a larger main jet and some adjustment to the needle is a must, but I just can’t figure out the right size for the main jet. Could you tell me your elevation where you live? That might help me understand which way to go. I live at 5300 feet above sea level. I pulled out the spark arrester junk in the exhaust. Just wondering what you did with your air box, and if you have a recommendation for a main jet at a higher altitude. Thanks in advance!
I see this is an old articl but I’m glad I could still access it and hope you can still reply.
I am a new rider of an ’09 klx 250sf. Trying to get an accurate read on mpg is tricky (if one is not inclined to drain, measure, etc.) w/o actually running out of fuel. (any walk home is too long:)
Can you help w/some accounts of what you’ve experienced or heard re: mpg on this or bikes similar?
Lee, I’d plan on getting 60 mpg. I always planned on getting gas at around 100 miles. But I think I went 120 once. The fact that you have a reserve petcock allows you to get a good idea of the range.
Great write up and awesome that u did all the upgrade urself.
I just upgraded to 300cc kit with a performance chip and new sprockets.
I tested the 0-60mph time before and after the upgrade using a GPS linked accurate app. Before was 10.5 secs after was 7.5 secs. A hell of a difference!
Will add a powerbomb full system today to get a bit more
Good commentary on the versatility of the KLX250s. Concise and to the point. I definitely miss blogging instead of making motorcycle videos…
A real common sense article for a change.. Except for ripping your bike apart and putting in a big bore kit when the wax isn’t even dry on the bike, this spot on… Good job .
I have a 2010 with Factory Pro jet Kit, FMF Q4 slip on. Bone stock air box and emissions. I was very impressed with the outcome. Following jetting instructions and finding the very best jet or setting for each step is crucial. Not only did it open up the bottom end but it increased fuel mileage by 15%.
I can appreciate the quiet stock exhaust but the usable torque is worth the noise. I’ve thought about trying the quiet core insert ( and probably will soon) expect to loose a bit but the quiet might be worth it. I used to be able to sneak up behind people on the trail but not any more. The FMF Q4 is quite a bit louder than I was hoping for.
Rode with a buddy a few weeks ago, he 09 350 bore kit, fancy pumper carb supposedly built and tuned for KLX with 350 kit FMF Q 4 and power bomb. I think some one made a mistake. I had no trouble keeking up on the highway or the gravel roads. As a matter of fact I had to keep slowing down behind him. His body language said to me he’s cranking the throttle but he’s not accelerateing like it should.
So my point is the pipe and a carefully tuned carb using a quality jet kit make a big difference to a stock KLX 250S. I had thought about the bore kit before I installed the pipe but no more. If I want that much power I’ll ride my DR650.
Great article thanks Jim