After deciding that my 2007 KLX250s was too slow to safely keep up with cottage traffic on the highways that lead to the neat adventure riding in my region, and my finances needing to be dedicated elsewhere preventing the purchase of a larger adventure bike I decided my KLX needed more power. After a few months of research and reading KLXSTER extol the virtues of the stock 250cc engines capabilities, with a proven modification scheme, I decided the most logical option was to bolt in a relatively unknown quality AHL 300cc kit and venture deep into the unvalidated areas of KLX modification. In retrospect it would have been a lot easier sticking to the stock displacement and the validated mod path, but the extra torque is NICE and I seem to need to do things the hard way.
I have only one brief ride so long term durability will have to be updated after some milage.

The 299cc AHL Kit
What prompted me to order the AHL kit was the (admittedly sparse) positive reviews online and the promise of a $250(CAN) bump in horsepower and torque. I figured I would give my 2cents from a product design background to help out other people looking at these kits.

The Good:
Dimensionally the components of the kit were good. There was what I suspect a mean shift in the tolerances by the Chinese supplier on the looser side of fitment compared to the OEM Kawasaki parts. Still in spec, but definitely on the looser side of things. The caveat here is that I have one component set to measure so I have no idea how tight in tolerance the supplier is making these statistically, but my kit seemed pretty decent. I didn't measure any taper in the bore and my final average bore diameter from 9 measurements was 78.02mm, which is slightly out of spec but I wouldn't be surprised if my cheap bore gauges threw off some of the measurements. Still within the service limit so I was ok with using it.
Machining finish on the cylinder seemed pretty good as well*, and I didn't have any concerns with the bores, deck or piston dimensions or finish.
Seems like a cheap way to purchase a new big bore cylinder for the KLX.

The Bad:
This kit is definitely NOT OEM Kawasaki quality. It probably shouldn't be a surprise at that price but the only component I ended up using was the cylinder. I ordered OEM gaskets, piston, wrist pin, rings and circlips (~$250 CAD). I think it was a wise spend after comparing OEM to the kit components.

CYLINDER:
I had to return the first kit to amazon (prime for a KLX big bore kit is WILD) since these kits are packaged stupidly. AHL sends the cylinder nicely protected in a solid carboard box with precision cut foam to prevent shipping damage, BUT THEN put the piston into the bore for shipping (and the wrist pin on the first one I received) directly into the cylinder. without protection. As you can imagine, 10000km later, there is damage from the vibration to the cylinder finish. The first kit I received had dents from the sharp edge of the wrist pin smacking into the cylinder wall, making the cylinder unusable. If you order one of these kits, be VERY thorough in your inspection and don't accept anything less than perfect. To deal with the damage to the surface finish on the second cylinder, I had my friend who owns a Corvette race shop quickly ball hone the cylinder to make a consistent cross hatch for break in. I do wonder how long the coating will last but it seems like people have had luck in at least the short to medium term.





The casting itself was acceptable but uninspiring. There was what appeared to be some shrinkage porosity on the machined deck surface. It wasn't severe enough for me to be concerned, but it was not the same quality as the OEM cylinder. There are minor differences in the casting which makes it evident these were not done on the OEM molds, but nothing that I found concerning. No idea on the heat treat, but when filing the lower tensioner hole to clear the spring (supplier is STILL not cutting these deep enough), it seemed to be fairly hard by aluminum standards.

Also note that you may want to paint the cylinder to match the rest of the engine as it is not painted from the kit factory. I was not patient enough...

Piston and Rings
The piston was what prompted me to really start scrutinizing this kit. For reasons I cannot find good engineering reason for, the supplier has machined horizontal grooves around the skirt. I spent hours googling to try to understand why these would be there, and I could not find any valid explanations. They look to me to be more effective at scraping oil off the cylinder walls than maintaining a proper oil barrier. Side by side with the OEM piston, there are some minor differences in bowl and valve relief geometry, but I don't think it would have made a huge difference. Had it not been for the grooves, I would have probably used the kit piston, although I later felt relieved that the part flying around underneath me at 10500 RPM was going to be an OEM part with likely very good QC checks.

The rings were loose and beyond the specifications of the manual. Not beyond the service limit so they would have worked, but not ideal.






Wrist Pin
The wrist pin dimensionally was perfect and external finish looked great until I looked closer and compared to the OEM part. The OEM pin has an even more refined external finish than the kit, probably not enough to majorly change function, but I would not be surprised if the OEM pin provides a longer service life. Where the kit wrist pin gets ugly is the internal machining. The OEM pin has a quite beautiful mass reducing design where there is a noticeable taper at the ends of the pin that smoothly transfer to the max wall thickness where the wrist pin rides on the con rod and needs the highest stiffness. This is a very elegant way to save mass on a part subjected to high G cyclic loading. The Kit pin on the other hand is a simple bore the entire way through adding mass and stress on the con rod. The machining is pretty ****e too, with a rough finish and right at the highest stress point, the bore shows evidence of being drilled from both directions and not matching up perfectly. I could see this being a stress riser and ultimately a point of failure.




Gaskets
After seeing the last few items I decided this was going to be an OEM build where it wasn't cost prohibitive (I wasn't going to fork out $800 for a new OEM cylinder when a bill blue 331 wasn't going to be much more). The head gasket also seems to not with the coolant holes when I held the OEM 250cc gasket up to it. OEM gasket fit perfectly.


THE BUILD
The build was fairly straight forward once all parts were collected. It took me a weekends to do since I was being particularly **** and cleaning everything and procrastinating the scary bits. The OEM circlips are substantially stiffer than the kit ones, so order OEM ones!
Compared to working on cars, motorcycles are calming and everything is solved with an 8 or 10mm socket. Follow the manual, work carefully and you should be golden.

TUNING
Tuning is on going - 2025-04-18
Current set up:
250m (820ft) elevation
Lidless
TBR slip on
OEM header
DJ 138 MJ
Kehin #38 pilot
DNO352 Needle - 3rd Notch
NO MCM (yet...)

2025-04-28 - Tuning update:
DJ 144 MJ
Kehin #40 pilot (38 is too lean)
DNO352 Needle - 1st Notch
Fueling feels decent, now for some break in mileage.

So far I have started the bike and done a brief ride not beyond 1/4 throttle (for the most part). I am using the DynoJet 2206 kit (KLX300R kit), which in retrospect would have been a lot cheaper to either order the Kawasaki performance needle and appropriate DynoJet main jets.
with a #38 Keihin pilot jet the bike is running quite well at idle, with approximately 2 turns. It seemed to have a slight bog when I couldn't help giving it a little more beans so I might have to move the needle once I get the main dialed.
I am looking to order a 14.7 SIGMA SLC FREE wide band O2 sensor and controller once they have wideband sensors available again to get that sweet sweet AFR data. I did look into using a Microsquirt ECU as a DAQ and patching into the crank POS sensor, throttle POS sensor (It might be useful after all!) and wide band to get some higher fidelity data but I am not sure I can be bothered to spend the time and expense figuring out how to get all those components to play nice with each other.

 

No need for all that other crap. I did that work for you.
Don't use big throttle until you've set up the carb correctly - what you have now will work for your low RPM break-in period.

Assuming you are near sea level, you will need a DJ144 main jet.
You will need to verify that the slide spring is stock and is stock length, and the slide lift port is stock in size. See my troubleshooting link below for help with this.
If the spring is not stock or you cannot determine such, you will need to order a stock slide spring.
If the slide lift hole is oversize, you will need to repair it back to the stock size.
Set the needle clip on the topmost notch with the 2 washers on top of the clip. ( To clarify: This makes that DNO352 as long as possible meaning that it will stick down into the needle jet at the bottom of the carb barrel as far as it can.)
Install the DJ144 main jet.

Using stock sprockets, enjoy pulling second gear power-only wheelies. Third gear with a tug on the bars. And enjoy running off and hiding from the newer EFI KLX300's. WIth the TBR+stock header and lidless airbox, you'll be a bit over 26 hp.

Assuming the carb is not leaking air ( a vacuum leak of some sort) and that the slide is functioning properly, a bog upon throttle snap is caused by the pilot system ( not the primary fuel system)
Such a fault in the pilot system is usually clogged ports and clogged parts. Anything within the pilot system that keeps the proper amount of fuel from flowing thru the transfer ports when the throttle is opened, will cause that bog.

Again, the link below is your friend with all this..

 

Also, do not do the MCM until you've fully immersed yourself in the new powerband. It will probably not be needed. Also, I am unsure of the piston, therefore valve clearances, of that kit.

It may be that doing the MCM on that piston is totally experimental - so it should be done with great care given to testing before firing up the engine.

 

Thanks for the feedback Klxster.
I decided against doing the MCM while I was in the engine since I couldn't find what I thought was conclusive data on its performance on the 300cc displacement. Marcelino seemed to imply that it might not improve much with the larger displacement. I am assuming he was thinking choked flow would occur before the VE advantage could be realized? I haven't done the calculations so I am not which is likely to occur.
With the stock header, and my need for highway RPM power I thought the stock cam timing and lobe angle might match my use case better anyways.
I used an OEM 300cc piston, not the kit piston so it should be OK, but I'll check clearance on your advice if I do go down that route.

I'll give your set up a try, I have the jets ready. Slide should be stock but I'll double check since the previous owner put a larger main jet on when they did the slip on. Everything else in the carb was stock and the original snorkel was in place too so I would be very surprised if they bothered to only modify the spring. I didn't see any evidence of it being cut either. Might have been my imagination so I'll run some tests this weekend to determine the conditions that subtle step up happens at.
Do you have an AFR chart with 300cc displacement and the 144DJ set up? If I have missed it my apologies. I've seen the 250cc development charts and AFR recordings. My concern is the larger pressure drop @ WOT with the same diameter carb, plus some anecdotal reports on here of running rich with the larger 140/144 DJ jets on 300cc displacement bikes or am I misunderstanding something?
I'm inexperienced in engine tuning but have enough book learning to be dangerous haha. I appreciate the wisdom.

 

Ahh, I apologize in advance as I've become old and grouchy.

You've drawn a lot of incorrect conclusions and positions - no doubt due to the usual reasons: You're eager to learn and have been digesting unverified information from unknowledgeable sources.

For instance: With the stock valves and induction tract, the MCM is a mechanical (not mystical) enhancement of VE from off idle to approx 7k rpm, compared to stock cam timings. However, VE is slightly reduced from approx 7k to 10.5k.
The size of the piston plays no part in this.
Here is Marcelino's quote:
Edit : Due to the shape and especially the size of the intake duct this mod will yeld the best results on a KLX250 , only moderate results on a KLX300 and it provides little to no benefit on a 330 or a 350 . The 330 and 350 allready makes more lows and mids so the cam mod is also pointless on these displacements .
His opinion regarding 300cc (and up) effectiveness can only be based on a belief that the induction tract will begin to further limit effectiveness at some point in the upper RPM band - such has never been verified as true or false. What is certainly logical is that BBK's already make biggy TRQ in the low to mid range and further enhancing this range may not be needed.
If it is ever proved that a BBK, with stock cam timing, cannot "breathe" above 7k, it would certainly be due to induction tract restriction - and, IMO, the MCM might as well be employed.

Pressure drop / vacuum increase and the effects on the CVK... Yep, I remember reading that crap too.. That is not the way the CVK's primary fueling system responds to increased "draw" - but now that you've been "corrupted" to think that way, perhaps it is best I point out that the lidless + full exhaust requires a DJ144 and same with stock header only required a DJ140, in order to keep WOT AFR below 13:1. The reason is because of increased "draw" thru the carb with the full exhaust mod combo.

There are no dyno charts for the two lidless combos on a 300 cc. Going up 2 jet sizes from what a 250 requires is logical - anything else is not.

My '07 KLX300R is Lidless+ full FMF exhaust, running the lidless CVK setup using DNO352 and a DJ146 main jet. It's a riot !

 

The effects of a shortened spring or a DJ spring, and/or an enlarged slide lift port, will be horrible overfueling in the low to mid range @ WOT.
Best to get a new spring from Kawa..

 

Thanks for the AHL kit review! You are very picky to details haha. Agree that AHL finishing is not perfect and shipping piston inside cylinder is not the best idea. But(!) my standard AHL kit did 7000km with zero issues and almost no wear. And I used everything from the box - gaskets, rings, etc.

 

I just finished installing the second kit on my KLX. I am very much a novice mechanic. The last time I was into an engine was in 9th-grade shop class. I am now 66--so it has been a minute.

I messed up the first install, and the middle ring got caught between the cylinder and the piston. The ring gouged a replica of the Grand Canyon in the side of the cylinder. Other than using a boatload of oil, it ran great.

I started again with a new kit and wrapped it up a few weeks ago. I am still in the process of fine-tuning the idle and slowly breaking in the engine. The weather has not been very cooperative.

I used the supplied base and head gaskets this time.

It has been a journey, but I've learned a lot and gained confidence in my abilities. I successfully tore into my Triumph Street Scrambler 900 last summer and replaced part of the internal shift mechanism. I would not have done that before installing the AHL kit in the KLX.

I purchased the 2009 KLX in 2020. It was running without a lid, an FMF Q4 pipe, and a stage one DynaJet kit when I bought it.

I wanted a quieter ride, so I reinstalled the lid with a KDX snorkel and went back to a stock pipe.

The only problem at the moment is a bit of idle wander. I just adjusted the air screw out 1/2 turn and it seems to idle much better in the garage, but I haven't been able to get out for a ride since I played with it. Lots of rain here this weekend.

 

Great review! Thanks!