What to expect with a 300 kit?
 

I'm not unhappy with the 250's power, but....

If I cheap out and buy a 300 piston and cylinder what kind of increases can I expect over the 250 with airbox and other free/cheap mods? 2hp, maybe 3 or 4 hp? Are the increases over the full rpm range?

More loft on the front wheel?

Like I said, I'm OK with the modded 250, but...

Thanks comrades.

I was exactly in your shoes when I bought my used '06. It had a full Muzzy exhaust, all airbox mods, and a DJ kit. It ran very good...much improved over the wheezy totally stock setup that I've ridden off the showroom floor for comparison. But...I rode a friend's KLX300 just around camp one day and realized there was quite a bit of difference...and not just from the reduced weight.

I knew about the aftermarket bore kits for the bike, but I had a great opportunity to get a factory 300 kit from the shop I used to work at for a great price. I also kind of liked the 11:1 compression ratio as I end up in places sometimes where the lowest grade of regular is all that's available. I liked the plated cylinder over a sleeve too, but this wasn't a very big factor in my decision. I've used plenty of sleeved big bore setups with no issue. I installed the kit, and it was very noticeably improved. I mainly notice it in having more grunt at low rpm. I can short shift easier and let rpm drop more while still having a strong pull when I whack the throttle from a higher gear. Climbing was most improved, as its ability to continue to tractor up a loose, steep climb was great without having to constantly slip the clutch and downshift.

It's a little amazing how only 42cc's makes this much difference. I have a little bit of a theory about the piston and cylinder head interface between the 250 and bigger bore kits. Those of you who have been inside the engine and have seen the stock piston probably noticed how the valve reliefs on the 250 piston go well down the edge of the crown of the piston toward the first piston ring land. My theory is that this is not optimum for the flame front and combustion process. The 300 and larger pistons have the reliefs more centralized on the piston crown which I believe improves more efficient combustion. I base this on observations from past big bore kits and modded engines on other models of bikes that I've seen over the years and not from a true engineering study on our engine. Regardless, the 300 kit was a big improvement.

When you mention "cheap out" on using the OEM 300 setup, I didn't really see a cost benefit at retail prices of an OEM 300 setup compared to the BB351 kit. I was able to get a kit through the shop I used to work at for a really good price, but at retail it wouldn't have been cheaper.

I did a 300 (actually 309) kit a year ago and love it. Bought a used cylinder, had it punched out to 80mm & plated, got a matching piston/rings. Runs great. If you think about it, adding 50cc (approx) is adding 20% (approx)
I'd do it again in a heart beat as your not stressing the bottom end as it was designed for the 300 from the get go.
This is what it should have been,.

Actually didn't the engine start out as a 250 in the KLX250R (or maybe 250D ?) from back in the stone age, was then punched out to a 300 and then back to a 250 in the S and SF trim.

But the engine was so overbuilt/engineered from the beginning that it seems that any size jug you can fit on it won't overstress the engine.

All i know is that my 331 kit totally transformed my bike, thing is a (relative) wheelie machine/torque monster, especially when compared to when it was a 250.

Thanks for the input. Very well said.

By cheap out with a 300, I mean buying used 300 piston/cylinder and making it work rather than an assembled BBK.

The truth is I had a KLX300 with a 331 BBK (from the PO) a few years ago and it was a little monster. Wheelied anywhere 1st to 3rd and in 4th with some technique - even better than my KDX on steroids (but heavier). I know I don't need quite that level of improvement over the 250, but I'd like to have a noticeable jump in "stonk" especially down low.

If I were gonna throw down the $$$ for a BBK, I'm pretty sure I'd go for the 351, but if a couple hundred for a 300 cylinder/pistion will get me where I want to be then I'll just do that. Top end, I'm happy with the KLX. In fact, the road that runs from town up to my house is a long straightaway with a steady modrate incline. My TE610 didn't like to hold 6th gear up that hill and I'd downshift to 5th but the 250 holds it just fine in top gear.

What I didn't want to do was throw down a couple hundred and then say - why did I waste that? If the cheap 300 mod realistically gets me halfway between the 250 and my old 331 then I would consider that money well spent and just about spot on for what I want out of this bike.

Thanks again for the comments and please keep them coming.

Not sure where you would find a used cylinder and piston for $200. Personally, I would hate to do all that work and then not be satisfied and do it all again.

I watched ebay for nearly a year looking for a cylinder to use as a core for the 340 kit. They kept selling for nearly the pirce of a new one. Finallly I gave up and bought a new cylinder and piston, expecting to use my original 249 as a core for the 351 kit. The 300 (292) was very disappointing, but then again, I'm an old fat man that lives in the high desert. I could feel the power increase, but it just didn't justify the time or cost. If I were to do it again, I would do it once, and go as big as it would go.

it will have more grunt but will spin up slower as it hast to pull more air in i personaly am not the kind of guy that throws displacment at a problem to fix it will it work yes but it's not the same as actually getting more from what's there, the cost is affective on a BB kit but just not the kind of motor i like to ride, when i first got the bike i rode it for a year then had the guy who buolds my xr650r's up at it it cost me what the BB kit does(not including parts) but it is a better bike in all areas because of it, he cleaned the head up (port& polish) cleaned the chamber up(he may have put a 300 head on i'll have to ask him), took wieght off the crank and balancer (big for making it snapy) but still smooth for street use, he found me a piston that keeps it around 250 but bumped it to 12.5:1, web cams and valves and lightened the flywheel, i have the pumper carb and full fmf megabomb and q4 and no smog stuff and the bike is fun to ride not quite an R bike but what it lacks there it makes up for in being bulletproof ave tons of hrs and miles of hard offroad riding on the mill and it still chugs maybe some day i'll put a 351 on it but it does what i need and thats what i want from it for now

Huh ?

the bigger piston will draw more air through the same intake port thus incresing the vacuum and that slows the motor dow because it has to work harder to draw air

Not necessarily. What you're saying has a generally correct assumption, but it really all comes down to how the OEM intake design works with the OEM piston size and stroke. You're assuming that the intake design with its valve size, port size, and other such factors was designed as an exact and optimum match to the piston and stroke. While Ferrari might spend the time and money to seriously optimize those factors in their latest V12 engine, it might be a stretch to assume that Kawasaki pursues the same diligence on all of its motorcycle engines...especially something as lowly as our KLX. I think we're mainly discussing volumetric efficiency here. Some engines have more optimum VE than others in their OEM designed state. Our KLX might indeed have its best base VE as a 250, but it could just as well be at 292cc.

I'm not bringing this up for argumentative purposes but just for discussion. And really, it's probably all a moot point, as we're aware that while base VE based on an original design might be optimum under the original conditions, the increased torque and horsepower that can occur when you make engine mods ranging from intake, exhaust, and bore size are real and just play a different tune with VE. I'm no engineer, and I've probably made bad hash of what I'm trying to explain as I understand it.