I am curious if KLX300 transmission guts will fit in a KLX250s transmission. I would like to get away from the big jump from 1st to 2nd gear. I have a 50 tooth rear sprocket and a 110 100 18 rear tire. I do more trail riding than road riding and find a huge jump from 1st to 2nd gear to be an issue on some hills where 1st is too low and 2nd is to high. Thanks for any input on this.

 

Can't help on the transmission question, but with the gearing, I run 14 front and 48 rear, but gave it a try as 13 and 48, which would be similar to yours assuming you are running 14 front. I found that 1st was almost useless, and all the other gears seemed too short as well. i.e. seemed to be changing gears a lot more. Went back to the 14 front and everything is better. Just suggesting that you may want to try a 15 front to improve your gearing a bit. Might find first better. I have not tried a 15 front, but others on here say it fits. I plan on getting one for when I have higher speed and less challenging rides. Same chain will fit with a 15 48, so should also fit a 50.

 

In a quick check of part numbers, the 06-08 KLX250 main and lay shafts match up to the KLX300, you would have to check gear part numbers to see if the gears are different or not, may not be.

Not so with the 09 and up KLX250, they don't match up with the earlier 250 nor 300.

Cases show different for both the early and later 250, not to mention the 300. Could be the finish or some simple change, I don't know. Only way you would know from there would be to actually get the gear box and try it. Or if someone here has done that.

 

OP, I'd be surprised if the the transmission parts or entire transmissions would have changed any that would prohibit a swap. I'd bet gear tooth numbers have remained the same too. That's speculation on my part, but on a model like this Kawasaki usually does little but fine tune certain aspects of the bike while leaving the foundation alone.

I would not be surprised that part numbers change due to something as simple as a change in the heat treating process of a specific component, but klx678 is right in that tiny things on the main shafts like a circlip location or dimensional change in a weaker cog or even the angle on the "dogs" on a gear will change the numbers without prohibiting swapping parts.

I would be somewhat confident that swapping the entire transmission gears and shafts from the 300 to the 250 would probably work just fine...confident not positive. I think most likely someone at Kawasaki would be the only one who could absolutely guarantee that question. Just looking at the issue of the parts having different numbers won't mean the parts won't interchange. Kawasaki has done little to change the foundation of this engine in all of its variations and years. It just wasn't economically viable for them to do so and remain at this price point.

Edit to add: I run a 14/50 gearing setup on my '06 250 with a 300 cylinder/piston swap and the 110 tire just like you. And like you I run way more off road and dirt roads than any pavement. I've never experienced an issue with the 1st to 2nd gear change you mention. Maybe it's the technique with throttle application or other elements coming into play here, but my KLX is one of the most competent climbing dirt bikes I've ever owned despite it being heavier than any competition bike I've ever owned. The minimal power it does have compared to competition bikes all seems to go to the ground with traction that almost never slips a tire. It's somewhat crazy.

I still ride occasionally with friends who have KTM's and other pure, competition bikes, and we often laugh how this stupidly inexpensive dinosaur of a motorcycle will climb some audacious, loose, steep climbs. While I'm a decent rider, some of them have skills beyond mine, so it's more of how the bike maintains traction with a useable powerband that is getting the job done.

OP, maybe it's tire selection or something in the technique you're using. I do have a 36mm pumper carb on mine, and going to a pumper is a common upgrade for this bike. I hated the 34mm CV carb on the KLX. I had four KLR600/650's over the years with the 40mm CV Keihin carb and really had no complaints about that carb on those bikes. The smaller displacement of the KLX just didn't get it for me with the stock CV carb, and I don't think it's a higher horsepower/torque thing as much as it is a power delivery thing with the pumper carb.

 

You can get the tooth count in the parts listing, I used Partzilla for that when I made my gearing spreadsheet to compare tire diameters and sprocket tooth changes. Which I will gladly share with anyone. Key in the tooth count of the bike, up to 6 gears, then vary the RPM and the tire diameters to compare for speeds in each gear.

 

Please can you tell me why you run a 110:100 on the back and what you run on the front?

I run a 140:80 thinking more width more grip. I have only been riding about 18 months.

Had my klx250s 2009 uk efi for the whole period.

Bike more capable than rider.

 

Think too much grip. Historically smaller MX bikes run smaller tires than the bigger bore. The 250s are running a 110/100 and the 450s run a 120/100, but the actual width is a bit beyond the indicated size on MX tire sizing. The reason for the narrower tires is too much traction can bog the engine, plus heavier tire takes more power to drive it. If your tire won't spin up when needed something has to slip and the only other choice is you slipping the clutch. It was an automatic for me.

I remember when some riders were running 5.10 knobbies on their 125 two strokes, no racers were running them, they were running 3.75 and 4.00. Even the 250 two strokes ran 4.00 and some would go 4.50, but the racers were running the 4.00. Put in metric that means some were running 130s on the 125s where the preferred tire was a 100 to 110 and on the 250s the preferred was around a 120. The KLX doesn't have the power to use the wide tires efficiently.

I am running a 4.10-18 dual sport tire, not the knobby style but more dual sport. First thing I did when putting on better tires. It allowed spinning the tire when needed versus slipping the clutch. My first experience needing that was riding in Michigan in deep sugar sand with the KLX. Instead of having to slip the clutch I could break the tire loose in deep sand pretty much as needed to avoid stalling or bogging. A real clutch saver. If I ran the knobby I'd be doing the 110/100.

There is one other benefit - the narrower tire is usually significantly lower cost. Twenty bucks difference between a 4.10 and a 120/80.