I bought a used 2009 KLX250SF (4000miles) last fall, it was low on power like it has a flat spot in the mid range, it had a little zing in the upper range but doesn't really pull much at all. I cleaned and rebuilt the carb, it started more easily but power did not improve. I removed the airbox lid and the engine will idle but completely stumbles anywhere above 4k rpm so I'm assuming it was running very lean.

Is this about normal for a stock KLX250SF, or is there something very wrong?

After riding for a few months like that I installed a dynojet kit this weekend, I used the DJ needle with the clip on the 3rd position, installed a 124 jet, used the stock diaphragm spring and did not drill the slide. This seemed to improve power somewhat but not drastically, then with the same carb settings I removed the airbox lid, midrange power was significantly improved but at high RPM the engine completely stumbles. Then I installed the 128 jet, it now runs smoothly at high RPM, but doesn't really pull hard, midrange power feels good but just runs out of steam above ~8krpm. The throttle response is also not very crisp, power seems to build in a slow surge, but doesnt really have a snap to it.

I'm really just looking to get some qualitative opinions on whether this is the amount of power I should expect or if something is wrong, I know it's hard to say if my bike is running right or nut but I would like to continue tuning (exhaust, MCM, etc) but don't want to move forward if there is something basic wrong.

Thanks!

Picture just for fun:

 

As long as you are running the stock exhaust system, the needed CVK setup is very simple. Assuming you bought the 2152 kit, reinstall the airbox lid and follow the instructions on the kit for a stock bike. Do not lose your stock components, you'll need them in the future. Power gains over stock are minimal.

If you bought the 2206 kit, things get a little more complicated.

If a previous owner hasn't already "jacked with" the slide lift port (by drilling it) or the stock slide spring ( by cutting it or replacing it with a lighter spring), you will be able to use the lidless recipes in the future without having to fix previous "hacks"... At any rate, Never Never Ever drill your slide, or modify your stock slide spring !

When you install a loud slipon or a full exhaust system, you can make the "biggy" power levels with a lidless CVK setup.

The MCM adds power regardless of all other mods, or lack of mods.

 

Thanks KLXster!
I have the 2152 kit. As far as "follow the instructions on the kit for a stock bike" I have a question about that.
The instructions have a Stage1 and Stage 2, Stage 1 says a DJ116 for stock exhaust, and Stage 2 says DJ124 for stock exhaust. On the first page of the instructions is says STAGE 1 "For mildly tuned machines using the sotck airbox, with stock or K&N filter". And then for STAGE 2 "For mildly tuned machines using the stock airbox with intake snorkel removed, with stock or K&N filter"

So, as per the instructions it seems like I have the option of using the DJ116 with stock airbox or the DJ124 with snorkel removed, which one should I do? Doesn't seem like I would want the Stage 1 with DJ116, that's even smaller than the stock 118 isn't it?

Also, following the DJ instructions I would use the lighter DJ spring. Is that basically for better throttle response? And then when I install a high flow exhaust I would want to go back to the original stock spring?

Thanks!

 

Do not fall into the "main jet numbering trap". The kit uses DJ main jets, your stock CVK uses Keihin main jets, they are very different animals. See my conversion chart link on my signature line for making comparisons. Use it to fill in this blank - K118 is equivalent to a DJ ___.

The DJ kit was designed for a KLX running the stock exhaust system. Choose and use EXACTLY the setup for what they are calling "Stage 1" or "Stage II"...

Yes, when you are ready to use lidless CVK setups, you must use a stock slide spring ( buying a new one guarantees good results), AND a stock slide lift port. Lift port information is among my Dyno Tuning (lidless recipes) links on my signature line..

 

Ok this sounds great, thank you! Makes sense about the jet numbering, so the K118 that comes stock with the bike is equivalent to a DJ112.

So what I have in the bike right now is wrong.
I currently have: no airbox lid, DJ128 main, DJ needle on 3rd notch with 2 extra washers above it, Stock Spring, Stock slide (not drilled)

Today I will change it to:
Replace airbox lid but remove snorkel (as the DJ instructions imply), DJ124 main, DJ needle on 3rd notch with 2 extra washers above it, DJ Spring, Stock slide (not drilled)

I also have a TBR slip on ready to go, when I install that I will go to:
No airbox lid, DJ128 main, DJ needle on 3rd notch with 2 extra washers above it, Stock Spring, Stock slide (not drilled)
OR, copying your Stock Header+DGR recipe I could do: No airbox lid, DJ140 main, DJ needle on 1st notch with 2 extra washers glued above it, Stock Spring, Stock slide (not drilled)

I am holding off on adding the slip on right now because I want to make sure it's running right first.
Before doing any of the jetting it just seemed anemic, I wasn't sure if there was something wrong or if its normal. In completely stock form I could max out at ~75-80mph, I could lift the front wheel in first gear if I revved to about 9k and quickly dumped the clutch, it just seemed surprisingly weak so I was suspecting something might not be normal. (My other bike is a Monster 821, so maybe I'm just jaded)

Does that make sense? thanks so much for the help

 

It's only a 250!!!!! You are not going to lay rubber for a block, let alone 1'!

 

@durielk haha, yes that makes sense

I'm just looking for a seat of the pants way to know if it's running normally before starting with too many mods.

 

Timed roll on run over a specific distance, same spot every time. Up hills work well for that too.

 

Are there any numbers available for comparison? Maybe a 1/8 mile starting in 3rd gear at 25mph and then rolling on.

I guess I could try a 0-60 also, I think the stock bike published numbers are around 9 seconds.

 

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If your carb is faulty, or valves too tight, or spark plug bad, etc etc, doing performance setups is counterproductive.
When buying a used KLX that doesn't run right, along with mechanic'ing, one has to be ready to become a "CVK guy" or hope that a dealership can fix a faulting CVK .
You say you rebuilt the carb, so how bout a test.. The aeration holes in the emulsion tube - all clear or were some clogged ? Did you lose and then have to find the needle jet ? Could your needle jet be installed upside down ? Is the spring indexed on the caps' tangs ? Diaphragm fully seated before the cap was reinstalled ?
Lots of mistakes can be made when "cleaning" a carb.. Usually peeps don't fully disassemble it, therefore they never see the component(s) that are faulting.

Never remove the airbox lid until/unless the CVK is running a Lidless setup. Never run a lidless setup until a performance slipon or full performance system is installed.