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Bought used klx140L (2011) with upgrades - need tuning advice

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Ok,  so pretty new to this site so don't be too hard on me here.

Just recently purchased a 2011 KLX140L for my son.   it was a little rough running and I just figured it just needed some tuning.

So,  got it home and started inspecting.

Current mods, changes made (that I have found this far)

- removed snorkel, holes drilled in top

- FMF powercore4 slip on exhaust

- 105 main 

- 38 pilot (stock, I believe)

- no washers under needle

- fuel screw at just a little less than 1.5 turns out

- idle screw at just a little less than 1.75 turns out

- CR7HSA plug (stock)

Was popping some and a little bit of backfire.

Needless to say when I checked the plug,  it was white.  I believe it is from running very lean between idle and 1/2 throttle range.


Recent changes (prior to me taking the carb apart and finding the jet sizes)

- put the stock muffler back on and it ran a little better. Probably since it had more back pressure. 


Here is where I think my sweet spot may be.  Looking for feedback on these changes.

- put FMF exhaust back on 

- change main to 100 (105 just seems way to big)

- change pilot to 40  (this i where I think my main lean condition was due to pipe and air box mods)

- put a shim (or two) under the needle to help 1/8 to 3/4 throttle


alright,  tell me where I am wrong and what I should do different.   Thanks in advance.










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I have a 2016 klx 150l and it came stock with a 112 main jet / 38 pilot. I'm now running 120, needle raised with two shims with aftermarket exhaust / snorkel removed and it's going great. As far as I know these little bikes are fairly similar, don't think a leaner main jet will be a good idea.

I also tried a larger pilot but never managed to make it idle properly with a 40.

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Thanks 78er.    looks like you went up 8 from stock on main jet.    me, at 105 is 10 from stock.     My carb is a Keihin PB20,  do you know what yours is?


Maybe I take the approach of one change at a time.

My main issue is the bike running very lean and I believe it is in the 1/4 to 3/4 throttle range.   Maybe I make needle height changes first and see how it goes.   Do you happen to know your shim size/depth?



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I got a Keihin NCV24 according to what's written on it. apparently there's more of a difference than I thought.

One change at a time definitely is the way to go. I raised the needle approx. 0.7 or 0.8 mm.

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ok,  I did the following and things seems to be working well right now.

- installed a Shindy carb rebuild kit

- Installed 100 main jet (jetsrus)

- installed 40 pilot jet (jetsrus)

- install 2 small shims under the needle.  (think it was about .30 mm)


bike started right up and is running great right now. Choke is now needed but it is 90 degrees here.   Still have the stock pipe on. seems to be slightly rich but it running strong.  I think putting the FMF pipe on will take away the richness a little and should be perfect.  we will see.

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