2009 KLX 250 SF Re-Jet W/air box mod

I have been searching for ever, but can only find jetting for exhaust mods and big bore kits.

I would like to open the air box on this bike to around 3" x 3" and re-jet. But no other mods.

It still has the factory setting and the bike is next to impossible to start, due to its lean configuration.

I believe stock it has a 40 Pilot jet, which i think I will go to 45. The main jet the numbers I find are all over? 170-180.

I could really use some help on this. I do not plan on a slip on or big bore, just a better starting bike that has an improved throttle response.

Thanks so much.


So, I removed the carb and found it has a #35 pilot and 118 main jet. I am thinking of going to a 40-42 pilot jet and a 128 main jet.

I would love to know what is working for others???????? (with only an airbox mod)

I ordered a #40 pilot and #122,125 main jets. Start from there, since I had to make a complete guess at this.

I also have a 36mm Mikuni left over from my carb swap on my DRZ400sm. I wonder if this carb would work????? I would think this is to big without any performance exhaust currently on the bike.

The larger pilot jet may help it start better, but many of us drill out the enrichment (choke) jet to 0.018" (just a little larger than stock) and that REALLY helps bundles with starting. I see others running #38 and #40 pilots, but I found the #38 to be too rich (fuel screw down to less than 1 turn out and didn't make much difference turning in or out from here, lots of black soot on the tail pipe, other rich-running issues). I went back to the #35. Sure, I have an HMF muffler. But if you're keeping the stock muffler, you may find more starting benefit from drilling the enrichment jet.

Search this forum and here and you'll find several threads on the enrichment jet mod.

Consider getting a better needle for throttle response. The stock ones are claimed to be pretty lousy. I have the dynojet needle from the #2206 KLX300 kit and it made a big difference.

Dyno Jet kit for a 250s

Removed backfire screen

KDX220 snorkel

Smog items gone

one smaller tooth front sprocket

128 main (DJ)

needle in mid (DJ)

38 pilot (DJ)

Runs great and no problems starting unless it sits for 3-4 weeks every once in a while. Then I tip it over for 10 seconds on the side and all is well. Same with XT225's! Sat over the winter and fired right up.

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