I get a lot of emails from here and at the track about jetting for the 2 strokes. Thanks to digger for pointing me in the right direction this is the currant specs. I have for everything. Its easier to start a thread With all the jetting in it to keep track of everything. Your jetting might need tweaked based on your temps and elevation, but this is a good start for you.
**Updated 4/19/23**
'17+ 125/150 jetting (‘22 bikes have a known bog, there is no jetting or carb swap combo yet that takes care of that problem. If or when it gets figured out I’ll update)
MJ 510
PJ 32.5
AS 1.75
slide 5.0 from JD Jetting
Needle 42-74 on the 2nd clip (Yamaha 284-14116-H4-00)
Needle jet S4 (KTM part #55431021400)
PV 2.25 turns in from all the way out.
Fuel mix 40:1
(credit to FGR01 for continuing to refine the small bore jetting)
On the '17+ 250's **Updated 10/13/2020**
Pump gas/race gas or straight C12
This is my current jetting. Its more refined all around and crisp down low with better top end over rev.
Main jet 500
Pilot jet 32.5
Slide 5.75 (Kawasaki part#16025-0003)
Needle 43-75 (KTM part #55431134400)
Clip position #2
Needle jet S4 (KTM part #55431021400)
Air screw 1.5-2.0
Fuel 55:1
OR
Solid jetting if you're someone that doesn't live on top end. Crisp down low out of turns pull.
MJ 490
PJ 32.5
AS 1.5 turns out
Needle jet S4 (KTM part #55431021400) stock is S1
Needle 42-75 on the 2nd clip (Yamaha part #284-14116-H5-00)
Slide 5.5 (Kawasaki part 16025-0002)
PV 1.75 turns in from all the way out
Fuel mix 50:1
300sx jetting
Pilot jet 25 and main jet 460 to start.
Slide 5.5 (Kawasaki part 16025-0002)
Needle 42-75 on the 2nd clip (Yamaha part #284-14116-H5-00)
5/32” chainsaw file to notch the front of the slide.
A/S 1.5 to start, you’ll need to adjust as needed.
The Shop
kx 125 fiche should have the 5.0 slide
42-74 needle start out in the #2 clip position https://www.motosport.com/oem-parts/part-number/284-14116-h4-00?mmysour…
S-4 needle jet
#5-slide, it's a must have item. IMO,
#5.5-#6 slides are to lean for the small bore bikes. You can either cut your #4 oem yourself, or you can purchase one from JD jetting.
https://jdjetting.com/product/jdmk02-mikuni-tmx-ktm-husq-carburetor-sli…
Main. 490
32.5--pilot
if you notch the front side of the slide, you will need a 30 pilot
You will also more than likely need a jet block gasket, as the oem ones deform over a very short amount of time.
https://jdjetting.com/product/jdmk05-mikuni-tmx-jet-block-kit/
air screw at 1.5 turns out to begin with.
I have a slide I cut that should be around 5.5 that I can put in my bike and test with and get some solid numbers. It will be a week or so as my suspension is off getting redone.
40:1 is more oil so it would be leaner fuel mix. I think so, digger posted in another thread 1.5mm deeper and wider on the cut to turn a 4.0 into a 5.5. I have one that I cut that next time I go ride I'll swap into it and see what happens and if I have to change any other jetting. I have a 500 MJ that I need to try also.
That setup has been used from 60f to 90f with slight air screw adjustments only. Sea level to 2000’
Pit Row
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