Upgrade to enjoy this feature!
Vital MX fantasy is free to play, but Premium users receive great benefits. Premium benefits include:
- View and download rider stats
- Pick trends
- Create a private league
- And more!
Only $10 for all 2026 SX, MX, and SMX series.
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
This is what is currently all in the bike now. Except Needle clip is on 3rd position at the moment. And it still falls on its face with air screw 1.5 turns out. I replaced the jet block gasket with the JD kit also. At 1 turn out it doesn't fall on its face but it's still sluggish. Runs best with air screw all the way in or almost all the way in.
The Shop
Luxon 4-Post Bar Mounts
$189.95 - $239.95
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
KTM 125SX Jetting. We have two 125s with the jetting listed on the first page and it works very good. I am just trying to fine tune it a bit more for the Temp. Trying to read the spark plug. It looks rich at idle and lean on the top end? This is my sons race bike and he lives on the top end most of the time. How do I know if it is lean on the top end?
You can check the main jet by either doing a WOT plug chop test or simply just keep going richer on the main until it gets too thick on top and you start losing the ability to rev out cleanly, then back it off 1 or 2 sizes to the sweet spot.
Bike runs flawlessly! Jetting has never been better. I almost pulled the trigger on a Lectron so glad I didn't. Currently running VP110 mixed with pump 93, 50:1. Thanks for the hook up!
It's better, feels very different.
I'd say crisp, smooth and not so "exciting" on 0 - 3/4 throttle, goes longer on high revs.
Plug color after three sand track sets was about perfect for my understanding.
Did my PB gps laptime on my local track so cant be bad...
Current setup:
RON95E10
50:1 Castrol
PV yellow 1,5 turns in
AS 1,5 turns out
Slide 5.5
MJ 490
PJ 32,5
Needle 6BFY43-75 , 2nd clip (42-75 in order but might not change)
S4 needle jet
Tested at 100m / 23 Celsius
Thanks for crawfishoo for overseas shipping of the slide&jets
Answer is yes, the jetting is a very good starting point for your bike. I have a 2021-TC125 and am running a close variation of page 1 (my exact specs on later posts throughout)
"Do I really need to buy the slide, etc ?" - YES, you need all the parts to do it right. The slide is the most important part. You are wasting your time dancing around the real issue if you try to shortcut without the slide.
Also, there is a known prob with the 2022 model 125/150. Some bikes just don't seem to jet correctly. Some users on here have tried swapping to known good carbs which seems to fix the prob. So, there is a hypothesis that there is something inherently wrong with some 2022 stock Mikuni carbs that will not allow them to run correctly no matter what jetting.
FMF system - yes, no problem. I am running full-FMF on my bike. I have also used stock, and Bill's. There is very little jetting difference between any of them. It's really just small fine tuning which you'll be doing anyway based on what fuel, oil, and ratio you use and environmental conditions.
Thank you.
Pit Row
Running the jetting specs on page 1, with the exception of 60:1, 1.5 gallons 93 pump with 0.5 gallons of VPR and Motorex cross power 2T oil
Bike has been running awesome, until yesterday….
About 800 ft elevation, central IL
Bike has a ProCircuit pipe, ProCircuit 304 silencer, VForce V4 reeds, vented airbox cover
Just did a top end at 58 hours, ran a few local races & practices, then a regional, then our motos at the ranch, the bike has not been babied, we are at 73 hours and ran into a overheating issue yesterday, pulled the top end and found signs of detonation on the head. Big score on the exhaust side of the piston next as shown.
Question is, are we jetted too lean or is it the mix 55:1 vs 60:1 an issue (I would think 55:1 would be more prone to detonation from a lean condition)
My first thought is you developed an air leak. Jetting doesn't change itself and suddenly "go bad" and unless environmental conditions had a drastic change that would explain it, it's likely an air leak. Being summer time, if anything the bike should be getting richer from the weather, not leaner.
The carbon pattern makes me think you developed this lean condition (likely air leak) rather recently and suddenly. It looks like there's the original carbon with some piston washing around the edges, like you'd want to see.. and then the clean edges started also getting carboned recently from the newfound heat.
Another thought... that wrist pin got really hot. I'd either swap pistons and pins more often and/or drill some oiling holes in the pin bores in the piston.
Mods: gizmo mods reed block
Fmf shorty that came with the bike
PJ: 32.5
Needle 42-74 pos 2
Needle jet: s7
Slide 5.0
Mj: 490
Amsoil dominator 32:1 Sunoco leaded 110/rec90 50/50
The track I was at is fast, and deep sugar sand. I’m at sea level in Florida temp was about 95*. I run B class towards the front in harescrambles, local moto, and the bike was wfo most of the time. I’m trying to avoid this again lol. Do I go straight to a 540mj and go from there? The bike never once hinted it was lean, and pulled clean from bottom to the top with zero bog. I prefer to stay with 32:1. The piston and rings looked great to be honest, rings were free as can be. Bike will be back together by the weekend, as I got the crank back today. Just looking for opinions so this doesn’t happen again lol.
That said, a few pages back there is some other 125 jetting that others have had luck with. You can put a 540 main jet in, but as digger has pointed out before that the carb isn’t capable of flowing that so it doesn’t matter if it’s in there or not that small bore is only able to pull so much fuel. One of these days I’ll go back and find it and update the first page.
You definitely will want to be in the 510-540 range on the main going WFO in sand at sea level. As AJ said, Digger will tell you that after 510 or so going bigger doesn't much help since the needle-jet or carb orifice itself becomes the limiting factor. But I will tell you I can definitely feel a difference from 510-520, 520-540.. and even a little bit 540-560.
Don't be afraid to try going a little richer on the needle also. Basically experiment so you know what's what. When the temps or elevation drop, I will run a 43-74 on (2) or even the 42-74 on (3).
The thing is all these settings are with an S4 which is slightly leaner than the S7. Combining these pilot or main suggestions with the S7 might make it too rich.
I do not thinks it’s a jetting related issue. I think it’s simply a manufacturing issue. They are all still using oem cranks. One of the 125’s is using a Keihin setup and the others are still on the setup Mikunis , similar to yours.
Done with heat cycles I’ll be doing plug chops this time. Do you guys think it’s necessary to bump to 40:1?
I’m aware of the big end bearing issue on the newer ktm’s. The guy that does my cranks also does them for a local shop that does a lot of mod bikes aswell. They noticed the 22+ had a metallurgy problem with the rods as both were getting trashed. They would install a pro-x before ktm knew they had a rod problem and the bike would be fine afterwards. Another local guy was having issues with other brands doing the same, he kept losing the big end bearing with motorex at 32:1. This guy is an ex factory Honda mech for the amatuer team. He had switched to a castor based oil and never had a problem since. Don’t know if any of it has any resonance or if it was all just a coincidence as it ranged from an 85 to a few 125’s also in about a 10 hour span.
Again, thank you guys for the response and help.
Post a reply to: 125/150/250/300 KTM/HUS/GG jetting thread