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Okay, I’ll just leave the slide alone. I see you recommend the 43-75 and 42-75 so don’t bother with the 42-74?
Also i can’t find a 510 main that is genuine. Any idea where to get one?
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_4-042_main_Hex.htm
Mikuni doesn't have them that size so aftermarket is your only option. I've ordered from here and have zero problems. Also you need to adjust your powervalve when you rejet as it does change things. On the small bore bikes we were starting at 2.25 turns in from all the way out and adjusting from there if needed.
I believe all the 510 I have are either ProX or EBC. They are also available on Rocky Mountain and other places in case you're already over there ordering needles, etc. Order a few sizes. I'd have 480-490-500-510-520-540 on hand if I was you. It will take a little tinkering to get it perfect and to adjust once humid summer comes.
On the needle, the 2022 crank recall and ECU issue ended up causing me to use the leaner xx-75 series needles. The xx-74 caused a rich bog when opening the throttle on the 2023 Gasggas 125 I jetted. Others have stated the same, that there was a bog or hesitation that was hard to get rid of with the xx-74 needle. My post is back on page 31.
Echo what AJ said on the PV. 2.25 in is a good starting point. Ultimately you need to get the jetting sorted first and then fine tune the PV to synch with the jetting.
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Thanks guys. Awesome info from yall! I did see the pro x, ebc, and jetsrus options but didn’t know if they were true to size. Will get all those main sizes/ needles and report back after install.
Finally got the carb parts and went with the suggested setup. It runs great! Im very satisfied with how it runs. I appreciate the info and help from you guys!
Like i said, I don’t have a ton of knowledge on jetting. Just basic knowledge i guess to get something running decent. As far as the humid summer goes. Do you have any advice of which direction id be going from the recommended setup? Currently 50F and these settings work.
Needle- 43-75 Clip 2
Main- 510
Pilot- 32.5 (could use 30)
When it gets hot and humid you might start developing a rich hesitation as you roll on the throttle right at the transition to the taper of the needle. That will be your sign to go to the 42-75 clip 2.
If you have some time on your hands, experiment with larger mains. Try 520 and then 540. Just to know the effect. These bikes really like a big main. I am typically always running 540-560, but I only do MX tracks hard on the gas and I also run avgas.
I run the same jetting from 40°-105°F+. When it drops below 40° I need to go up one size on the pilot, down one clip on the needle and adjust the air screw.
Just an update as i finally have my 23 GG 125 sorted out. Brief history is i started with the stock Mikuni (and while i sometimes had it jetted pretty good) it would often need constant tweaking to keep correct. Sometimes i would get it running good and other times i would seem to go backwards. The jetting recommended here really helped but i feel my particular bike is on the worse end of sensativity if that makes sense. I literally tested a ton of needles/ clip positions / pilots & AS combinations....
Anyway, i ditched the MiK for a Keihin AG from TSP (with their jetting specs) out of frustration and it was much less sensative but still had the off idle / transition rich condition. I chased it with pilot jet / AS & clip changes but it was never totally clean. Dont get me wrong it was ridable on a track but it would begin to load up if riding slow like through the pits. Member FGR01 really helped me get this sorted out with the much leaner needle that i ended up with. Can't thank him enough as i wouldn't have gotten there by myself seeing how much leaner the straight section is from the OEM diameter.
Here are my final settings. 182m 48p N3CK 4th ~1.6 AS 50/50 110 sunoco/pump 93 40:1 Maxima K2 0-1000' elevation tested@ 40-65*F
Thats exactly what happened at camp coker and just put the 42 in and its fixed. Great advice and i appreciate it very much! For my reference is the 42 a leaner needle?
Glad that worked as intended. Yes, 42-75 has grooves that are cut 1/2 clip leaner than 43-75.
2021 tc125 vforce4r reeds FMF fatty and Powercore 2 silencer. I’m having a bog when I crack the throttle open.
#5 slide from jd jetting
S-4 needle jet,
42-75 second clip,
32.5 pilot
510 main
air screw 1.5 turn out.
Not sure about pv I have the tool ordered. I’m going to try a 35 pilot tomorrow.
What fuel are you using. Make sure you are mixing 40:1.
Make sure you swap the jet block gasket for the JD one and also make sure you verify the float height at 8-9mm.
After all that, put in the 42-74 needle on clip 2, 35 pilot, and 520 main... 540 if it's cooler and not too humid.
I am mixing 40:1. I have replaced the jet block gasket. I’ll have to order the 42-74.
Thank you for the help.
You should really adjust the power valve before you make any more changes as it will affect the jetting. I just use a flat blade screwdriver to adjust them. After getting that set if you still have a bog swap out the jetting. FGR's small bore specs. are pretty spot on.
Just to add to this- if you want an easier way to play with your power valve setting prior to fine tuning the jetting based on the “hit”- check out Kreft Powerdial 3.0 it makes this adjustment doable by hand. I use it on all my bikes. Great if you bounce between track and trail
This is more of a curiosity post: I have ran AJ’s spec on 2018 KTM 250sx and a 2019 KTM 125sx with great results using Amsoil Dominator 50:1 (per oil manufacturer spec)with 93 ethanol free pump gas due to the low flashpoint and clean burn.
Question is what’s everyone else running for fuel combo? I ride with my son a lot who is intermediate so a lot of on/off throttle and pilot to needle jet zone….not really much main jet riding haha. As he progresses and when I’m riding harder should I look for a 40:1 mix to get more life out of my top ends? Curious on logic from others.
Pit Row
The 125 you should run 40:1. In my 250 I run 60:1 and it’s wide open a lot. I have 175 hours on the bike and have done 1 bottom end and 2 top ends in that time. Actually the only reason I did a bottom end is because I got ahold of that bad batch of T4 and the fuel gelled and trashed the crank thrust washers.
In my 125 and 250, which are now 150/300, I run 38:1. Partly just for the simplicity of 1 gas jug. But also I just don’t really trust the higher ratios, especially in a 125. We’ve all seen the pics of the 250/300 scattered engines when they stick a piston and yank the wrist pin out. I’m have a theory that the 250/300 has that huge oiling slot in the rod because of the 60:1 ratio. Keep in mind the 125 does not have that big slot and is somewhat prone to the wrist pin getting hot.
I finally Buy a clean 2020 150 sx with 45hs . I will order 5# slide , 500,520,540,560 mains and 32.5,35,37.5 pilots . Sea level , 40:1 Motul 800 /710 . Fuel pump 93. And Jet block gasket and float needle oring.
I'm a little confuse on the needle , 42-74 already on the list but i would like to order the next richer needle for low temp and Sandy situation . 43-74 or 42-73 ??.
Get the 43-74 as an optional richer needle. The clip grooves are 1/2 a clip richer than 42-74 so you can make small adjustments going between the 2 needles.
Thanks and the float height 8-8.7mm should be set with the carb at 45° or the upside down method ?
About 45 degree. Basically you need the tab touching the plunger on the needle, but not depressing the plunger in.
#5 slide
S4 needle jet
42-74 second clip
35 pilot
520 main
Air screw 1.5
I use 90 ethanol free pump gas mixed 40:1 with castor 927. I adjusted the power valve to 2.25 turns in from fully out. The jetting change from above helped a lot but I am still having a slight bog when rolling on the throttle. I also have a question about what spark plug you guys are running. When I bought my bike it had a br9ecm in and I have not changed it. I appreciate the help I am new to tuning carburetors.
What are current temps and elevation?
Have you verified the float height at 8-9mm? Many seem to come around 11mm which is a little too lean.
Before you do anything else, try adjusting the air screw to maybe get an idea of which direction to go on the pilot. Go in to 1.25 turns from closed. Try that. Then go to 1.75 turns and try that. If 1.25 is better, you need a 37.5 pilot.
The BR9ECM should be fine, but I run the BR9ECMIX. You can throw a fresh plug in just to rule that out.
Currently 70-80f at around 500f of elevation. I tried to measure the float height with a digital caliper but I couldn’t get a super accurate measurement. I measured it at around 7-8mm. I’m honestly not for sure where to take the measurement. I have seen a video of someone doing it by blowing into the fuel inlet and my understanding is when you are not able to blow more air in then that’s when you want to take the measurement?
In this video, it's a different model of carb, but he explains the process pretty well. https://youtu.be/Qk5_wTJbTHQ
So, to make the #4 slide into to#5 slide, i have to cut 1mm in the red mark with a round file ( by hand or use a demel) ? . Also i need to take some on the green mark ? .
This is my first time doing it. Any tips where to start ? . make a 0.5/1mm mark with a caliper , start cuting in the middle of the arch and then file to the sides maybe ? .
I apreciate Your help .
@AJ
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