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I put the optional needle in it in the 2nd from the top position and it ran great!!!
Stock jets on everything else...
I think it could be a little leaner on the pilot, but overall I was happy with it!!
"In the past on other bikes I've generally had to go a few sizes bigger on the main when running AvGas" AvGas is non-oxygenated. What fuel did you switch from to require larger jets?
I have a 2021 KTM125 and I'm in the process of installing diggers jetting specs. I plan on running VP T4 fuel, does anyone have a pilot/needle/main recommendation for T4 with 6% oxygen content? We ran the bike on Sunday with stock specs other than I installed the 520 main and by the plug reading, it looked light brown. But we do plan on racing this weekend at a sandy track at sea-level and 90-95F temps.
The Shop
JD jet block gasket installed
S4 needle jet (factory)
42-74 needle in #2
#5 slide
35 pilot
520 main
1.5 AS
VP T4
32:1 Motul 800
90F @ sea level
Definitely a good starting point from the few street passes I made this afternoon. Hopefully we'll get good results at the track too. Thanks for the help.
You are now on almost the exact same settings as me except I have the next richer 42-73 needle in. Our temps are similar but you are at lower altitude but I am probably at lower humidity.
I recommend you try the 42-73 needle. I have no experience with VP-T4 but I suspect the needle might be a good match with that oxygen content. I understand the AvGas I am using is not oxygenated but I have a consistent history of it needing bigger jets. What little reading I have done seems to indicate this is due to the specific gravity of the AvGas but I am not fully read up on it. Oh, and I noticed this switching from VP MS-100 which is 3-ish % Oxygen. Needed to go up 1 or 2 on main or the bike revved out too cleanly on top. Felt borderline dangerous.
Here's my plug. This is not after any specific chop test or anything. Just after a day of hard motos.
I'll certainly order the 42-73 needle as well, especially since the temps will drop soon. And next time I have the carb out of the bike, I'll press out the needle jet to confirm on the S4 size listed in the manual.
I'm curious about the statement digger made about the main jet circuit being maxed out at 510-515 jet range. Hopefully that is not the case, especially since KTM included 520, 540, ,560 jets and they recommend a 560/580 main for sandy track conditions. Maybe KTM doesn't understand the carbs they are installing on their bikes?
I'll report back after a track test this weekend.
As far as the TMX-x main circuit goes, I think you'll find that you are all done by 520. Meaning you can remove the main jet and you still won't see an increase in fuel flow. The internal circuit flow is simple maxed out at around what a 520 will flow.
As for mine, temps were 20-25 degrees cooler this weekend due to the smoke cover from CA blocking out the sun (70-80 degrees) Bike was definitely leaner. Going to try a 540 main and possibly 37.5 pilot if I get similar temps on the next ride. Might even try raising the 42-73 to clip-3, see what that does. Might have to start really considering notching the slide.
I finally got to ride mine hard and from mid rpm up it screams!!! But it could use a little crisping up in the lower rpms.....
The bike ran pretty good. Only slight bogging when not in proper gear or if momentum was lost during cornering. Sea level track with temps in the lower 90's but the track is sandy so I'm curious of how it will run on a tacky track without much wheel spin. Bike pulls hard enough that I may try a 50t rear from a 51 to help calm down 2nd gear in the corners. Plug looked good but I did not do a plug pull, just normal track and pit riding. I did just receive the 42-73 needle today that I will try next.
An interesting note, my buddy has the exact same 2021 KTM 125. The only difference is he received the vented side air box cover with his while mine has the solid cover. After having success with the jetting on mine, I gave his a test ride fully expecting to have the 1/3rd throttle bog but his bike ran completely clean regardless of throttle position. His bike is completely stock jetting and he runs pump gas/110 at 50:50 while using Motul 800 at 32:1, while I run straight VP T4 and with the same Motul 800 at 32:1. Would you think the vented air box cover is responsible for the clean jetting? Or do you see jetting differences bike to bike normally? I'm wondering if the jetting was spec'd for the vented side cover?
Pit Row
The other issue is we plan to race the limited class (250 limited) and I do not think the vented cover is legal for that class so I do not see any reason to jet for the vented air box if its eventually not going to be raced.
That Yamaha part # is the 6BFY42-74 needle.
Digger has said earlier that with the S4 and this series of needle the Main circuit maxes out around 510-515 which would explain why going to a much larger main had no improvement for you.
Need Digger and AJ to chime in but I suspect you'll be needing the S7 and/or a much richer needle and maybe notching the slide.
What fuel are you running and what is it mixed at?
On my TC , I run 50:1 and it was feeling a bit lean down low and at startup
Bumped the PJ to 35 and it took care of it.
Needle clip is at 2.5 Position (using a JD washer/spacer ring) and I might move to #3 as it's finally cooling down here (at least early in the am, afternoons still in the 80F )
I like my jetting on the richer side to tame the beast...
Beside the PJ, i just tweaked a few times the A/S to fine tune for temp/humidity changes
But this is really being picky - the key was 5.5 slide /Yamaha needle changes
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