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New to VitalMX here and getting into the dirt bike scene... first, thank you, digger, and everyone else for consolidating all the testing/information. Going through the 10 page thread and my desire to always fine tune/get the best out of things, I was curious...
Wanted to verify that the 1st page specs are the most updated since you've been testing. i.e.
I picked up a 2021 KTM 250sx, only mod is the FMF Turbinecore 2 silencer added. Brand new... put 3 hours in the bike so far and can't wait to really get this bike going, as I understand it these bikes come rich from factory (being conservative for them makes sense).
Live in PNW (Oregon)
Elevation is basically sea level 50ft.. up to say 1,000ft depending on the riding area for majority of riding.
Temps: 50-90f will be the normal range, and mostly 65-85f realistically as we warm up
Fuel: Sunoco 93 Octane no ethanol (they sell 110 race fuel I just was trying to stick to factory recommended 95...)
Mix Ratio: 60:1 Motorex
Stock jetting currently from factory...
ASO 1.5
IJ 35
NDL 42-71
POS 3
MJ 450
Also will say I was given both the solid cover airbox plastic and the vented airbox plastic from dealer when purchased. As I am breaking it in just have been using the solid for now, will try vented when I get to a more open area. Has there been any further tweaks or changes you might suggest given the parameters I've listed as a starting point from your recommended settings? Appreciate the insight and will be happy to provide all the testing feedback here with data points to keep this growing.
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
Just fired the 350 and picked up a '19 250sx, have also moved and it's now very similar to StevenMX above at pactically sea level and generally 40-60 degrees at the moment.
Pump fuel and not wide open all day (skill level wouldn't accept that) and no sand to speak of.
Pro Circuit works pipe and full length 304 being the only mods, yellow PV spring 1.5 in.
It doesn't seem too bad, certainly not as bad as expected from all the Mikuni talk around, but just got that feeling it could be a lot cleaner.
Would you still say the needle, slide and jets on the first page considering the pump fuel and elevation/temp?
Thanks again man
The Shop
I run pump gas in mine with that jetting with no problems.
Grabbed a few mains either side of 490.
Probably gonna stick a Rad Valve into it which from memory called for wanting to be a touch leaner, so may work out okay.
Thanks again for putting the time in folks.
AJ what temp ranges have you ran this jetting under?
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
Feel your pain Steven, parts deliveries are a shit show lately, either super fast or slow as it comes... I never know which!
As far as the temp range goes, you should be good from 60f--to 90f,with just air screw adjustments, but that slide may be a touch lean in cooler conditions. this is assuming you are riding from about 0' to 2500'
Let's say keeping altitude 0-2500' the same but riding from 40-60f and say 90 -100f+ ... what combos would you suggest... Trying to have the additional pieces needed given the conditions as far as needles/slides/jets.
Temps 50-55f and sunny
Elevation: 500ft
Main jet 500
Pilot jet (35)* ... I will put the 32.5 in next week, I actually forgot to throw in the 32.5 when I was doing all this late at night
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
Consensus: The bike is a rocket ship, instant throttle. Pull and hang on!
Pit Row
Per AJ's baseline settings to get you going...
'17+ 125/150 jetting
Pump gas/race gas mix, C12 straight
MJ 490
PJ 32.5
Needle Jet S4
AS 1.5
slide 5.0
Needle 42-74 on the 2nd clip (Yamaha 284-14116-H4-00)
Needle jet S4 (KTM part #55431021400)
PV 1.75 turns in from all the way out.
Fuel mix 40:1
Update on the jetting. I've been playing with all sorts of combinations. Nothing seemed to help. I finally took the plunge and filed the notch in the #5 slide and was very impressed with it. I can now influence the jetting with the air screw. Before it didn't seem to make a difference.
It was in the low 50s when I last tested it up and down the road but I'm now at the #5 slide with a notch, 30 pj, 480 main, 42-74 needle in the second clip, and AS at 2.5. I might go back to what is on the first page and try that with the notched slide and see what it does.
"(1) Preload. The main power-valve spring can be preloaded by turning the Allen head adjuster in the center of the power-valve spring cap. The stock spring preload is set at the factory to allow the power-valve flapper to open at 5600 rpm. If you turn the Allen head adjuster one turn out (counterclockwise), the power valve will open 150 rpm sooner (at 5450 rpm). If you turn it in a full turn (clockwise), the power valve will open 150 rpm later (at 5750 rpm). The longer the power-valve flapper is closed, the harder the power will hit?because it will have built up more rpm. Most MXA test riders run the stock 5600 rpm setting. There is one caveat, though: once you change the power-valve adjuster setting, you have to count turns to keep track of where the stock setting was."
This leads me to believe that even with tight tolerances... there might be some variances from bike-to-bike, given that lets say the 1 additional turn adjusts the range roughly 150rpms either direction per turn. So if you had those 3 ktm 85s within 2 turns deviation, you are talking about 300rpm variance at 5600rpms. That is a delta of ~5% when tuning to that specific rpm. I'm just speculating of course with the information I can find, but I imagine they do tune these as they do run them at the factory before delivery.
Use the 32.5 pilot, 490 main and stock needle 2 or 3 and no need to change the s4 nozzle in there.
You could cut the slide slightly, I did my own and also tried the oem 5 slide and the 5 slide was a little too lean.
I don't see massive changes with the T2 fuel.
I've got a fresh can at home i will try soon but I can't see me making big carb changes based on experience.
The silencer is critical on the 150 and many more restricted silencers can mess up the jetting.
Pc shorty or stock.
Back in the day, I would see CR125's with powervalves that opened very differently from bike to bike when brand new. I believe KTM knew how critical the PV setting is and made the preload adjustable on their two strokes for the past two decades.
You'd be surprised how finicky the PV timing is and also how it affects jetting.
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