2012 250sx jetting, stutter, idle surge

So i bought the sx from a friend, hopped on it this year, thing is still on stock jetting, bike basically brand new with only 60h on it, 10ish hours on new top end.


With stock settings, 158 main, 42 pilot, N1E1 needle, 2 turns out on air screw.  This matches up to the stock settings for around 1500ft altitude and 15-25 degrees ontario canada.


Checking the plug, it was a very light brown so clearly running lean.


Reading on MXA their review of this bike, they go to 160 main, leave on 42 pilot and put in N1EH needle on 3rd clip, 1 1/2 air screw, this jetting definitely helped but the thing has a really bad idle surge still, and if you hold the throttle steady, anywhere on the throttle except wide open, once the bike reaches its kind of equilibrium with very low load it stutters like crazy.  My rm 250 2006 runs amazing, bought it with current jetting so i by no means have any experience jetting this but i know how the bike should run which this 250sx clearly isnt.  I read going with a suzuki NECJ needle is a good fix for the low end stutter so i think im going to go with that and give it a try.


Going with the MXA jetting, the needle is richer, and main jet is richer.  So my plug is slightly darker brown but is still a fairly light brown, but the plug was very wet around the outside.  I kind of assume the wetness could just be from when i shut it off with the engine not firing?  Im going to go buy some larger jets and that suzuki needle and give it another try.  


As for the idle surge, i read that means bike is lean so hopefully all of what i have read is correct and i just gotta give this thing more fuel.  


one last thing is the bike is supposed to be mixed 60:1, and im running it 50:1, i don't like the idea of going with so little oil.  Could this be part of the problem?


Any suggestions for how much larger of a main or pilot to go with?


thanks for any responses.

The air screw setting isn't a fixed value, it has to continuously be re-adjusted

to fine-tune the pilot jet circuit depending on ambiant temps. 


If beyond it's normal adjustment range, say less than 1 turn or more than 2.5 turns out, you either have to lean or richen the pilot jet.

Try turning it 1 turn in and see if it surges less, if so a richer pilot is required.


By going from 60:1 to 50:1, you've also leaned out the fuel mixture a small amount, more oil = less fuel.


Check for air leaks, perhaps it is more apparent at idle since that circuit is lean,

but less apparent at other throttle openings if your jetting is a little on the rich side (needle/main)


Don't go by electrode tip colour only to read your plug, here's a handy guide : http://www.braigasen.com/howtoread.htm

Edited by mlatour

Aright so i thought i knew something about how to read the plug but i clearly didnt know.  So i cut the current plug to see where it was at.

2015-04-29 20.15.23.jpg
It is black and well over the 2mm which means its running too rich.
Also the flat around the plug was black which also indicates too rich.
It and the heat line was definitely showing it to be too hot.


So i put the main jet back to 158 (stock and recommended for my location), and it still has the same pilot.

I warmed up the bike and started playing with the air screw.  Tried at stock 2 turns out, bad idle surges, at 1, bad surges, at 1/2, seemed like it could be better and at 0 which i dont know if you should run it at that but i tried anyways and it surged way less but was boggy.  Also tried at 2.5 turns out and still same issues


I rode around for about 30 minutes on stock setup, and 1.5 turns out on air screw.  The new plug looks like this.


2015-04-29 20.13.03.jpg
2015-04-29 20.12.51.jpg


Honestly i dont know what to read from this.  I figured 30 min of riding would give me something on the flat surface threaded external but it doesnt. I guess 

Im thinking i need to drop the pilot to 40 from the 42 and see how it works because i cant get it to not surge at idle.

2015-04-29 20.12.51.jpg

2015-04-29 20.13.03.jpg

2015-04-29 20.15.23.jpg


By turning in the air screw, you richenned the idle mixture, if by 1/2 turn out from fully closed you did not see any difference in idle quality,

that indicates that you are out of the air screw's adjustment range and require another pilot jet size.


Your air screw experiment has shown that a richer pilot jet could be used.

Less air/more fuel reduced the surging, but having the air screw near fully closed messed up the pilot circuit's operation. (became boggy)


As a precaution, perhaps keep your main jet richer (160) until you've determined that you don't have an air leak

and during these cooler spring ambiant temperatures. (current highs of about 15°C)

New plugs will take a while to get enough 'colour' on them to read.

Edited by mlatour

After i wrote the response i realized what i said was incorrect.  Definitely need to go with a richer pilot like you said.


Tonight i will check for air leaks, i guess the only thing i wont know is air leak through crank or if a reed pedal is broken but thats doubtful with how new it is.


So if i dont find any air leaks, the best check for the main jet is to warm up bike, swap to new plug, hold it wide open through he gears then kill it, pull in clutch, swap back to current plug and that way i can verify the main jet.  See where its at then move to pilot issues.  Which ill have to order a couple sizes down and play around with it.  This is fun stuff hahahaha

The pilot and needle circuits are adjusted more by 'feel' than by plug colour :

throttle response, surging, how crisp it rolls on the gas etc.


As mentioned, a brand new plug will take time to get some colour / deposits accumulated on it,

it's often more accurate to use a used plug that wasn't run with too rich of carburator settings as your test plug.

Instead of chopping up a plug each time, a magnefying glass/flashlight will also let you inspect the base of the insulator.


Try to test/adjust your carburator circuits only one at a time, by accelerating under load up to their relative throttle openings.

First the pilot/air screw (0 to 1/4), then the needle profile/clip position (1/4 to 2/3rd), keeping the main jet for last. (1/2 to full).


The pilot and needle circuits still feed some fuel at high rpms / WOT. 

If you adjust the main jet first, leaning out either of the two other circuits afterwards will also slightly lean out your WOT fuel quantity.


If used off-road, keep the main jet richer than 'perfect' as a safety margin if long straightaways / high speed dirt roads are encountered.

Having too rich a main jet on an motocross track will result in less overev capabilities.

Edited by mlatour

So i tried, stock pilot is 42, and from that i have tried 38, 40, 45, and 48.  Going with 45 and 48 made it run very boggy off the bottom so clearly they are too rich.  The 40 and 38 run fine but all of these still have a bad idle surge.  So after speaking with a friend we figured out im over reacting and this is just normal on these bikes.  I have watched multiple videos of 250 sx and they do the exact same surge, but some don't at all, which is why i was concerned.  


Like after he revs it and its decreasing rpm and surges is what im talking about.  case closed i guess, i still hate that idle surge

Edited by kozakcon

What's the squish clearance? If that's too wide then you'll get hesitation and surging.

Plus the jetting don't be versatile for varying conditions.

No point tuning jetting without first checking the squish is in a sane range.

Edited by numroe

Squish clearance would be stock because it hasn't been modified.  I feel like at this point its fine, changing squish means expensive machining doesn't it?  If stock bikes are running like this it must be OK, i just didnt want to blow the thing up.  I fixed the stuttering using a different needle so i think im ready to rock throwing the 42 back in it from stock.


Thanks for the help and good explanations

Edited by kozakcon

You wrote that you feel the motor is running quite poorly. You seem to have exhausted your jetting options. What you described is consistent with a wide squish gap.

I guessed yours is stock. Which means little. You still have to measure it. Stock will have some range. Due to mass production machining tolerances. KTM or Yam, etc, it makes little difference.

First thing you should do with a new or unknown 2-stroke is measure it, then get it corrected if necessary. It'll cost less than $200. Sometimes much less, like $100 for a head or cyl base shave, or $30 for a thinner base gasket. Head correction and jetting, is a way better way to improve your bike than spending on a pipe, or stickers, etc.

A very safe figure would be 1.3 to 1.4mm for US 91 pump fuel. I'll guess that the KTM 250SX stock range might be 1.5 to 2mm. Someone else might want to confirm that. In my opinion, anything above 1.7 is not acceptable for a 250.

If your squish clearance is too wide, and your rod small and big end bearings are healthy, then tightening the clearance will not hurt reliability. This clearance makes much more difference to how efficient a 2-stroke runs than say valve clearance on 4-stroke. The 4-stroke equivalent would be more like having the timing chain out by a tooth. Yes, wrong 2-stroke squish gap is that bad.

If detonation is a concern, then you can get the squish reduced, but have the dome vol machined to retain the same compression.

No need to remove the head to measure squish. On a KTM you don't even need to remove the tank. Maybe yours is OK. But check it. See http://www.thumpertalk.com/topic/1024037-my-surging-eg295-is-driving-me-nuts/page-4#entry12319000

What is your wide open throttle kick over compression value? At present and when new rings are installed?

Edited by numroe

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