KTM sx 125 jetting issue? Video!

Could be several things to look at/check over. Have you checked the air filter for cleanness and if it is over oiled? What is your oil/gas mixture? What does the plug look like after an acceleration run? Have you contacted BUD RACING for jetting recommendations? And have you set the air mixture screw?

how does it run when you are riding it? under a load? may have rev limiter ! ride the bike & do a proper throttle chop test! you want the limiter to work when you hit neutral!

How many hours on the top end?

Paw Paw

That seems like a lot of hours on a single ring.
Have you done a compression test?
If so, what was the reading?

Paw Paw

You might try disconnecting the kill button just to see if it is failing from viberation

A stuttering or hesitation on the top end at WFO is usually a main jet that is too rich. It needs to be lean enough on top to allow the bike to rev out cleanly.

I would throw the stock pipe and previous jetting back on just to verify the symptom and make sure it isn't something else. If it runs the way it used to with the stock pipe and jetting , put the HGS back on and try a 1-step leaner main.

I would have to agree with FGR01. Try leaning the main jet. But first I would also make sure your spark plug isn't bad in any way (I had corrosion on the contacts recently resulting in the same problem,) clean the air filter, and do a compression check. The YZ125's have single ring pistons as well (what I run) and I have had no problem going 20-40 hours on a piston and I race the bike hard. It's generally from other problems such as dirt getting through the air filter which caused me 3 top ends until I bough PC Racing Pre-Filters. Anywho, make sure those three things are good, then try leaning the main jet.

Make jetting changes one size at a time, not in large increments. A pipe should not require more than two sizes up on stock jetting. Not sure if that carb has an air correction jet, but on some models it is the most overlooked jet in the carb, and can make quite a difference in overall performance.

And for what it's worth, a lot of Motoplat ignitions were installed on 83-84 Honda CR125's ,as the Motoplat that fit these bikes had no rev limiter, as the stock unit did.

Cutting the blue wire on 78-79 CR250 Honda's also bypassed the rev limiter on those models.

Not many manufacturers are going to release a model that will rev freely to the moon, or until the rod turns blue.

Two strokes are just not as obvious when the rev's come up against the limits.

Total final advance in the ignition timing is controlled through the CDI unit. There are limits on most bikes.

Do a compression test as worn rings create lower compression that acts like a rich condition. Like I said before....25 hours on an single ring setup is a lot.

Paw Paw

Not sure if you ever corrected this?

I have had the same issue twice now on two of my 150sx bikes.(2011 and now 2014)

Its the reed vavle.

On my 2011 the bike would feel like it was hitting rev limiter or someone was hitting the kill switch really fast when wide open on throttle.

I spent countless hours jetting, bought every electrical part there was and still no fix.

Finally I just bought a new bike and swapped the parts I had not replaced yet,( carb and reed valve)

Turned out to be the Vforce3. Not sure what was wrong with it as reeds looked new with no fraying or lifting.

I still have the Vforce3 in my garage and was looking at it again when I started to have same issue with my 2014 150sx at the 12hr mark.

Not sure what exactly is going wrong with the plastic reed valves(2011 Vforce3, 2014 KTM copy of vforce with boyesen reeds) but I have purchased a Boyesen rad vavle and hope to never have the problem again.

Thanks for posting that. I'm having the same problem on my 2013 150sx. Does the rad valve give up any power compared with the vforce?

What elevation are you at? temp?

I am starting to realize these bikes don't need the rich jetting everyone seems to think they need.

Rad valve was really rich so cant really say right now,

I went straight to a race without testing and had some bog/burble issues I played with jetting a bit(I tried a 175 main but think it will need a 170main) but its hard to do on race day.

I was so frustrated that I pulled the the FMF fatty/shorty & rad valve off and went back to stock with a new 2014 reed cage (yes the piece of crap plastic one) and went testing today.

Stutter was gone except if I held it in the over rev for a really long time.

Then I got it to run really clean by leaning the main to a 178 (from 180) and the needle to 2 clip from 3rd( 2014 needle is still 1/2 clip richer than your stock 2013 needle in 2nd clip). 2400ft / 24 Celsius

Honestly if I were you I would just by the 2014 reed cage it still plastic junk but its really cheap,here in canada I payed $118(not sure where you are?) so is a good way to test if it really is your reeds(it will be).

Like I said earlier though these plastic reed cages don't last for me. I have 2 vforce3's & 1 2014 reed cage siting in my garage that look like new but make my bike stutter really bad(I even tried new reeds on Vforce even though they looked new and same issue.

When this new reed cage starts giving me issues I will put rad valve back in and take the time to jet it properly I am just tired of working on bikes right now and just want to ride.

Picture of 2014 reed valve

When I first got the bike I had to go richer just to keep it from knocking. Now it's developed this wide open sputter so I was trying leaner main jets without much success. Fluttering reeds definitely sounds like a possibility. Although it's hard to believe they've worn out so quickly.

Did you say that new reeds on the old cage doesn't fix it?

I tried new reefs and still have the reed cage its still looks new.


The 2014 cage fixed my issue.

I would guess a new vforce would also.

I am hoping that the rad valve fixed for good after this reed cage goes bad

I tried the 2014 cage today but didn't get any improvement. I'm about due for a fresh top end so I guess I'll do that before anything else.

Ever get this fixed?

Me? I've just been living with it until I replace the piston. Once I get around to that I'll chase the jetting some more.

I think you're bike is detonating or pinging, a common problem with those bikes.
I would suggest the following:

1) Never use wossner pistons, go for Vertex or KTM OEM (which is a Vertex in an orange box)
2) look for KTM SXS cylinder head. I know they exist for 150s, but don't know if they have them for 125s.
They have a larger chamber (slightly less compression) and makes the bike breathe much better (worked wonders on my 150). If KTM doesn't make one for 125s, you could try a VHM head which does the same thing and has replaceable chambers.