2003 KX125 - No mid throttle?

Hey guys,


Newb here, just bought my bike yesterday - 2003 KX 125. The bike has some minor mods - FMF Fatty and shorty, Athena cylinder and piston kit (stock size) with around 20h on.


While riding the bike, it seems like the bike do not like to mid throttle at all, the bike is jerking pretty bad and power goes on and off.

I'm not new to 2stroke mx, I rode my friends CR250 and 125. 


We cleaned the carb (pilot jet had crap in it) and thrown a new plug, but still doing the same thing.


As part of our troubleshooting we checked compression - around 120 psi, seems a little weak? 


Do you guys have any idea of what I should check next? We tought of the powervalve maybe not sealing properly?


Sorry if I did some grammar/syntax/spelling mistake, french canadian on the keyboard  :ride:

Did you do the compression test with the throttle wide open?

Did you do the compression test with the throttle wide open?


Yes we did!


I would also like to add, when I hit the powerband, the bike is smooth and pull great.

Remove the powervalve arm cover on the side of the cylinder. Run it, the arm should move when you rev it.  But it's time for a top end.

Remove the powervalve arm cover on the side of the cylinder. Run it, the arm should move when you rev it.  But it's time for a top end.


Will give it a try!


Can low compression cause this issue? Also, do I need an Athena piston kit to match the Athena cylinder, or can I throw in any aftermarket piston?

Edited by lp216

Friend of TS here,


I'd also like to mention that we pulled the carb and cleaned it. Inside we found:

400 main

6CHJ11-84 needle 3rd clip

35 pilot


But I'm not sure how that compares to stock. I was also under the impression that we got 110psi out of it... either way, not good. We also fiddled with the idle and air screws. I had to really tight up the screw to get it to idle. But then again, it's probably a lost cause with a worn out top end.

And again.... 120psi, you're luck it even starts.


Anything below 150, it's time for a change.

So we did the piston today, but we are still having the same issue...


We noticed bike was spilling gaz when on the stand, so we checked and adjust the float level to service manual specs. Even after we adjusted the float the bike was sometime spilling gaz when on the bike, sometime not. The bike sounded rich at mid throttle so we moved the needle and it really helped, but a few run later it was running lean.


We are running out of idea what to check/adjust since it is really inconsistant, so if anyone have an idea of what is off it would really help!


Also, my friend checked the reed and said he could see light through it, so it's defenitly time to change. I'm wondering how bad it could affect engine tuning/performance/reliability?

I'm on a budget and would prefer to wait 'till winter if I can.

Any cracks or missing pieces to the reed petals

Any cracks or missing pieces to the reed petals

No, but the 4 corner ones are concave (bent inwards) enough that you can clearly see daylight under them. It leads me to believe that the previous owner might have already flipped them.

You can't tune a broken engine, so fix the reeds and go from there.  


Also, good luck if that still has the p.o.s. TMX carb still on it.  I did Keihin swaps on both my 125's no carb issues since.

You can't tune a broken engine, so fix the reeds and go from there.  


Also, good luck if that still has the p.o.s. TMX carb still on it.  I did Keihin swaps on both my 125's no carb issues since.

Yup, still has the POS TMX. Tell me about it, I have a late model CR.


But since we're on a limited budget, we'll have to make this one work.

I made money on my swaps.  I bought 2 used Keihins off Ebay for less than what I sold the TMX's for.

I did the Kehein swap and I can only hope no one goes through what I went through.



Well at least some I like. And I don't like too many people. But Jeekinz helped me tremendously. Thumbs up!!!

We'll change the reed petals and keep you posted.

If the reeds are as you say..they will never seal right so that is the first step. Then as stated either toss that tmx carb or keep it leaned out as much as possible. You could try the JD kit but I would opt for a real pwk or atleast a good pj keihin if possible.

You guys swap with the 38mm PWK right?


We'll make sure the fuel needle valve is not worn out 1st.

I used a 38, Jeekinz used a 36. He had much better luck than me. I spent 4 months figuring out the jetting. His took him about 4 minutes. Considering it's the same carb as the 250 minus the electronic stuff, the 38 on a 125 get a whole lot of air and will want much, much more fuel to compensate.


I use a 50 pilot and mains from 178 to 185 depending on weather. But I also run 110 octane fuel.


If you do a search on youtube s/n 21Nsapper, you'll see an array of vids on my 125 fiasco. Almost came to a 44Mag to the head of the bike.

No, I used a 38 on my bike and a 36 on my stepsons bike.  The 38 from a 97 KX250 was a direct swap, I'm still ising the 250 slide, just jetted a little different.  The 36 I needed to fab spacers to fit.


Just search "TMX" in this forum, you'll find all my info.


Besides the TMX leaking, I could never jet the 'bog while turning' out of it.  I bought a pound of brass and tried multiple combinations.  I put the Keihin on, after a couple minor jetting changes it was dialed in.  Haven't been into the carb since then.

So. I'm wondering why my jetting is so much higher than yours. Even on pump fuel. Maybe the difference in 03 to 04-5. 2 weeks ago I was getting a little bog in it and tried going up and down on the pilots. Up or down from a 50 and it did not like it one bit.


Now the compression is down to 140 so a new top end is in order. Maybe I'll order it this weekend.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By luke8500
      Hi all,
      I have a 2004 Kawasaki kx250 that died suddenly on me the other day. all i can get it to do is backfire every once in awhile. Tested the resistance and everything of the magneto and stator with a multi-meter and all is good.  Coil is new oem Kawasaki the reeds are brand new vforce and the top end and carb have been completely rebuilt.  key on crank is intact and all the timing marks are lined up. I hooked a timing light to it and spun the motor over really fast and found that it is sparking way to late.  The service manual says that it is supposed to fire 14 degrees before TDC but it is sparking way after TDC.  Im thinking its a fried CDI box since its the only thing i wasnt able to check.  Any help would be appreciated
    • By luke8500
      Hi all,
      I have a 2004 kx 250 2 stroke and it wont start.  it only backfires.  I was out riding it one day and came up to a stop so the bike was just idling and it started to try and die so i revved it up a few times then let it die and now it wont start back up.  It only pops or backfires when you kick it.  Its getting good spark and i just put a brand new plug and coil in it. I took the reeds apart and they look like they're brand new.  It was running great before so i dont know what happened.  Im thinking it could be a fuel problem even tho the plug is a little wet if you take it out after a bunch of kicking.  Any help would be greatly appreciated 
    • By Zach121
      Alright, found three bikes all located where i live in Texas. I know they're all very different, but if you could tell me what you think each bike is worth, and which one would be the best deal. Thanks.
      Notes: Both 2 strokes have new top ends. I think the front brake lever is broken on the kawasaki.
      KX250 2002 $800-$1000?

      YZ125 2000 $1250

      KTM 400SX 2002 $1200

    • By Ryan Turner
      So I'm at a bit of a cross roads, I currently own a 06 rm250 and I love it. I also have a 02 kx250 which I'm selling with intent to buy a newer two stroke. I can't afford a newer ktm and the older ones aren't that great and kawasaki never impressed me. so I'm stuck between 06 yz250 and 05 cr250. I've heard the yz and rm after 06 are almost the same bike just the Suzuki handles better and the Yamaha has better suspension. So I'm not sure if I wanna buy carbon copy of my bike with slight differences. That's why I'm interested in a cr250 I heard the 02+ models RC valves are shit garbage but was mostly fixed in the 05-07 models. I ride at a track and at some bogs down the road from me. So from people who have owned both, what would you recommend. I know the cr is known for a hard hitting power band which I enjoy since im an aggressive rider. Should i just see which deal comes my way first.
    • By dirtbikesrule
      If you own a 2 stroke dirtbike,please reply once. Im just wondering how many people on this site have them.