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2003 KX125 - No mid throttle?

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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:

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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.

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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

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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.

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

 

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

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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.

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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.

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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.

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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.

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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!!!

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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.

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You guys swap with the 38mm PWK right?

 

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

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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.

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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.

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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.

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