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Mid Throttle Sputter?

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My buddy bought a 99 KTM 250 EXC with a 98 RMX 250 carb and now it sputters at steady mid throttle.  2 strokes usually sputter a little at a constant throttle, I mean they are a race bike not a cruiser, but this sputters like no other 2 stroke I've rode.


So I am pretty comfortableicon1.png working on carbs and diagnosing them except for this one.  The problem is that the 99 KTM EXC starts up no problem, idles great, runs great WOT through all gears, but when mid steady throttle it wants to sputter and foul a plug.


1. Disassembled and cleaned carb, tank, pep cock, and remix fuel and instal new plug.

2. I thought that the needle needs to be replaced since that controls mid throttle. 

3. He swapped the carb tonight and put back the orignal KTM carb and it rode better but still the same issue jsut not as bad.

4. How would the CDI or other ignition issue be happening to make the same problem and how do I check it?


This is what the RMX carb on the KTM currently has:

I think it is PJ Carb

Needle - R1472M 1L

3rd Position

Main - AB175

Pilot - 55

AF screw at 1.5 turns out


Not sure what type the KTM carb is jut I am told it is the same PJ styleicon1.png but here is what is suggested:

45 pilot @ 1.5 turns out on the air screw 

170-172 main jet 

NOZH needle in the 2nd leanest clip (2nd from the top)


Thanks in advance

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I had mid range splutter on my 150 when summer came, I went one leaner on the needle and it sorted the problem for me

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Yup jetting could be the problem, being that the needle is to rich, even if you switch carbs, if the other one is the same jetting it wont change anything unless your old carb is dirty, but how was your spark plug, like the colour and what oil ratio are you using

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Here is the jetting chart for the bike but since the carb has been swapped out I don't know how much help it will be.


My bikes don't sputter at any throttle no matter how long I hold it there. They even idle for ten minutes without problem while I run them out of gas after I am done riding.

Edited by Larry63r

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