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03 kx 125 carb swap

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So i have an 03 kx 125. Bike runs pretty good but has a notorious flat spot. It's got the stock mikuni tmx carb and word on the street (or this forum) is that they're junk. I have an extra pwk off a 05 kx 250 laying around that I was thinking about putting on to see if it would help out performance a bit. Has anyone ever swapped a pwk in one of these bikes?

Thanks in advance!

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The 05 250 carb won't work. It has the electronic stuff on it. We too have an 03 125 and an 05 250. The 125's is a Mikuni whereas the 250 has a Kehein. Yes, with enough ingenuity, you could make them work. But not worth it.

Your flat spot is due due jetting. Ours had the same issue. bad flat spot and sputtering. The best swap you could do is to give the 125 a 34mm carb. Otherwise, you need to lean it out and work with the needle clip position. I eneded up jetting the bike with a 370 main, 37.5 pilot and clip on top position. Even i pos. down on clip creates bad sputter. I was only one step away from ordering the leaner needle.

Also, what pipe do you have? It originally had a PC works pipe on it when we bought it. My son begged for the FMF and it made a night and day difference. The FMF rocked.

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hmm, i know the 05 carb has the tps and solenoid controlled powerjet, but none of those are needed and they shouldn't interfere with anything. I have a pc pipe and remember you posting about swapping to the fmf. This spot i have definitely seems like a lean one. I'll have to dig into it a bit more.

Sappers- Any interest in a wiseco pro lite for that 250 you just blew up? I a brand new extra one if you are interested. Pm me if you are

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On 11/15/2011 at 6:35 PM, SAPPERS said:

The 05 250 carb won't work. It has the electronic stuff on it. We too have an 03 125 and an 05 250. The 125's is a Mikuni whereas the 250 has a Kehein. Yes, with enough ingenuity, you could make them work. But not worth it.

Your flat spot is due due jetting. Ours had the same issue. bad flat spot and sputtering. The best swap you could do is to give the 125 a 34mm carb. Otherwise, you need to lean it out and work with the needle clip position. I eneded up jetting the bike with a 370 main, 37.5 pilot and clip on top position. Even i pos. down on clip creates bad sputter. I was only one step away from ordering the leaner needle.

Also, what pipe do you have? It originally had a PC works pipe on it when we bought it. My son begged for the FMF and it made a night and day difference. The FMF rocked.

I see a #470 Main but not a #370  ???

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Posted (edited)

EDIT: I just realized this thread is 8 years old...oh well my infos could perhaps still be of interest :ride:

 

The Yamaha YZ125 has successfully been running the Mikuni TMX38 almost forever, even the new '19 still have them.

Since 2007-on, in stock form the factory settings are just about ideal for 95% of all riders.

Other bike manufacturers have used the TMX on their 125's but have never seemed to find as good baseline settings for it.

 

Typical YZ settings are:

(2007-10 models with the standard silencer)  

 

2-1/4 turns out on the air screw

#40 pilot jet

6BFY43-74 needle, 3rd clip

#410 main jet

fuel level (not float level) 10mm +/-.5mm above the bowl's mating surface

 

I'm sure they'd be a decent starting point in other 125's

Those are for MX usage, for trail/off-roading, leaner pilot and needle settings and a richer main jet could be beneficial.

 

The bad reputation the TMX gets is mostly with larger 250/300cc engines as it doesn't have a power jet circuit.

Edited by mlatour
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I'm running a 6DGY26-66 needle from a CR125 in the TMX on my 03.  I have no complaints.  It runs crisper than the PWK I have on my other KX125.  Seems like it is difficult to get the Keihin leaned out enough to work well on a small bore, at least at my elevation.  The PWK is set up as lean as I can get it whereas the Mikuni TMX is in the range of adjustment.

The stock 03 TMX needle is too lean in the 3/4 range.  The 6DGY26-66 has a richer taper and runs much better toward WOT.

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