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35 or 38mm Keihin? TMX swap KX125

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This carb is a POS!

 

If I tune it to run decent, I develop a bog, mostly railing berms.  If you richen it up, you foul plugs.

 

I'm going to swap it out for a Keihin but cant decide on a size. 

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I have a TMX on my 02 KX125 that screams. Sounds like somethings not right. And I would take a Mikuni over a Keihin any day so you can just mail me your TMX.

Edited by Devil Tiger

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jeekinz the pwk 38 mm airstryker is an awesome carb, its not really to big for the 125 how ever it may get on the pipe at a little higher rpm,s than a smaller carb or not be quite as responsive just off idle but the clutch compensates, it does fine .

ktm 125,s come with a 39 mm carb

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Jay,

 

when we raced 125's, one of the first things we did was swap out the 36 for the 38 carb. in the early 90's the 125's came with a 38 and a big reed cage.  big carbs make them run like a raped ape on top at the sacrifice of bottom end, so it may hurt you in the bermed corners if you let the rrr's fall off. the 97's were a handful to ride, no bottom end, power valve spriong changes helped, reed spacerhelped and a slightly lighter flywheel also helped. it is really the whole package to make a 125 run good. If you can keep it wicked the 38 is a good option.

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WOW, just absolutely, ironically, freekin WOW.

 

We are having the exact same issue and just did about 2 hrs of research on this. I diagrammed both carbs and found that carbparts.com sells everything we need. Including the adapter from carb to airbox since there is almost a 18mm difference. The Kehein being shorter. I'm going to call them and see what they recommend. Also the 35mm carb has different dimensions from the K38. So we'll see what they suggest. Or if yo do it first, please post up.

Edited by SAPPERS

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The 36 and 38 Keihin are nearly identical in measurements tothe tmx....from what I found onthe web. Only 1-2mm difference around. Im going to try a 38. I dont really ride the bike WFO, but like overrev sometimes. Id rather deal with no bottom than loosing it in turns. Never had the problem with my 96 RM or 02 YZ.

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Heres the diagram of the Keihens

http://www.carbparts.com/keihin/pwk_jetting_and_dimensions.htm#Dimensions

 

 

Then the dimensions of the Mikuni are

 

A - 44 or 47  depending on exact model   (PWK35 = 42, PWK36/38 = 46)

B - 62    (PWK35 = 59, PWK 36/38 = 59)

C - 76    (PWK35/36/38 = 91)

D - 164     (PWK35 = 164.3, PWK36 = 176.6, PWK38 = 171.6)

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long body or short body carbs - if I remember right pwm is the short body one and pwk is the long body?

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I don't think the 15mm difference in length will be an issue.  The is some room in there.  But that's the carb I was looking at: 38mm PWK airstriker.  91mm long.

 

Could also put a wing in there if the size becomes an issue.

Edited by Jeekinz

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This carb is a POS!

 

If I tune it to run decent, I develop a bog, mostly railing berms.  If you richen it up, you foul plugs.

 

I'm going to swap it out for a Keihin but cant decide on a size. 

As I understand the 99&00 KX 125's came with a Keihin PWK36. My used 2000 KX 125 came to me with a TMX35. I want to start from scratch and buy a new carb and jet it right from the start, however I'm not sure if I should buy another Mikuni or if I should go with the Keihin it came with originally. What do you guys think?

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2stroke... what we are saying is the Mikuni is junk. So replacing old junk with new junk is still junk.

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For what it's worth the TMX worked okay on the 01 CR250 but from 02-07 they were horrible. If you have the flat side TMX try the S7 nozzle - it's a Yamaha part number from a 98 or so YZ125. Some guys file the slide on the TMX to make it work better. I bought a new PWK for my CR250 2002 model but it barely improved overall power and carburetion. That bike has other issues from the factory so hard to say the PWK was no good on it. 

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Disclaimer: I know my bike is f#^%ing filthy, I let the kid "clean" it.  So no busting balls. :busted:

 

 

 

Picked up a 38mm.  funny how everyone on Ebay thinks they only came on certain CR's. lol  Even if I don't like this carb I can double my money from it just saying it was from a Honda.  HA!

 

Straight swap.  I'm running what came in it, 250 specs which are a little lean, different slide and needle.  Going by a '98 KX125 should run a 160 main and 42 pilot, #6 slide.  I'm currently running a 155 main, 38 pilot and #7 slide.

 

Rode in my lot, seems to run ok but won't tell until I'm in some dirt. I'll bring a small flat head and spare plug.

 

IMG_5983.jpg

Edited by Jeekinz
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So the bog while turning is finally gone! I need to fatten up the main and maybe lower the clip one notch, but the pilot seems good. It actually runs real good as-is, I just want to see if I can wind out the top end a little more with a fatter main.

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So, Did you still use the existing cable or have to get another one?

 

This is on my must do list after the rear wheel is relaced. It looks like the boots fit perfect despite the measurements in the pics. I'm guessing no adapter is needed.

 

BTW, you need to wash that thing before working with carbs :p

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So, Did you still use the existing cable or have to get another one?

 

This is on my must do list after the rear wheel is relaced. It looks like the boots fit perfect despite the measurements in the pics. I'm guessing no adapter is needed.

 

BTW, you need to wash that thing before working with carbs :p

 

lol

 

No, existing cable works fine.  As far as the boots go, the PWK is only 1mm smaller on the reed side and 1mm larger on the filter side.  The largest difference in size is the width, the PWK being 15mm longer.  It still installed without issue.  Installing the subframe assembly with the stock TMX is sort of a pain in itself as you have to sort of stretch the airboot onto the carb.  So it works out with the PWK being a little longer.

 

My only real concern is the slide.  The bike runs good now, I don't know what the benefit would be running the 6 instead of the 250cc's 7.

 

BTW, this is a 97 KX250 carb.  38mm, quad vent air striker without the powerjet.  The 98's have powerjet without TPS.  

Edited by Jeekinz
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I'm gonna probably order a new 38mm from carbparts.com.

A good friend and I were discussing this. We're both the engine dynamics type guys. Given the bore/stroke/swept volume of the 125. The 125 would actually like the amount of air given by the 38mm. Whereas the 250 would begin to choke itself having over a 40mm and that a 38mm is probably the sweetspot size for a 250.

 

Most likely, Jeekinz, you're probably gonna need to try the #6 slide. If it doesn't work out, at least you can't say you didn't try. (guinea pig :p )

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Long time lurker here.  Kinda bringing this back from the recent dead but...

 

I too am getting a little tired of trying to get my mikuni to run right so I am seriously considering swapping for the pwk38 for my 03 kx125.  It insists on overflowing fuel even after meticulously adjusting the float, I just replaced the o ring under the float needle as described in another thread so we'll see if that has any affect.  It also boggs if i lean it too far over in corners, and I can not seem to get the pilot jet dialed to where it does not spooge a little while in low rpms. 

 

I have a 98 kx250 so it would be kinda nice to have the same carb on both bikes so I only need to stock jets for one type of carb.  But my question is this, my 98 carb has a plug on it which i presume is the TPS?  if I can find another 98 quad vent, airstriker with the tps will it work on the 125 without the TPS connected or is that going to change how the 125 runs?

 

I guess I'm just confused what the TPS (throttle position sensor) does on a carbureted vehicle?  does it controll ignition timing advance? or is actually controlling something in the carb itself instead of just "sensing"?

 

Thanks for any info.

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