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06' YZ125 carb swap question

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As you are probably aware, the 06' YZ125 comes stock with a 38mm Mikuni TMX carb. My application is off road and I wish to enhance the low/mid capability of the motor and am willing to sacrifice some top. I have heard previously, especially on KTM 125's, that the route to take is going to a 36mm carb and seem to be indicated on other MX 125 conversions.

1) Is this swap accurate and a worthwhile mod or should I have the 38mm machined/divided (ala power-now and power-now plus) and call it good?

IF viable/reasonable mod, then:

1) I note with some curiousity that a 35mm TMX is manufactured and that the carb inlet/outlet side apear to be the same diameter as the 38mm TMX. Is this accurate? Bolt-on mod in replacement with same cables, etc?

2) Comments as to going to a 36mm (machined and divided) Keihin air stryker carb in place of the 38mm TMX?

Many thanks for your expertise...

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Have you tried rejetting your stock carb? Yamaha has 13 or so optional needles for this bike and 1 optional slide. The slide is a 4.25 up from a 4.0. Not that big of a difference. Leaning the slide and needle should produce much better low end. A Kx125 uses a 5.75 slide and has an available 5.5 and 6.0.

We just got a 06 YZ125 and just from starting it and watching the smoke pour out it appears to be way way rich. I will eventually try a 5.0 slide with a stock KX125 needle as that may be the ticket.

Going small on the carb may make the low end too bright not to mention spending tons on brass to get it jetted properly.

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Thank you, I am working with JD Jetting and I have become pretty happy with my jetting now. I am looking at alternatives and the 35/36mm route had been spoken about and made me curious.

I will likely go ahead with motor mods (porting/milling head) and have the divider plates machined into the stock 38mm carb, unless I can get some positive info about a carb swap being a dramatic change.

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I too am turning this MXer into a off road bike for my son. The mods will be:

Two teeth larger back sprocket (Ironman)

V-Force 3 reed

FMF fatty w/spark arrestor silencer

Phat Head Racing head w/two steps higher compression dome

3.0 gallon Clarke tank

and a few more goodies.

When tuning his YZ85 we went to a 3.5 slide (from 3.0) and a stock KX85 needle (NAPE).

This really added a bunch of low end.

It seems that Yamaha jets its two strokes super rich on the slide and needle.

If you examine how Kawasaki does it their slides and needles are much leaner but top it off with a larger main. This is probably one of the reasons why Kawasaki's were known for there low end 85 & 125 motors.

Yamaha was the only one to use a 3.0 slide in their 85. All of the others used a 3.5 (except Suzuki which uses a cheaper carb than the PWK).

Hence the reason I want to try the KX125 needle and slide in the YZ125. It worked before and maybe I am on to something.

Now if I could just find a TMX needle demension chart to decode all the needles. So far nothing even from Sudco :thumbsup:

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Extremely good insight. I would like to hear what you come up with. I will do some checking on needle spec's, etc and post up.

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We haven't even ridden the bike yet as desert season doesn't start for a few months here in San Diego. So it will be awhile before I start fiddling with the bike.

These are the available needles from Yamaha for the 06 YZ 125:

6BFY42-74 STD

6BFY42-72

6BFY42-73

6BFY42-75

6BFY42-76

6BFY43-72

6BFY43-73

6BFY43-74

6BFY43-75

6BFY43-76

6BFY44-72

6BFY44-73

6BFY44-74

6BFY44-75

6BFY44-76

Here are the needles from a 2004 KX 125 (KX125 is no longer in production)

NEEDLE-JET,6CHJ10-80 1 (OPTION)

NEEDLE-JET,6CHJ10-81 1 (OPTION)

NEEDLE-JET,6CHJ10-82 1

NEEDLE-JET,6CHJ10-83 1 (OPTION)

NEEDLE-JET,6CHJ10-84 1 (OPTION)

NEEDLE-JET,6CHJ11-80 1 (OPTION)

NEEDLE-JET,6CHJ11-81 1 (OPTION)

NEEDLE-JET,6CHJ11-82 1 (OPTION)

NEEDLE-JET,6CHJ11-83 1 (OPTION)

NEEDLE-JET,6CHJ11-84 1 (OPTION)

KX125 slides are:

VALVE-THROTTLE,CA5.5 1 (OPTION)

VALVE-THROTTLE,CA 5.75 1

VALVE-THROTTLE,CA6.0 1 (OPTION)

Yamaha slides are:

4.0 STD

4.25 Optional

I am pretty sure the last two numbers of the needle are the diameter at the largest point. This means that all the KX needles are way leaner. I can't figure out the other dimensions without a dimension chart but if I have to I can guess.

Standard KX jetting:

Pilot 40

Slide 5.75

Needle 6CHJ10-82

Main 420

Standard Yamaha jetting:

Pilot 40

Slide 4.0

NEEDLE 6BFY42-74

Main 410

Interesting eh!

Whats even more interesting is that the Kwacker needle appears to be leaner than the leanest Yamaha needle and Kawasaki even has way more leaner needles available than standard. Not to mention their slides.

Honda has some optional needles too but not many.

None of these needles are available from Sudco. They appear to be made exclusively for their respective OEM partners.

I just have a sneaky suspicion that the good ol' boys at Kawasaki have this jetting thing down way better than the Yamaha camp. Yamaha seems to be afraid to lean out the slide and needle which will produce good low end. Kawasaki leans the heck out of the mid range circuit but tops it off with a big main to keep things in check. Makes sense to me.

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As you are probably aware, the 06' YZ125 comes stock with a 38mm Mikuni TMX carb. My application is off road and I wish to enhance the low/mid capability of the motor and am willing to sacrifice some top. I have heard previously, especially on KTM 125's, that the route to take is going to a 36mm carb and seem to be indicated on other MX 125 conversions.

1) Is this swap accurate and a worthwhile mod or should I have the 38mm machined/divided (ala power-now and power-now plus) and call it good?

IF viable/reasonable mod, then:

1) I note with some curiousity that a 35mm TMX is manufactured and that the carb inlet/outlet side apear to be the same diameter as the 38mm TMX. Is this accurate? Bolt-on mod in replacement with same cables, etc?

2) Comments as to going to a 36mm (machined and divided) Keihin air stryker carb in place of the 38mm TMX?

Many thanks for your expertise...

I was recently told that a 2mm oval overbore will yield a big boost in low end. has anyone ever tried this mod?

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