All 2st About the Mikuni/Keihin swap:

I don´t really understand what makes the Mikuni that bad. Isn´t it a pure jetting thing? I mean, what is it that makes the difference if it´s not a jetting issue? Is Mikuni that bad after making carbs for a hundred years? It would be intresting to see the results if a jetting specialist put some time on a Mikuni and compare the results with the Keihin. It would also be intresting if someone knows what it is that makes the Mikuni so bad. I understand that people feel a big difference when they put a dynotested and correctly jetted Keihin on their bike if they had a bad jetted Mikuni on it before. Don´t misunderstand me here. I don´t say that people can´t jet their carbs, but it´s easy to buy a jetted carb and then forget about jetting anymore. Personally i like to learn about jetting and i think my bike is running fine, but i will continue to test with different jettingparts just to learn more about it. Maybe i´ll end up with a Keihin myself one day, but not before i have tested the jetting on my Mikuni more. I talked to a carb specialist today and he said that i should come to his shop and we should do some testing, so i get back with more info on this! Intresting stuff i think!

That would be cool to work with a carb specialist just to see how they go about jetting. I have my 08 CR125 close now but I think it could be better but I really don't understand enough about jetting to get it perfect.

Hi John. Yes, it will be intresting to see what he comes up with. To hear his theories and test them afterwards.I´ll probably go there tomorrow and i will giva a report after that!

Johnny

I am by no means a specialist.

With the PWK on my WR250, I now practice things like not using the clutch on singletrack. While this might sound weird, it saves me time during a race because my bike will pull cleanly from barely above 0 rpms up to max rpms (no need for the stinkin 4 stroke!). Maybe a pro rider doesn't need this because they are always wide open, but for me it is very nice. I never change jetting. In the winter I tape some of the airbox shut and it runs fine. When I had the stock Mikuni carb on there, it ran poorly with the minor temp change that would happen from morning to afternoon. The Mikuni worked fine if I kept changing jets - but I'd rather just ride.

I will sell you my mikuni if you'd like a backup.

Maybe i can use two!!

I don't miss the Keihin. I had my 125 running good. The 144 kit played some tricks on me but it's just a jet change away from working good.

I had lots of other issues with the Keihin when I owned ktm's.

The best carb I have in my fleet is the Mikuni on our ktm 65sx. It just works at any temps, no bogging, no leaking, no spooge,
easy to work on.

Its not so much a jetting question...but one of signal strength

When the TM flat slide carbs came out way back when....the TMX came out soon after.....its been used over the years on many bikes as you know....we have all become very familiar with the internals in this carb

I would never say the Mikuni TMX is a bad carburetor... but over the years it has gotten a reputation for being fussy for driveablity and jetting...and this has everything to do with signal strength....pulled air(velocity) from the engine is routed over the various orifices in the carb and this is referred to as "signal"... anytime you have a massive carb on a small displacement engine your going to have a weak signal and it must be jetted jetted just right.....

The Keihin PWK has features that have been optimized to have this signal more concentrated at key areas to increase the velocity or signal..... particularly at low RPM areas where it translates to much better driveablity...and its much less fussy to jet

Personally I have stayed with the TMX over the years....and I have learned to deal with them and the new version on the WR125 has much better progression than the previous one....but the new WR is a very sensitive beast when it comes to carburetion as we know......

am I switching to a PWK...??? Yes

I too have stuck by the TMX, but when all the CH Racing Factory 2 stroke race bikes have Keihins.... it's not like you can go to them for the hot TMX jetting tips. If they think the Keihin is the best carb for performance, I'll give it a try. Motosportz has a great deal on the PWK with JD supplied jet kit, so I ordered the kit from Kelly!

A J.D. Jetting 38 mm PWK air striker with a power now rear insert is as close to F.I. as it gets!

The 01 and up Honda CR250 came with the same TMX that the 08 and earlier Husky 125 did (I m not sure if the newer 250/300 still have this carb). Anyway, it is very common for Honda riders to switch to a PWK also. I really wonder how much of the poor low end from the last generation of CR250 was due to this carb.

But I thought the 09 WR125 came with the same Mikuni as the YZ125? If so I thought it would work better since I do not hear of YZ riders switching to a Keihin. Or maybe the limited needle selection works with a YZ? So I take it the Mikuni on the newer 125 is not a shorter body then the old one? I'm just curious since I would like to know if I should keep my current PWK if I get a new 125.

I do not have a lot of trust in Mikuni which is one thing that turns me away from the FI 4 strokes. I don't know why Husky avoids using Keihins.

It is a shame that if I buy a new 125 I have to add the price of a carb, spark arrestor, and tank (if one is ever on the market). That adds quite a bit to the cost.

Agree on the lack of TMX needles. I used the term "jetting tips" more broadly, really meaning complete carb set up, including needle, clip position, etc.

On a side note....

I was looking at some pics of CH Racing 2009 2 strokes. Guillaume's WR300 has a PWK and Oblucki's WR250 has a PWM. I haven't seen a good pic of a WR125.

NWRider,

The new TMXX is about a half an inch shorter from the the intake flange to the airbox flange. It is a really tight fit to get the 38mm PWK airstriker to fit in there but it is well worth it. Keihin does make a 38 mm PWK airstriker that is the same dimension as the new TMXX and the keihin PWM, but it is a proprietary KTM product. The designation is 38 PWK AG. Try finding one of those around. They are only $275 from the cheap KTM parts outlets. If you find someone parting out a late model KTM 200 grab the carb.

When I made the switch to the JD kit PWK from Kelly all my jetting issues went away. One slight tweak to the packaged jetting and I probably will never open the carb up again. It just works flawlessly and runs great.

One thing is that the jets that work in my 144 are 4 sizes bigger on the main than I had in my WR250 and considerably richer on the needle and one step richer on the pilot. The Signal strength comments make complete sense.

Walt

Interesting. I see KTMs being parted all the time. Most of the newer ones have 36mm carbs though. I have looked at some KTM models and it seemed like the float bowl area is really big and I was not sure if it would clear the case. I am not sure if I was looking at a PWM or the PWK AG though. Does the AG look like a normal PWK and have a screw top? Is this carb only on the 200? Munns racing with a ktmtalk discount would probably be the cheapest. Maybe I'll check it out, or keep my 36mm PWK, or get a new 38mm PWK from Kelly and live with the tight fit. Don't know what to do.


Even with the PWK I can not just leave the jetting. The 125 really needs to be perfect. Too lean or too rich and it will take a lot of clutch to pull on the pipe. Of course I am on a 06 and I was running avgas which wanted really lean jetting. I could never get a JD needle to work at all with that fuel so I alway ran C tapers. Now I am running 50% AV and 50% pump and the JD needle works well and it seems to not be as picky with jetting. I have not run this fuel in really cold or hot weather though so I can not be sure.

The AG is a screw top airstriker 38mm. The one I saw that has this model was on an 06 200 xc-w for what its worth. I don't know squat about the orange bikes.

Walt

I am going to take the engine side carb boot off and see if I can create a little more room for the carb spigot. Just another 1/4" would make the fit much easier to get on and off. Of course it runs so well I might not have to take it off again.

Walt