Home Forums Tech Talk Carb jetting questions

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    • #46711

      I have a kart powered by a 97 yz125. I bought the kart used with a Kehin PWK 72 39mm carb attached. It seems to be running a little fat so I am trying to gather info to jet it correctly with the help of my buddy. BTW I am not too familiar with carb jetting.

      Looking at jet sizes online it seems Yamaha OEM carb stats list the main at 350 and the pilot at 50..45.

      OEM Kehin for the PWK 72 list 160 for the main and 60 for the pilot/slow jet.

      In the carb now I believe the main is a 150 and the pilot 50.

      First off my question is the PWK 72 too big for the yz125 and secondly why is there such a difference in jet #’s between the two manufacturers?

      I cant seem to find a conversion chart that makes sense. One will not load and the other has very similar ranged sizes between both.

      Maybe I will just slowly step it down and keep checking the plug using Kehins #’s.

      Any help would be much appreciated. Thanks in advance!


    • #46736

      Just double checked. It actually has a #180 main and a #50 pilot…

    • #46767
      bo rougeou

      First you need  to look at the literature available on how a PWM  works and the relationships of the needle and jets over  RPM.

      And be careful,use only Kehin jets so that the taper fits the needle.

      I am no carb expert, but knowing how it should work will point you in the right direction

    • #46813

      Yeah, if been reading all about it the past couple days.

      Did a compression test and it was good.

      Dropped the jets down to Kehin OEM specs. 160 main and a 60 pilot.

      This things top end now is a beast and pulls the front wheels off the ground!

      Off idle it’s a dog though… Stuttering and bogging. Need to mess with the Air adjustment screw and if that’s not the case start looking into the pilot more.

    • #46832
      Greg Wright

      A check of the needle and needle position may be in order.

      Greg Wright
      Rapid Racing Inc.
      Vintage B-Stock Pilot
      "When in doubt Gas it, It won't help but it ends the suspense."

    • #46846
      Matt Dixon

      Sounds fat on the pilot.

      We have a track that the AA gets over 7000ft. We dont usually get down to a 160, but close, but our pilot is much lower then yours.
      But that is with a 99 CR125.

      Good Luck


    • #46847

      The clip on the needle was in the lowest/richest slot out of the 5.

      It’s now in the 3rd/middle and I will just go from there.

      I’m not sure why it was setup to run so dang rich…

      Maybe it was a carb off something else and he just threw it on to sell it… Who knows?

      Learning a lot though! So that’s a plus.

    • #46859

      I’m stationed in Oklahoma and our field elevation is roughly 1,200 ft.

      Just throwing that out there if that helps. Thanks for the replies!

    • #46926

      From all I gathered from the past few days of reading kart & bike forums, articles and sites for both the yz125 and cr125; is I should be in this ballpark:

      Main: 170 – 180
      Pilot: 40 – 45
      Clip: 2nd or 3rd slot
      Air screw: 1-1/2 to 2 turns out.

      Like Matt said, fat on the pilot with the 60.

      I’m throwing in a 170 and 45, 3rd clip, 1-1/2 turns out and see how that goes.

      Like I said, the 160 and 60 was just a starting point seeing how that’s what the Keihin PWK 72 came with out the box.

    • #46931
      Matt Dixon

      Fatter pilot to start, those are mighty lean.
      If it boggs off corner lean it out, if it hesitates then fatten the pilot.


    • #46936

      45 is lean for the pilot?. Most have been 42’s and 45’s I’ve came across for 125’s running Keihins (38 & 39mm)

      The richest I’ve seen the main was 185, most were 170 – 180.

      It’s apperant I’m confused… haha!

      The 60 in it now I know is too rich. Even with the 50 it had in it before seemed a little rich as well.

      It doesn’t like to idle and the idle screw seems to do nothing.

      It bogs out of a corner for sure. Falls flat on its face… However, up top in the band it rips like I said.

      I sent JD jetting an email as well, and I also know every kart/Bike setup is different.

      Just trying to get it running right.

      It’s been one thing after another and this is the LAST step. Everything else is paractically brand new…

    • #46940
      Matt Dixon

      Remember the 2 jets work together at WOT.

      If the 2 motors should be jetted close then try 168/55/dgj2

      We ran that just fine with the AA of 2000ft


    • #46972
      bo rougeou

      What Matt said.  Be sure the pilot jet hasn’t picked up any trash.

      That will make it behave like you described

    • #46973

      The jets in it now are brand new (60 & 160) and I cleaned the carb.

      I’m about ready to just order a OEM carb seeing how I know nothing about the history of this one.

      Is the PWK 72 39mm to big for a 125? Also, are karts jetted differently than bikes?

      Sorry for being a PITA. Just trying to figure this all out… Kinda frustrating. Lol!

      All I know is it has a big carb on it, it doesn’t want to idle, the idle screw does nothing, it bogs hard when you first get on it, and it’s a rocket ship up top in the band.

      I can bang gears all day long tbat way and it pulls hard. Just that horrible bogging down low when your not ripping it.

      101 octane @ 32:1

    • #46974
      Matt Dixon

      We run a 38mm in Stock Moto so 39 will be fine, save your money.

      I have told you what you need to do to get it to run, we ran pilots in the 40’s when we ran a restricted Jr125.

      It should not idle, the motor fuel requirements are much different then a bike.

      Jets are cheap, put them in and you will be close.

      New jets get clogged just as old ones do, run a filter.

      Trust me go run it and report back, I gave you a fat set up but will “rip” at all rpms.

      edit: 32:1 is what we always run


    • #46975

      I really do appreciate it. I’ll head to the shop and grab the jets you recommended or order them if they don’t and a few other close in that range.

      Thanks again. I’ll let you know how she runs. I’ll run through the carb again as well.

      I should stay with the 3rd clip setting right?

      I installed a new pump, lines and filter as well last week.

    • #47314

      Update: ordered a JD kit. ended up installing a 55 pilot, 175 main, blue needle with the clip in the 3rd notch. Air screw 1-1/2 out.

      (Kit didn’t have a 168 main, but came with a 162,170, 175 & 180)

      17O was a little lean up top my buddy said according to the plug and 180 was a little rich.

      She idles now and the idle screw actually works!

      Low end cleared up nicely. No more bogging/hesitation. It’s now acting like a typical light switch 2-stroke. Lol!

      Plugs is a light tan color after a WOT pull.

      Thanks again for all the help guys!

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