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2018 yz 125 jetting help

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I've not had to experiment much with needle settings but here

are some infos I found online and keep in my files :

 

clip position : affects the whole range but moreso in the 1/4 to 1/2 throttle range

 

needle diameter:  affects the low end of the range, say off-idle when the slide just opens

 

needle taper angle : affects upper end of the range, say around 1/2 throttle

 

A bit complicated to sort out at first but again the YZ125 service manual has a very complete description

of the various taper/diameter OEM needles available, reason why I doubt the infos from a JD kit would be as comprehensive for this application.

 

What is the optional '1/2 clip richer' needle in my 2010 is actually the stock needle on the 2011+ models, 

richer settings required when they switched over to the longer world spec silencer.

Edited by mlatour
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I should perhaps also explain, I run the optional 1/2 clip richer needle with the clip in position #2

which makes it a 1/2 clip leaner setting than my stock needle in it's nominal position #3.

 

I've tried the stock 42-74 needle in 2-3-4th position and it simply doesn't respond as well as the optional 43-74.

In theory they both have the same diameter and taper, only the clip notch height is different.

Edited by mlatour

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Knowing how to jet is best.

However the yz125 comes from factory jetted quite good for sea level and 70f.

For that reason I take the lazy way and use the iPhone app for yz 2 strokes,it uses your locations god weather and adjusts your jetting accordingly. No need to guess on anything humidity, altitude,temp all compisated for.

 

IMG_1525.PNG

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Again like any chart it's a suggestion or starting point, but doesn't compare to fine tuning to your specific conditions:

 

-load (type of terrain)

-type of riding (slower trail / faster MX)

-engine characteristics

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Decide on your premix and octane then start to jet.

Start with your PJ - if you can't seam to get good motor response with turning the air screw drop a PJ size until you do.  I'm running a 37.5 (factory was 40) for my 05YZ144 by E.G.

Once you settle on the PJ and get the air screw adjusted focus on the needle.  If you're rich and you are getting a sputter -  lean it a position until it is gone. Remember you are only Looking at 1/3 - 2-3 throttle.   I actually changed needles from blue to red but I'm running a JD Jetting kit (love it).   Their needles dump gas fast!!!! And impact the MJ size you will run.

Next focus on MJ.   Now you can do plug reads (not used for PJ and needles) but make sure the plug is not brand new. It must have a good half Hr on it.   I actually run a little rich on top.  I ran a 420 MJ and the plug looked perfect but felt like I could get more overrev and I wanted to be safe side and settled on the 430.  Always settle for the richer jet unless you have negative side effects.  I trail ride but have plenty of areas and hills climbs to hit 12k rmp's and who doesn't love that sound on a 2stroke.   I have never fouled a plug yet. I shouldnt have any MJ change for fall-just needle and air screw.

Remember to properly jet the motor, reeds, and pipe&silencer need to be in good shape.  

I struggled a bit at first with Jetting due to the whole144 change but it is a great experience when you dial it in and full power comes out.  The 144 is much stronger than the 125.  Eric Gorr did a great hub and was a huge help in the decision process with porting.  My old cylinder was sleeved or I would have kept it a 125.  $500 included a cylinder, big bore kit, plating, porting, power valves, piston, ring and life time of honing.  

Nothing magical Jetting once you get into it- Try and test.  I kept plugs reads for specific set ups at specific temp/humidity.  By the way I'm running Maxima 927 32:1 with 93 octane.  

Good luck.  

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The kit for 125 has factory Main jets ranging from 400 -430 and has 2 needles - blue and red.  Blue is for colder conditions and red for hotter. The needles are what make their kits special.  They give you a instruction sheet but they have great customer service.  I called several times and they were always willing to help.  I got mine off Amazon.   I decided to use them off a tip from Eric Gorr.   Happy I did.  I did try factory needle 125 needle but didn't like the results.  I'm sure I could have found the right setup eventually but I already had purchased the JD Jetting kit.  Good luck 

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Thanks alot man i think im going to purchase one tommorow ive heard there needles are pretty good and if it saves the time of me fooling with jetting when the weather gets colder ill pay the 80 bucks lol

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By the way mlatour was helpful on a post of mine and really knows his stuff.   I was reading his post and he is spot on.   

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It was only saving me from swapping main jets for cooler weather as I'm a little rich in the hotter weather - I'll take that assurance. I will still do a plug read when it get cold.  Never want to be lean on top end, besides the 144 has plenty of power.   First thing I had to do is get an oversized front rotor!!!!

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Thanks alot man i think im going to purchase one tommorow ive heard there needles are pretty good and if it saves the time of me fooling with jetting when the weather gets colder ill pay the 80 bucks lol

I couldn’t get the jetting down on my 2018 300 for the life of me I bought a keihin carb from JD jetting and they &%$#@!ed up and sent me the wrong one so now I’m out about a week what kind of carbs come in the YZs?

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The yzs have a mikuni tmx 38mm
I know this off the top of my head because i was researching those jd jetting kits

The michuni is horrible I hated mine I even had the slide modded it was way to sensitive to changes like temp and alt even with the JD kit granted the kit helped a lot... the keihin is great that’s why I’m switching I’m at 4500 feet if your at sea level you will probly have it easier

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Yea im at 634 feet

They do run really rich stock mine was awful because it was setup for sea level at 4500 feet and jd gives you a jetting chart so it’s not a guessing game, the issue I had was 0%-10% throttle it had a bad bog and couldn’t fix it for the life of me but mid to top was amazing and since you said you will be in the band a lot it should be perfect and if you ever feel it’s strugging switch to a keihin air striker 36mm or 38mm game changer

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Exactly the troubles i was having they run really rich

Yea stupid rich, just go with the jd because the stock jetting is gonna have you biting nails and then your gonna know the michuni inside and out which isn’t a good thing, I have taken that damn thing apart so many times and every time it’s a whole new situation and when I finally get it dialed and the temp changed 10 degrees it’s like it resets
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