Jetting 2013 yz 125

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7/3/2017 9:08 PM

This is probably a stupid question but jetting has always messed with me and have been scared to try it cause I feel like if I get one thing wrong and don't put enough gas into my motor it is gonna seize it up or something. But I wanna try it. So say a #50 pilot is a bigger jet (richer) than a #40. Right? Just wanna make sure. And my bike runs fine it has good power and everything but when I'm cruising low in the rpm's and fully open it up as fast as I can it wants to just die and sounds like bwoooooooooop and won't get back up. I can baby it and it will be fine but I want it to be perfect. That's usually a lean pilot jet. Correct? And also I took my bike to get jetted and he said the main jet was a little rich so he leaned it out. But now I am going to have a lean pilot jet and a lean main jet. And is that not a little weird to have everything that LEAN?
Also what if it's has been changed before. Are their numbers on the jets that will tell me what's in it? I'm planning on buying two jets lower then what's in it but idek what's in it. Like I said, carbs have always been a big headache but I'm tired of giving somebody 40$ to change out a 1$ part. And I like to be able to do as much as I can to a bike. Also what makes a bike have that Crisp/raspy/crackling/popping noise? Jetting? You hear all these two strokes and they have a ring ding ding sing ding noise but then you hear that one that sounds mean, NOT LOUD, but mean and cracky. I'm not looking for the no silencer noise. Also should I run race gas? I know that will mess my jetting up but I will put it in before I buy these jets. It's not bored, it's not ported, it's not for any reeds or nothing. Stock pipe and an FMF shorty. That's it. Is it worth the race gas? Sorry for the long post but I want my bike to run clean and right now I just feel like I'm torturing it riding like this. Although the jetting is so little off where if somebody else rode it they would not notice it, I still like my bike to just feel comfortable and new when I'm on it. There is a link attached that shows the sound I am talking about.


7/6/2017 3:52 PM
Edited Date/Time: 7/8/2017 12:39 AM

Yes, bigger number, bigger jet. It makes a difference where you are, alltitude, and weather also changes things. Feul does matter! Yamaha calls for 95 octane, you can but it from VP and probably other feul company's ,or you can make your own. I used to use about 3 and a half gallons of non oxegeated 91 octaine mixed with one and a half gallons of 110 octaine race gas, if you do the math, it's close to 95. and of corse oil .Don't use any gas with ethonal! !
It's been awhile since I've worked on a yz 125, i can't remember what I ran, but your bike needs a bigger pilot jet, 42 or maybe a 45. Also maybe move the needle up 1position (move the clip down one grove. , it should have at least a 460 -490 main jet, I've had as big as a 510 in fall and winter here in Minnesota!


7/6/2017 4:21 PM
Edited Date/Time: 7/6/2017 4:25 PM

yes the bigger number Jet, the more fuel it will allow to be atomized. So more fuel would make the mixture richer.

The 2013 YZs were jetted pretty good stock. Shouldn't need too much adjustment at sea level to 1000 ft.
Stock jetting was:

Mainjet: 430
Pilot jet: 40
Needle: 6BFY43-3
Clip: 3rd from top
Air Screw: 2-1/4 turns out

What you are describing sounds more like a needle issue than a pilot jet issue. A pilot jet in simple terms really only meters fuel from the 0-1/4 throttle position. The best way to tell if it is your pilot jet is by adjusting the air screw. If optimum rpms is less than 1 turn out you need a richer pilot jet. More than 2 1/2 turns and you need a leaner jet.

I suspect that your 40 pilot is on the money, and your air screw could be adjusted to find the sweet spot for the pilot.

My guess would be that your needle is too lean from what you describe. As Dan Robin said above try lowering the clip on your needle a notch or two. That will richer the mixture from 1/4-3/4 throttle, and should eliminate the bog. If it makes it worse try one notch the other direction, but what you describe sounds lean so clip goes downward. If it runs better with the clip downward to the bottom of the clips but still not good enough, then you may need a richer needle. Try the easy stuff first, get your air screw optimized, move the clip, and see how it feels.

Remember you can't properly jet a worn engine, and you should start with a clean air filter, and a new plug and fresh gas. Good luck, it seems scary but it's not that hard. Small steps, and only change one thing at a time.

Edited to add. Take notes. It sounds dumb, but it helps to record everything you do. It helps you to hone in on what you are trying to achieve.


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7/6/2017 4:50 PM

A couple other things to help in understanding your jetting:

-The main jet is mostly for 3/4 to wide open throttle, but remember it also feeds the needle.
-The needle is responsible for most throttle openings: 1/4 to 3/4.
-There is some overlap between the two items above, as the needle blocks the main jet when the throttle is closed. Moving the clip down makes the mixture richer because it allows the throttle slide to lift the needle out of the main jet's way earlier in the travel of the slide. Moving the clip upward makes the needle leaner.
-The pilot is one half of the "slow circuit," referring to what's going on at lower RPMs and throttle openings. 0 to 1/4 open.
-The air screw is the other half. Turning it all the way in blocks air from the slow circuit. Turning it outward adds air (leaner.) I know my pilot is good when the idle is highest at around 1/5 turns out. This means full rotations: 360 degrees and then 180 additional degrees.


Braaapin' aint easy.

7/6/2017 7:24 PM

Blackjack31 wrote:

This is probably a stupid question but jetting has always messed with me and have been scared to try it cause I feel like if I ...more

What temperature and elevation are you riding at? That's the first piece to this puzzle.


7/6/2017 7:47 PM

Def surprised no assholes posted on this thread
Good deal you guys posted some good info

All I have to contribute is
Start stock
And make small changes
and first thing
Is get needle set for off idle