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Yz144 jetting questions.

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Hey guys, just picked up a super clean 06 yz125 with a 144 BB kit!

Some backstory. This bike has a stock jug modified/bored to a 144. I bought it and the previous owner said he put a fresh ring in it about 2hrs before I bought the bike.
I put about 20 hours on the bike after I bought it and decided to do a top end, correctly...
I used a Vertex piston which I ordered after measuring the cylinders bore. Set the ring gap on the loose side of the spec.. ETC. He had some Boyseen reeds in there (which I did not know) and I put some vforce 3's in because I liked how they were responsive on the initial throttle application. Rest of the bike is stock, pipe, silencer, etc... I Mix 91 and 100 octane 50/50 and I run that at 32:1 with yamalube, stock spark plug.

The bike was jetted: 450 main (which is correct after doing a few chop tests), 35 pilot, stock setting on the air screw and the needle was at clip position 2.
 

So to the point... The bike ran good before I did the top end, it has way more mid-top pull now due to the fresh top end.
Before the throttle response was not as crisp or responsive as I would have like from closed throttle to about1/8th throttle. But it did not feel too lean nor too rich, I just compensated for this by using the clutch out of corners. No big deal, it is a 125...

So I started messing with jetting, 450 main still. but I went to a 45 pilot and clip position back to stock (3).

Now from closed throttle to about  1/8th throttle, maybe a little more, it is laggy, less responsive and kind of sounds like it is misfiring like when a plug is being fouled.
So should I go back to a stock pilot (40) and try that to begin with or should I move my clip position? I am not sure if it is running too lean or too rich at that throttle opening.

NO air leaks, loses zero psi for over 15 minutes. Thanks in advance! I love this thing and just want to get it right, I just started out so I am learning so please forgive my lack of knowledge!
 

P1000003.JPG

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The stock settings for a 2006-2011 YZ125 should be :

 

air screw 2-1/4 turns out

#40 pilot jet,

#42-74 needle, 3rd clip position

#410 main jet

 

Most stock bore 125's respond well to having the pilot jet and needle clip position leaned out 1 step.

 

In theory with a big bore you shouldn't have to run any richer than these settings

but rather have to go leaner since the increased vaccum signal pulls in more fuel.

 

As you've described, the off idle 'missing' or cutting out indicates a rich condition, if it was lean it would simply bog.

 

In your case the #35 pilot and the needle clip on the 2nd position sound about right,

it's the #450 main jet that is opposite of what most 144 owners are experiencing, many running a #390 or #400.

Edited by mlatour
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Try going the other way with the pilot jet

 

So try leaner? That is what I was going to start with. I was going to revert back to a stock 40 pilot and see how it does. My buddy said to change the clip position but I do not believe that is the issue I am having due to where I am having it at the throttle position I am at!

 

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The stock settings for a 2006-2011 YZ125 should be :

 

#40 pilot jet,

#42-74 needle, 3rd clip position

#410 main jet

 

in theory with a big bore you shouldn't have to run any richer than these settings

but rather have to go leaner since the increased vaccum signal pulls in more fuel.

In theory yes, but a 410 main was WAY too lean when that baby is pinned... 450 seems perfect, tried a 440 and a 460 when I first got it, but the previous owner said it was jetted perfect and it very well may have been for where he rode.

I am going to try this I think then: leave the main alone. Revert back to a 40 pilot and try messing with the air screw to see if it alters how it feels where I am having the problem and leave the needle alone at clip pos 3 for now.

Thanks for the feedback dudes!

It did not have this mis/bog before with the 35 pilot so it would seem that it in fact does need to go leaner on that pilot, from a logical standpoint.

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Also, the needle clip position affects the metering exactly where you are experiencing a bog, 1/8 -1/4 throttle,

it's the needle's profile/taper angles that alter the fuel metering at 1/2 - 3/4 throttle.

 

The air screw simply fine tunes the pilot jet circuit but works within a narrow range.

The nominal setting is 2-1/4 turns out,  if it needs almost a full turn in either direction that indicates you need a leaner or richer pilot jet.

(example: engine responds better with air screw opened more = more air = a leaner pilot jet required)

 

OEM pilot jets come in #35, #37.5, #40, #42.5, #45 size increments, the air screw allows to fine tune in between those increments

say like when a #38 or #41 pilot jet would be ideal for the day's ambiant conditions but not available.

Edited by mlatour

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Also, the needle clip position affects the metering exactly where you are experiencing a bog, 1/8 -1/4 throttle,

it's the needle's profile/taper angles that alter the fuel metering at 1/2 - 3/4 throttle.

 

The air screw simply fine tunes the pilot jet circuit but works within a narrow range.

The nominal setting is 2-1/4 turns out,  if it needs almost a full turn in either direction that indicates you need a leaner or richer pilot jet.

(example: engine responds better with air screw opened more = more air = a leaner pilot jet required)

 

OEM pilot jets come in #35, #37.5, #40, #42.5, #45 size increments, the air screw allows to fine tune in between those increments

say like when a #38 or #41 pilot jet would be ideal for the day's ambiant conditions but not available.

Ahh, I see! Thank you, that makes a lot more sense than what I have been reading lol.

So if it runs better turned all the way in, then I need a richer pilot, but if it is opposite then I would require a leaner pilot.

So would a quick test be turning the air screw all the way rich and see the result and vice versa? That way I could have an indication of where to go.

Do you think I should leave the needle position alone for now or also alter that?

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Idealy you only change one setting at a time or else you'll never know which one positively or negatively affected how the engine runs.

In your case perhaps go back to what initialy was known to work, #35 pilot, needle 2nd clip, and start from there.

 

Do your testing from the bottom up: adjusting the off idle throttle response, then midrange, ending with the top end,

first tuning the pilot jet / air screw, then the needle's clip position, and finally the main jet.

 

As mentionned the air screw only has a limited range of adjustment, after those points are reached 

say turned in/out beyond 1 full turn from the nominal setting of 2-1/4 turns out, it becomes much less effective.

 

As you've mentionned is a good starting point, if it runs better turned in a full turn (less air) it needs a richer pilot jet,

if it runs better backed out a full turn (more air) it needs a leaner pilot jet.

 

Don't know about your local weather but if fall temperatures vary a lot from cool mornings to warm afternoons,

if you fine tune the pilot circuit in cooler weather it will be slightly rich when temps rise during the day.

If the temperature differential between low and high isn't too great, that's where fine tuning the air screw

can compensate and avoid a pilot jet change to maintain the right fuel metering off idle.

Edited by mlatour
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For what its worth, I had to go way leaner on the pilot with my 144 and conditions.

One step at a time, air screw setting can be used to tell if you are rich or lean on the pilot.

Make sure the engine is nice and warm.

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So try leaner? That is what I was going to start with. I was going to revert back to a stock 40 pilot and see how it does. My buddy said to change the clip position but I do not believe that is the issue I am having due to where I am having it at the throttle position I am at!

yes you are correct as its bogging down early......40 primary is a good start....and i would go from there...eliminate 1 thing at a time

Remember the bigger the bore the smaller the main needs to be

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I ended up figuring it out. I settled with a 440 main. Aftermarket needle. 35 pilot. Had a weird ignition problem, intermittent weak spark under certain load and throttle conditions.

All fixed! Also put about 30 hours on the bike since with a stock bore 125 cylinder on it. Sold the 144 kit and went back the stock 125 bore. Bike is now for sale as I got a new bike.

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I ended up figuring it out. I settled with a 440 main. Aftermarket needle. 35 pilot. Had a weird ignition problem, intermittent weak spark under certain load and throttle conditions.

All fixed! Also put about 30 hours on the bike since with a stock bore 125 cylinder on it. Sold the 144 kit and went back the stock 125 bore. Bike is now for sale as I got a new bike.

 

What kind of 144 kit was it? What parts did you sell and how much did you get for it? Reason I ask is I have an EG 144 kit that I may sell soon (since I have two of them and only one 125)

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What kind of 144 kit was it? What parts did you sell and how much did you get for it? Reason I ask is I have an EG 144 kit that I may sell soon (since I have two of them and only one 125)

Im not entirely sure I bought the bike with it. I'm pretty sure it was an EG kit. Used a stock cylinder and it was bored to 58mm. I sold it with the jug, powervalves, head, new OEM gasket kit, new vertex piston kit, Jets and a 14t front sprocket. I sold it for $450 which I thought was a pretty good deal.
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we have a 2013 YZ144 , its a new Athena kit that we have had on the bike for a few months . we run a 45 pilot , needle 3rd clip and a 490 main at sea level Queensland Australia and the bike absolutely rips everywhere ,

I have read the mxa wrecking crew version of the yz144 build they did and as soon as I read they left the jetting stock I just laughed , they couldnt work out why there Athena kit only made two more horses thru the rev range . I have also read where people are going leaner on there main WTF

 

Our bike has a brand new bottom end in it aswell so its a brand new engine , I will also add that the top end does not drop off like some people suggest  the bike just keeps on pulling , we can now hold gear without having to change up in  some sections of track , the midrange and top end pull of this bike is sensational , the bike was fast before now its a missile

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we have a 2013 YZ144 , its a new Athena kit that we have had on the bike for a few months . we run a 45 pilot , needle 3rd clip and a 490 main at sea level Queensland Australia and the bike absolutely rips everywhere ,

I have read the mxa wrecking crew version of the yz144 build they did and as soon as I read they left the jetting stock I just laughed , they couldnt work out why there Athena kit only made two more horses thru the rev range . I have also read where people are going leaner on there main WTF

 

Our bike has a brand new bottom end in it aswell so its a brand new engine , I will also add that the top end does not drop off like some people suggest  the bike just keeps on pulling , we can now hold gear without having to change up in  some sections of track , the midrange and top end pull of this bike is sensational , the bike was fast before now its a missile

 

seems very odd ,

clip position i agree with

the standard pilot was a 40 ( i would not have changed it )

and standard main was 430 ( richer you sacrifice overrev) 

have you done a plug chop with the 490 main ? , just seems like your missing out on the top end

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From what I know from the 144 'click'.......

 

35 pilot

400 main

stock needle/clip position

 

Look out !!!! :thumbsup:  :thumbsup:

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+1 Bogger.

 

It's common for 144's to run leaner than a 125.

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From what I know from the 144 'click'.......

 

35 pilot

400 main

stock needle/clip position

 

Look out !!!! :thumbsup:  :thumbsup:

Also timing advanced all the way ....

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My 144 would lock up solid on leaner jetting then stock I know cause it did it with the first 45 mins of owning it long ago.

I'm running a 450 main in mine

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