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2018 yz 125 jetting help

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So ive recently posted ab my troubles with my old 2017 yz 125 fouling plugs,spooging, etc while looking back i can tell how stupid i was not to take a crack at the jetting on it when all signs were showing it was running rich, regardless that bike has been stolen (still working to find out who did it) but i have recieved the money from insurance for a new one.. knowing what i know from the 2017 yz 125 and from talking to people on fourms, that the newer 2 strokes come jetted rich from the factory so that you dont blow your top end on the yamahas waranty.. ive looked at my old manual and i want to try to get my jetting down before i purchase the bike ( as im waiting for the dealer to get one in) im in kentucky, tempatures rn at about 80 degrees (which is what i normally ride with, i dont know my altitude and humidity and dont know if i should look into that for jetting.. the manual says from the signs that the carb is running rich (judging from the plug) to drop mainjet no. Until a difference is discovered, however it also talks about raising the clip on the needle.. id like to say itd be as easy as dropping the main jet and going on with it but from my mechanical knowledge nothing is usually ever that easy.. any help I can get would be greatly appreciated

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Side note: i would usually always foul plugs on the 2017 when ive just rode it for an hour, went in to get a drink, or stopped riding for 20-30 minutes then take off, if i wouldnt take off immediately going into the band it would foul the plug which riding a engine hard when its cold is really bad for it.

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With initial settings for 68°F, by 80+°F you are also nearing the point that slightly leaner settings are required, moreso if it's very humid.

On top of typical slightly rich settings, quite often the float/fuel level is set on the upper limit of the range which makes each metering circuit richer.

 

If you are not riding this MX race bike like 'an MX race bike' (accelerating hard corner to corner on a race course)

meaning rather slow paced, trail / off-roading with the very occasional wide open burst, you will continue to foul plugs on the stock settings.

Using a 'racing' type premix oil will make it worse.

 

Even for recreational motocross, unless your skill level allows you to keep the engine singing all the time,

you could no doubt benefit from leaner pilot jet and needle clip settings and, running the fuel level at it's minimum spec. height.

Main jet only affects 3/4 to full throttle.

A low flash point premix say like Amsoil Interceptor, Lucas etc. is also helps to prevent spooge & fouling.

 

If you got this bike stricktly for slow trail riding,

since you have the budget to buy new

pick something more appropriate like a WR250F.

 

Edited by mlatour

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I have not gotten this bike strictly for slow trail riding nor have i said i did, im going to be in the power band alot and riding normally, not always full throttle but by no means always going slow out of band, id just like jetting that i can ride on without fouling plugs, loading up, and loading up when idling for a period of time (2-5 minutes) if that

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Yamaha 'plays it safe' and finds basic settings (meant for wide open MX racing)

that should work for most people but, each engine and usage is different.

 

My YZ125 is strictly track ridden and although I'm no pro my engine gets revved and rung out quite a bit.

Even so, it responds best with a 2 sizes leaner pilot and, going 1/2 clip leaner needle settings than stock.

The carburetor's fuel level set in the middle of it's range.

Premixed at 32:1 the silencer's tip is barely moist at most, it doesn't spooge at all.

 

The only time I've ever fouled a plug in 5 years is when experimenting a much richer needle in near freezing temps.

 

Proper jetting (and fuel level) is key for best driveability, each bike and rider is different.

The service manual gives basics settings to get you in the ballpark but you have to fine tune for best results.

 

Pilot jet and needle are best tuned by 'feel', the main jet by plug coloration.

Many 'settle' for good enough, keep fouling plugs etc. but once you've experimented a bit and seen the difference it can make, 

you are rewarded with a crisp running and responsive engine.

Edited by mlatour

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I'm a woods rider/racer, at similar temperature and altitude as you.  Currently I have a 390 main and I believe the pilot is a 35.  I use the needle clip to adjust for seasonal changes in temperature.  Don't forget that you can also use a hotter plug.  I normally run an 8 for the woods, but if I am riding a track or in deep sand I'll put in a 9.  No need to run the expensive plugs.  The NGK BR8ES and BR9ES are great two stroke plugs, and they can be purchased at any auto parts store for $2.50.  

32:1 is the ratio you want.  

Another option that is simpler is a JD jet kit.  While I have never used one for my yz 125s, I have used them on several other bikes with good results.  It comes with a couple of custom tapered needles, main, and sometimes pilot, jets.  The instructions are simple to follow for your temperature and altitude.  A kit costs $75, but it's a lot easier than swapping brass and testing.  James has already done all the testing for you.  

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Yea ive thought about the jd jetting kits but id really like to use those in the future when ive got the stock jetting down lol i think its the mechanic/curious side of me wanting to get this down and know how to do it on my own, ive always ran 32:1 and i think im going to start out with dropping the main and checking it out then moving on to needle and pilot jet because i will have to adjust idle and pilot jet

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Ideally you fine tune from the bottom up:

pilot jet

needle

main jet

 

but only after setting the carburetor's fuel level, note that the manual specs actual measured fuel level which requires a special tool, not float level 

(tool can be fabricated with a spare bowl plug)

 

 

Reason is, albeit a small amount but the pilot circuit is still feeding fuel at high rpm.

 

If you fine tune the main jet first with say too rich pilot and needle settings,

if you considerably lean those two out later on, you've also leaned out your wide open throttle.

 

Myself ride on a rather tight MX course with many slower 180° corners

can really feel the difference in throttle response with only a change in pilot jet.

Edited by mlatour

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Thats exactly what ive just read online, so instead of just tweaking the main jet, noticing it stops fouling plugs and going on i should do

Pilot, and idle screw

Needle, and clip

Then main jet?

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The air screw fine tunes the pilot jet circuit,

it gives you a small range to lean or richen the circuit before requiring either a smaller or larger pilot jet.

 

Say the nominal setting is 2-1/4 turns out with a #40 pilot jet.

On a warm day (air less dense), you would turn out the air screw to add some air to the circuit,

if by 2-3/4 to 3 turns out your engine is still running rich at low rpms,

you would then switch to a #37.5 pilot jet and re-set the air screw back at it's nominal setting.

 

Beyond 3/4 turns or so from the nominal setting of 2-1/4, it is out of it's effective range of tuning.

 

You fine tune the air screw daily, depending on ambiant temps and humidity,

even doing mid-day corrections as temps get warmer.

 

The needle is done by 'feel', a bog / slow to rev, takes longer to hit the powerband would indicate too lean.

A 'stutter' like hitting a rev limiter would point to a too rich needle setting.

 

Too rich a main jet will kill top end overev, 

On the verge of too lean gives the most power for short bursts but...you're taking chances at this point !

Slightly rich main is the safest, especially for those who off-road and ride open terrain

where the straightaways are longer than MX tracks.

 

 

Edited by mlatour

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About what you quoted / posted,

The main jet affects 3/4 to full throttle, needle about 1/4 to 3/4.

I'd be wary about judging needle settings by how well it revs up in neutral while the bike is on a stand.

Actual riding conditions and load (deep sand, hills) is how the right settings are found.

 

You'll notice the manual recommends richer settings for deep sand which puts extra load on the engine.

Edited by mlatour

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So on the air screw how do i tell if it is running rich at low rpms and i should lean it out? Last time i just had the air screw turned to about 1 turn out and had the idle screw set and that made it idle steady for the most part.. i was most likely doing that wrong but i like my bikes to idle steady and when i had it on the stock air screw setting with the idle adjusted it would climb (sometimes when i came to a stop it would idle perfect but as soon as i gave it gas it would die down and almost die out) so about one turn out on the air screw and the idle screw turned in a bunch fixed that for me.. should i be doing that different

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Once the engine is fully warmed up (always warm up a cold engine gradually, low load before riding it hard)

you set the air screw to obtain highest idle speed, you then re-set the idle screw for proper idle speed.

 

You can later experiment a bit richer or leaner for best throttle response.

 

If your pilot jet choice is in the ballpark for ambiant weather, you should not have to re-adjust idle speed (I've never have)

Edited by mlatour

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As mentioned earlier, even in stock form my engine has always responded well with leaner settings than stock: in ()

#35 pilot jet (#40)

the 43-74 '1/2 clip richer' needle - position 2   (42-74 - position 3)

and most of the time the stock #410 main jet  

 

but I've since reduced the compression squish band height (commonly known as 'milling the head')

which in it's self usually requires leaner settings and, blend about 15% race fuel for a total of 94 octane.

 

The squish band correction makes the engine run crisper at low rpms 

and the higher octane is mostly a 'safety margin' for peace of mind since I tend to jet on the lean side.

 

If I would judge by sound alone, my pilot and needle settings would seem way too lean 

as the engine has a gnarly / raspy sound (but no detonation) coming out of corners like it's on the verge of blowing up but,

as soon as I richen the settings it just doesn't pull as hard.

Not saying these are the ideal settings for everyone but with careful plugs readings (magnifying glass, plug chop)

and observation of the piston's crown carbon wash pattern over the years

have shown what sounds  way too lean is actually just about perfect for my usage.

Edited by mlatour

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One more thing if you could help me on is how i should adjust the pilot screw according to the weather that day? Example should i lean the air screw out a little as it gets colder in the weather or day, or should i richen the air screw when the weather or day gets a little colder, thanks for the help

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All that info is in the service manual, but in general:

 

-colder temps, low humidity, lower elevation = richer settings

-warm temps, high humidity, higher elevation = leaner settings

 

Say you tune the air screw first thing in the morning at 60°F and by noon it's warmed up to 80°F,

it would require a bit of turning out (leaner) to compensate.

 

Once you've found the best settings at which your engine runs, note them and the ambiant temperature.

When ambiant temps rise or fall by more than 15°F or so from your baseline settings,

you can then figure going one step leaner or richer on all metering circuits.

 

 

Edited by mlatour

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Sounds good! I thank you so much for all your help, ive always really wanted to know how to correctly jet my bike so i thank you for helping me learn all of this and for the skills!

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