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2018 yz 125 jetting help

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Firstly, in your original post you point out that you didn’t know your average regional humidity or altitude. Those two factors are more important to consider than your riding style, pre-mix ratio, or spark plug initially.

 

Moisture in an engines intake air is arguably the most influential variable for tuning. You need to know your humidity, or be willing to spend time tweaking until you find your “sweet spot”.

 

If you’ve decided on your pre-mix ratio, stick with it and follow these instructions to the letter. Don’t cut corners. You’ll have your bike ready for any track or trail for weather matches to the time of setup:

 

Before beginning tuning, install a new plug, set float to OEM specs, and have bike at operating temps at all stages of tuning.

 

1. Close air screw, then open 1.5 turns.

 

2. Start engine.

 

3. Turn idle screw in until slightly fast idle occurs (to stay running).

 

4. Turn air screw in and out until fastest idle is found. (If screw is less than 1 turn from closed, install larger pilot jet. If 2.5+ turns from closed, install a smaller pilot jet).

 

5. If pilot jet is changed, tune pilot and air screw again for fastest idle (repeat steps 1-4).

 

6. Reduce idle with air screw to pre-stalling level and ride bike with closed to ⅛-¼ throttle intervals. Adjust air screw for sharpest response.

 

7. Mark throttle grip at ¼ and ¾ turns of open. Ride bike in this zone, and if bogging occurs, lower the needle clip 1 position. If pickup is rough, raise needle clip 1 position. Continue adjustments until sharp response occurs.

 

8. Install fresh plug, and perform full throttle run, through all gears. Once at top speed, engage clutch, kill engine, and coast to stop.

 

9. Remove plug and inspect insulator closely for condition. White/Grey is Lean, and Brown/Black is Rich. Correct colour is Tan. Adjust main as required to set correct level.

 

If the weather changes even marginally, minor turns of screws will be needed - and this is why it’s ideal to have jetting which allows for screws to be roughly in the “middle” of their ranges, as it will allow room for slight enriching or leaning for desired results.

 

Remember that as a general rule, in heat we lean out, in the cold we enrich.

 

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Any carb can run rich, even stupid rich.   I started off with the JD Jetting and had a significant sputter at 1/3 throttle while the top end was great.   I even had a thread going and had mlatour me out.  Bottom line, you learn to lean them out.  There are tons of needles, pilot jets, and main jet to switch to deal with endless weather combinations.  

Once you dial in a combination write it down.   Keep a log.  You don't have to reinvent the wheel every time.  Also the manual gives some great info on how to switch up Jetting given temp changes and carb symptoms.  If you don't have one download it - amazing source of info.   

 

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4 hours ago, ItsJuniorJ said:

1. Close air screw, then open 1.5 turns.

The nominal setting for the Mikuni TMX38 is 2-1/4, 

by starting at 1-1/2 you're already nearing the limit of it's range of effectiveness.

 

Many seem to favor Keihin or Lectron carburetors in general but in this application (YZ125) the TMX38 actually functions quite well.

The factory service manual has no less than 6 pages dedicated to carb settings and technical specs on jets and needles,

if only people took the time to read and understand all of it, they'd realize an aftermarket 'kit' probably isn't necessary.

 

Edited by mlatour

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The nominal setting for the Mikuni TMX38 is 2-1/4, 

starting at 1-1/2 you're already outside it's range of effectiveness.

 

I won’t lie, I’m a Kawasaki guy, so I’m generally conditioned to Keihin setups/numbers Lol

 

I presumed the Blue bikes ran Keihin’s as well, my apologies! Regardless, the setup process is the same, simply add in your OEM dial-ins as per your manual.

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I actually got a little nervous when relying on top end meeting based on color.  I was using a 420mj and I thought the plug was perfect.  See attached.   I decided to jump to the 430 as my nephew thought it could use more gas on top.  I made the change and thought it pulled strong and didn't notice any reduction to over-rev.   I decided to leave it and now the plug is more brown.   I'll take protection over perfection as long as there is no noticeable negatives in how it runs.   Mlatour and juniorj - great info above.  I read it and think if Jetting is new to someone the info should be invaluable.   

Whenever do a top end and bottom end rebuilt, do a card rebuild.   

IMG_6438.JPG

IMG_6439.JPG

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A coffee coloured plug like that is a sign of running a little lean, but you’re not far off, if there can ever be such a thing as “perfect” setup with manual carbs. If your plug is coming out like that after chop tests, dry and coffee coloured, I would richen it slightly but not so the plug comes out wet. Chop tests are a must.

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To richen it I went from a 420 to a 430mj which require swapping needles from blue to red.   Funny thing is I ended up going with the last suggested setting of JD.   It all good now.  I've not done a chop test but will before next ride. I will note the weather conditions the plug was used for and save. Thanks for the info.  

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Thank you guys for all your help i think im going to start going through those three processes with the air screw,needle, and main jet and recording my results then when the weather changes or i make a change to the bike (reeds, exaust system, etc) ill go through the processes again and record my results

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Also generally would higher elevation mean richer? And if i do go to higher elevation or lets say lower what jets would i need to change? Or settings would i need to change

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I have a friend that lives at a higher altitude ( 902 ft ) while i live at 630 ft if my jetting was spot on for the weather would the jetting be the same if not close to right for that altitude

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On 10/6/2017 at 11:36 PM, mlatour said:

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

Mentioned on the first page of this thread,

once again, it's all in the service manual...

 

Figure the same 'perfect' settings are good for a range of 1000ft or so.

So going from 600ft to 900ft is considered a very minimal change (300ft) in elevation.

Edited by mlatour
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Also generally would higher elevation mean richer? And if i do go to higher elevation or lets say lower what jets would i need to change? Or settings would i need to change



Air gets less dense at higher altitudes, so tweaks could be needed depending on the amount of altitude change, combined with temperature and humidity.

A 200-300ft change in elevation could be worked around easily enough.

As a “rule”, at higher altitudes, the same amount of air contains less “fire power”.

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On 10/10/2017 at 4:14 AM, ItsJuniorJ said:

 

 


Air gets less dense at higher altitudes, so tweaks could be needed depending on the amount of altitude change, combined with temperature and humidity.

A 200-300ft change in elevation could be worked around easily enough.

As a “rule”, at higher altitudes, the same amount of air contains less “fire power”.

do you guys mean 2000-3000? i ride 5000 feet and make my way up to around 8000 at times, a few hundred feet wont do anything

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do you guys mean 2000-3000? i ride 5000 feet and make my way up to around 8000 at times, a few hundred feet wont do anything


It’s still only one variable. The most influential is humidity.
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12 hours ago, Hucks said:

do you guys mean 2000-3000? i ride 5000 feet and make my way up to around 8000 at times, a few hundred feet wont do anything

Right, the difference between 600 and 900 ft elevation won't make a real difference.  Can you imagine someone saying "boy my bike sure runs rich at the top of that hill over there." :) 

Just follow the process, and regardless of your specific conditions, you'll get it sorted out.  Once you get it jetted right for your location, many adjustments you need to make from day to day can be done with just the air screw.

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On 10/8/2017 at 11:16 AM, medalheatpistons said:

I have a friend that lives at a higher altitude ( 902 ft ) while i live at 630 ft if my jetting was spot on for the weather would the jetting be the same if not close to right for that altitude

the michunis tend to be more sensitive about elevation and temp, i just got my kehin 38mm with the fins and quad vent and cant tell a difference when temp changes and minor elevation changes! plus it was a lot cheaper then a lectron

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Hey guys so ive got the 2018 and its turning around winter here and ive noticed that its running pretty good not fouling plugs and stuff because i think the stock jetting is so rich on these that its basically set up for cold weather like this (60-75) degrees so i havent changed much on the carb, just the air screw setting because it was bogging at wide open throttle so i richened it up and it quit but i was wondering, i have the bike set up to idle and it idles when its warming up and when you come to a stop fine but if you give it a little.. or alot of gas it will come back down and idle really low like its about to die.. now when i usually come to a stop it has a nice steady idle but when you crack the throttle at all the idle goes down like ive described.. any help? I think im gonna mess with the pilot jet today just to see. Thanks in advance

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9 minutes ago, medalheatpistons said:

Hey guys so ive got the 2018 and its turning around winter here and ive noticed that its running pretty good not fouling plugs and stuff because i think the stock jetting is so rich on these that its basically set up for cold weather like this (60-75) degrees so i havent changed much on the carb, just the air screw setting because it was bogging at wide open throttle so i richened it up and it quit but i was wondering, i have the bike set up to idle and it idles when its warming up and when you come to a stop fine but if you give it a little.. or alot of gas it will come back down and idle really low like its about to die.. now when i usually come to a stop it has a nice steady idle but when you crack the throttle at all the idle goes down like ive described.. any help? I think im gonna mess with the pilot jet today just to see. Thanks in advance

dont fight the michuni it sucks

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Perhaps not in all applications but on the YZ125, the stock Mikuni TMX38 functions quite well.

 

quoted: just the air screw setting because it was bogging at wide open throttle

 

Medalheatpistons, you haven't been paying attention to all the infos posted up to now,

the 'fine tuning' you did with the air screw has absolutely no effect on the main jet / wide open throttle

but certainly effects idle / off-idle where you current problem is.

 

I don't mind helping out folks that need explanations but they still have to do their part and do their homework.

Read, re-read and read yet again your service manual, it has about 5-6 very comprehensive pages on carburetors settings and tuning.

Edited by mlatour

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