jetting for 140cc yx with 26mm mikuni

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Well-Known Member
Nov 8, 2008
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nyaaaa hi.

So ive put a new YX 140 into my pit. But im using the old 26mm mikuni from the old 125cc. Its a fake mikuni i presume (bike is atomik)

the main is 95, and the pilot is 15 (from what i can read).

What would you all recomend for a YX 140cc that is running a cam kit and rated around 14hp (from JR Racing).

I ran the motor in last weekend, and only today i pulled the plug to see wusup, and the motor is running lean. But i also found one bolt on the inlet manifold was semi stripped, so i removed the plastic inlet manifold spacer, and bolted the manifold back on + gaskets only.

I had to re adjust idle as it was way too high. now it idles like a mad kant.

I still havent played with plug chops yet, as the motor was only just run in. I'll do that this weekend. But in general, are the jet sizes listed OK for my motor?

Or should i go bigger?

the jets i can get easily are:
mains 102.5, 105,92.5,97.5

pliot 20,25,,17.5

oh and yes i will follow the carb tuning guide. But i thought id ask first as im lazy to search.:D
first up... 14hp. Bullshit. lol. trust me, stock motor, you'll be lucky to get 8.5-9hp out of it. with cam and nothing else, with a fakeuni carb... maybe 9.5 on a nice sunny day, with no clouds and chirping birds etc etc.
jetting sounds pretty right... perhaps go with the 20 on the pilot, and 97main, possibly the 100 if it's still running lean, needle jet start on the middle clip...
oh, and put the carby/manifold spacer back in...
yeah the motor isnt 14hp, but i like to quote it to feel important :) thanks man.
ha ha, yeah the quoted power figures from manufacturers and sellers tends to be a bit optimistic at best..
but yeah, hopefully your jetting specs arent too far off. :D
My NEW XY motor was qouted as having 12hp out of the box, after takeing it to the dyno it only had 6.5 hp ahhaha - pissed my self laughing - sent the people i got the motor from the dyno figures and told them they should refund half my money cause it only produced half what they said... Recieved no reply from them :)

On the jetting side of things, XY140 worked runs very well on a DHZ 24mm Mikuni carbie, out of the box it got 9.somthign HP tuned 11.7 or something, all i did was timing and moved the clip down one notch and tweeked the jetting screw a touch. Never really got it to run well with the 26mm Mikuni carbie that came with it.

Just play with it you'll figure it out, every single motor is different in the carbie setup.
yeah i bought a $30 fakuni jet pack 3 pilots and 3 mains from DHZ.

I tried the adjusting the screws and notched it down rich, but still lean. When these jets arrive, ill go up two sizes up.
I ran down the same road as you roughly, and have gone up one size in both. Whilst it now pulls like a train, the air/fuel screw isn't quite right, so thats my next tweak, but I'd thoroughly recommend going up at least one, and I'm 2 clips down from middle so I'd almost go the next jet again. But you are on the right track, or we're both on the wrong track ;)

Tell you what tho, I'n well sick of ripping the carb off to adjust pretty much anything!
i hate tuning this fakuni, cos i have to take it off all the time.

currently running 17.5 pilot, with the largest main from the DHZ multi jet pack. 1XX main.. cant remember

plug is now a tan color on the porcelain, and cream/ more whitish on the metal arc bit thingy.
Does this mean its still a tad lean? The whole plug should be tan souldnt it? including the metal arc arm thingy?

Fuel screw is 3 turns out. But pilot is like 5 turns out!

The carb guide says
"An air screw out more than 2 turns needs a smaller pilot jet "
but even if i put the smallest pilot, the air screw is still 5 turns out!?
When i screw it in, the idle rises, when i screw out, the idle drops.?

the circlip is also on the 2nd lowest.

Bike goes good though. it has stopped the crackling and backfire on off throttle.

what should the ideal fuel screw settings be? remember that i have to take the damn cab off to adjust it, so is there a default number of screw outs that it should be?

Ill probably screw it out 2 more times, and go up one more on the pilot cos the plug chop showed it still a tad leanish.
try a 20 pilot. It sounds like it's running ok everywhere else...
But yeah, air screw should be set around 1.5 turns out. further out, or further in than *roughly* 1.5 turns out means the pilot jet is wrong... i'd go with the 20 pilot and see how that goes. :)
try a 20 pilot. It sounds like it's running ok everywhere else...
But yeah, air screw should be set around 1.5 turns out. further out, or further in than *roughly* 1.5 turns out means the pilot jet is wrong... i'd go with the 20 pilot and see how that goes. :)
When you say "air screw should be around 1.5 turns out" is that the idle mixture screw?

Also, how can I tell if my Mikuni 26mm is genuine or not??

Sorry to hijack the thread...
creole I think you're getting confesed about the fuel, air, idle screws.

now don't quote me on this but I think the fakuni has a FUEL screw underneath as opposed to the AIR screw described in the tuning guide (an air screw is typically of the side near the airfilter on a china carby while the FUEL screw in underneath on the manifold side)
so if you have to screw out the FUEL screw 3 turns then you're pilot jet is running lean meaning you need a richer pilot jet.
the other screw on the carby on the side facing towards the middle of the bike is your IDLE screw, this screw simply raises or lowers the idle speed, screw in to raise the speed.

so, you DO NOT have an air screw. you have a fuel screw (underneath) and an idle screw (on the side).
Screw the fuel screw out to richen the mixture, screw the idle screw in to raise the idle speed.

Now I'm not an expert so I may be wrong, hope this helps.
I hate the fakunie's too, they're a pain in the ass to adjust the mixture so I ended up buying a manifold adaptor so I can face the manifold outwards and easily get to the fuel screw with a normal screwdriver.

DHZ sells these for about $25
Fakunis have only the air/fuel mix screw, and the idle screw. air/fuel effects the amount of fuel going into the pilot jet circuit, and the idle screw physically raises the slide of the carby to raise or lower the idle by opening or closing the slide by a small amount.

Air screw and fuel screw, in regards to the fakunis are one and the same... ie air/fuel screw...
Also, how can I tell if my Mikuni 26mm is genuine or not??

you probably have a genuine mikuni 22mm oval bore carby, the reason it's called a fakuni is it's advertised as a 26mm
Don't under estimate yourself DvDRip ... You're spot on in everything you've said ... the Fakuni's have a FUEL screw ... so having to have it screwed out too far means the pilot is too small ...

Your adjustable carb adaptor idea is excellensio ... Between YOU and Thump*140 ... you are both doing a spot on job for Mini Riders ... It's great to see !!! ... I can just sit back and read most of the time now ... thanks for saving me a heap of typing ... LOL ...

Crackling and popping could be caused by valve sealing or ignition problems ... ie unburnt fuel is exploding in the pipe after leaking past a poorly sealing exhaust valve ... popping thru the inlet is mainly caused by a poorly sealing intake valve or erratic ignition timing ... In a LOT of cases ... people are tuning their carbs to get around another problem ... much like a tall guy taking headache tablets everyday instead of raising the height of the doorways in his house to eliminate the true cause of his headaches ... ie constantly smacking his head on the tops of the doorways ...