jetting for 140cc yx with 26mm mikuni

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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...

ohhhhhhhhhhhhhhhhhhhhhhhhhh i see now!!

I was following this Carb tuning guide by Gawain, thats whay i was a bit ( 0.o)


thanks guys. top help.
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The FUEL screw underneath 4 stroke carbs works by bleeding fuel into the inlet tract AFTER the slide ie they're a bypass ... 4 strokes have half the intake pulse frequency of a two stroke since they induct fuel/air every 4th stroke so they have less vacuum and air speed at the same rpm ... Leaking fuel in on the manifold side of the slide supposedly helps with low rpm "on and off" the throttle response similar to having the choke on slightly ... Fuel screw carbs are supposed to be less sensitive to humidity and temperature changes ... BUT ... everyone knows that rich mixtures don't run too well once the engine heats up to operating temperature ... so whenever you cop spluttering bogging etc when your engine gets hot ... always think of the mixture as being too rich ...
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
That’s your problem bro, manuals tell u to stay away from Chinese stuff , jets will be wrong too