Forums 🚥 PIT STOP 🔧 Hyosung Technical Help [Tutorial] How to swap Carb Main Jets – Hyosung GV & GT 125/250 Reply To: [Tutorial] How to swap Carb Main Jets – Hyosung GV & GT 125/250

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Nov 30, 2017
North UK

So had another quick look at the bike, I took the air filter off so I could access the carb. I thought I would start it up just to see what it was like and in my opinion it ran better!? I also noticed some black bungs at the bottom of the carb, so removed them to reveal screw heads. What are these for? Also choke is working better with air filter removed. Do I have a mixture problem here? Any help would be appreciated


Bungs hiding gold screws = Air / Fuel Mixture = Only the factory tunes that. Dealers are told not to touch it whatsoever , otherwise re-tuning is a serious abyss here. I agree with the koreans.

If it runs better without a filter = Investigate air filter (clogged?) or spark plugs (glaze brown means they are healthy)

Glaze brown plugs also mean = carb / filter / electrics are doing well.

To increase mixture = Either add 1 step up from a factory main jet (90/92.5)

Pilots remain size 15 each but must be clean = smooth rolling idle is dependent on pilot jets

Again, investigate your black CV sliders on the carb
Idle and revving up to 7k rpm etc = they slide up
Dropping revs to idle = they slide down

Particles on the slider will grind against the carb walls = certain rpms will be bouncy, certain rpms will be jerky –> carb sliders may be hanging

Cleaning them fixes this.

If the sliders are “retracting” fast & (as quick as your throttle moves) – they should be working on

This clip will hint , why its critical the sliders work nicely … (idle , through gears , warming up)


The real key to a free flowing bike is to attack these at once:
Coils , Spark Plugs , Clogged Jets , slow retracting black tube sliders, good fuel pump working , air filter is new (PAPER FILTERS LAST 2-3K before they start to give you less air for carbs)
5k would be pushing it.

If its a fabric / cotton gauze filter – it generally is okay until approx 7-10k , the filter will hint how dirty it is.


“UNLESS” – Someone in the past ever messed up the mixture screws (which were hidden behind black bungs) , then it can explain cause not mixing air intake & petrol properly inside. Like i said, it would be an abyss to retune it back as Koreans never revealed the adjust settings, they do that before shipping the bike away. Depends on country & market its going to.

So the home owner, may go through few spark plugs until its right (plug chopping business)

But honestly, the spark plug colors will tell you how rich or lean the bike is.

// Meditation doesn't mean you have to sit still....

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