How to swap Carb Main Jets on Hyosung 125/250cc v-Twins

Carb Main Jets one of the most common upgrades to do on these 125/250 carby twins.   This tutorial will show you how to swap carb main jets the easy way.  #SwapNGo!

First things 1st, if your bike has warranty, consult your dealer before attempting this. This may void warranty on new bikes.

Anyway, Let’s cut to the chase!

0. The tools required:
– Large Mole Grips  (& a small one too)
– Philips (+) Star screwdriver – Regular Length
– Philips (+) Star screwdriver  – STUBBY LENGTH (Shortie)
– A small amount of cooking oil
– Hex Bits (& screwdriver Bit)  (if you have stainless bolts on the carbs)
– 10 or 12mm Spanner or Socket Tool.
– Flat-Head ( – ) screwdriver

1. Whenever you’re ready, unscrew the bolts that hold the side-panels of your bike. Then take off your fairings!

Owners of Naked Bikes = Skip to #2.  (GT Comet)

Tip for GTR Owners  = Place a rug under the bike,  and when your fairings drop to the floor as “ONE piece”.  Carefully push one side under the bike, so the whole “one piece” can come out safely off the bike.   One Piece means “left/Right Side Panels & Middle Engine fairing attached as one.”

2.  Get the tank off the bike next…
* Unscrew #10 (fuel tank bolt) pictured below & lift up the tank “slightly up” so you can see the fuel pipes.
* Disconnect the fuel pipe safely from under your tank  (marked in red circle!)
* Don’t forget to also disconnect the “small” vacuum-air hose that is also attached to the tap
(Some GT Naked bikes use a manual tap on/off Tap instead of a vacuum fed tap)
* When the pipes are off,  remove the whole tank away from the frame. 
(Watch out for item #3 – These rubber things fall out sometimes)
(Save Picture to device to view it much larger)

3.  Loosen the airbox clamps
On the LEFT side of your bike, directly under the airbox.  Grab a stubby/shorty (+) screwdriver and unscrew loose the 2x clamps on each side of the carb.
They are marked in the red arrow as reference.  They are black in colour.

4. Remove the airbox away from the bike.
Take your airbox off & put it aside for now.  Jump to #5 next.
* Don’t forget to remove the 2 bolts holding the airbox to the frame!

5.  Your bike should now be exposed like this…

6.  Next up, let’s loosen the rubber intake clamps…
On the red arrows pointed -> unscrew loose the clamps holding the carburetors down.

7. Lift up your carbs out of the intake boots   (just a little!)
Yes leave everything still connected (cables & pipes)
We still have to drain the fuel out , so jump to #8

8. Get a bowl & do this on a VENTILATED area or outdoors! (Fuel Vapor)
* On the red arrows , unscrew these 2 bolts about half-way out.
* Fuel will start dripping down the small pipes under the bolts
(fuel is draining out)
* Grab a bowl or bucket and capture the fuel  (watch for spillls!)

9. When the fuel is drained completely…
UnHook the idle-screw adjuster.
* On the left side of your engine, there is a black “hand” screw that controls idle speed.
* Unscrew the STEEL bolt that holds it down  (Look at red circle picture)

GT Comet Owners = Skip to #10.

10. Phew…. Let’s remove the float bowls now!
You can do this SIDEWAYS  or  FLIP the carbs over a bit, as pictured below…
Remove the 8 screws that hold the float bowls down.
* Be warned! = Some original screws can be harder to unscrew loose. So be ready to use some force!
* To be honest,  Mikuni should really start using stainless bolts anyway!

11. Struggling?  Grab a pair of mole grips!
* When a screw is being difficult to remove with a screwdriver.
* Adjust your mole grips to bite a small bolt head
* Make sure it’s strong enough , so it’s doesn’t slip!
* Bite the bolt head –> Turn left. It should squeek loose.
* When the “bolt” is loose, use your original screwdriver to remove the bolt entirely.
* * * It’s common for these carbs sometimes to have rusty looking screws.  * * *

12. Finally,  we get to the real thing.  Your carb’s undersides should be exposed now!
* Next  up , get your “Upgraded” Jets ready!
* Take a 5 min break!
* You are going to swap your standard carb main jets jets for the bigger ones.

 Information On Carb Main Jets 
Depending on which model 125/250 v-twin , it is important to remember these points:
* Stock GT125 / GT125R & RC = 87.5 Front Main Jet & 90.0 Rear Main Jet
* Stock GT250 / GT250R = 92.5 Front Main Jet & 95 Rear Main Jet
* Stock GV125 Aquila = 87.5 Front Main Jet & 87.5 Rear Main Jet
* Stock GV250 Aquila = 90.0 Front Main Jet & 87.5 Rear Main Jet

All 125 vTwins = Size 15 Pilot Jets (Front & Rear) = Don’t change it.
All 250 vTwins = Size 20 Pilot Jets (Front & Rear) = Don’t change it.

• If you change exhaust pipes to “free-flowing” (aka. stainless pipes/straight through) = Then it is even more necessary to upJet the v -twin carb main jets.
  If you have an aftermarket Air Filter such as “K&N” =  You also need to upJet
  (more air = more fuel!)

13.  Ready? Let’s upgrade!
Swap the front & rear carb main jets for one size up each.
GT125/GT125R = Upgrade to 90 FRONT  & 92.5 REAR
GT250/GT250R = Upgrade to 95 FRONT & 97.5 REAR
or
GT250/250R = 95 & 95    ( For Aquila 125/250 = One Size Up each. )
* Okay, see the red circles ?
* Unscrew each jet  & replace them with a bigger one.

14. Double check that the jets are secure & tight before closing up!
* Take note! – These jets are brass, so don’t over-tighten them!
* Make sure your carb is also clean from any visible dirt or dust particles.
* The Jet Holes are very tiny! …(So a clean area is a must)
* A Carb Cleaner helps clean up old carb parts & carb main jets !

15.  Grab some cooking oil (or old motor oil)
* Drop a “SMALL AMOUNT” of oil on your FINGERS
* Use your fingers to SLIDE OVER the black gaskets…
* Keep doing this until both gaskets have a nice coat of oil-film over them.

16. All done?  Time to close everything up,
^ let’s do things in reverse now ^  #12 -> #1…  
Start from #12 going back to #1  –  Always double check each step you take!
Fit all your parts back together.

17. In the end , this should be the final outcome

18. Install the Fuel-Tank (& also connect pipes!)
* Also, check that all screws are nice & secured  such as:  2x Intake Rubber Clamps, 2x Airbox Clamps, Carb-Float Bowl Screws & 2x Carb-Fuel Drain screw closed up too.
* Inspect your throttle cable (Twist your throttle grip to test!)
* Ensure your Choke Line is working   (Test your choke = The carb  will slide its choke rail!)
* Check your fuel drain cable is AWAY from the engine  (zip tie to frame & let it point to ground)
* Lastly,  make sure the “ENGINE IDLE” HAND-SCREW is also attached properly 

19. You should now be ready to start!
This quick ten second video was taken, while the bike was still cold & we swapped carb jets.
This 125RC was only 600 miles old, no choke was used.

20. Trouble starting? 
* If it doesn’t start the first time around.  That means the fuel inside the carbs hasn’t reached “capacity” yet.
* Put the CHOKE lever on max setting (towards YOU)
* Then Crank the bike for about 5-7 seconds (while TWISTING the throttle a few times).
* Then it should fire in to life.   Let it idle for few mins to burn off any excess fuel that could have made the spark plugs wet.

Still got some trouble? Get a can of Bradex EASY START – Spray lots of it inside your air filter hole. Now crank the bike again & it should fire this time round!

Factory Troubleshooting Guide for starting issues.
(Save the images to your Device to view it much larger)

21. The end!🏁

You should be miles away in to the countryside or your other scenic spot!

I hope this tutorial was easy for you to follow.  As always, if you have any questions or feedback,  just reply below!

– M