
How to Swap Carb Main Jets on Hyosung 125/250cc V-Twins
Carb Main Jets are one of the most common upgrades on these 125/250 carby twins. This tutorial will show you how to swap carb main jets the easy way. #SwapNGo!
Important: If your bike is under warranty, check with your dealer before doing this. Doing this may void warranty on new bikes.
Anyway, let’s cut to the chase!
0. Tools Required:
- Large Mole Grips (& small one too)
- Philips (+) Star screwdriver – Regular Length
- Philips (+) Star screwdriver – Stubby Length
- 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. Remove Side Panels & Fairings
Unscrew the bolts holding the side-panels, then take off your fairings.
Owners of Naked Bikes: Skip to #2.
Tip for GTR Owners: Place a rug under the bike. When fairings drop as one piece (left/right side panels + middle engine fairing), carefully push one side under the bike to remove safely.
2. Remove the Fuel Tank
- Unscrew #10 (fuel tank bolt) and lift tank slightly to see fuel pipes
- Disconnect the fuel pipe safely (marked in red circle in pictures)
- Disconnect the small vacuum-air hose attached to the tap
- Remove the tank from the frame. Watch out for small rubber parts that may fall out

3. Loosen Airbox Clamps
On the left side under the airbox, use a stubby screwdriver to unscrew the 2 clamps on each carb.

4. Remove Airbox
Take airbox off and put aside. Remember the 2 bolts holding it to the frame!

5. Exposed Bike View

6. Loosen Rubber Intake Clamps
Unscrew the clamps holding the carburetors down.

7. Lift Carbs Slightly
Leave cables & pipes connected. Fuel will be drained next.

8. Drain Fuel Safely
Do this in a ventilated area or outdoors! Fuel vapors are flammable.
- Unscrew 2 bolts halfway (marked in red)
- Catch dripping fuel in a bowl

9. Remove Idle Screw Adjuster
Unscrew the steel bolt holding the idle screw (left side of engine).

10. Remove Float Bowls
Remove 8 screws holding float bowls. Some may be tight; use force carefully.

11. Use Mole Grips if Needed
Bite bolt heads carefully if screws are stuck, then remove completely.

12. Swap Main Jets
Install upgraded jets: one size up front & rear, depending on model.

Stock Carb Jet Info:
- GT125 / GT125R & RC = 87.5 Front, 90 Rear
- GT250 / GT250R = 92.5 Front, 95 Rear
- GV125 Aquila = 87.5 Front & Rear
- GV250 Aquila = 90 Front, 87.5 Rear
Pilot jets remain stock: 125cc = 15, 250cc = 20
13. Install Upgraded Jets
GT125/125R = 90 Front, 92.5 Rear
GT250/250R = 95 Front, 97.5 Rear
Aquila 125/250 = One size up each

14. Double-Check Jets
Ensure brass jets are snug but not over-tightened. Clean tiny holes with carb cleaner.

15. Oil the Gaskets
Use small amount of cooking or motor oil on your fingers to coat gaskets.

16. Reassemble Bike
Reverse steps #12 → #1. Double-check all screws & clamps.
17. Final Outcome


18. Reinstall Fuel Tank & Pipes
Check all clamps, throttle, choke line, fuel drain cable, and idle screw.

19. Start Bike
If it doesn’t start immediately, use choke, twist throttle a few times, crank 5-7 seconds. Use Bradex Easy Start if necessary.
20. Troubleshooting
- Fuel may take a few seconds to reach carbs
- Use choke lever fully if needed
- Crank while twisting throttle 5-7 seconds
- Let idle for few mins to burn off excess fuel
21. Done! 🏁
You should be ready to ride. If you have questions or feedback, reply below!