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[How to Tutorial] How to Swap Upgrade Carb Main Jets Hyosung GT GT125R GV250 GV125 GT250R

Hyosung Carb 250 125 GT GV Pilot Jets Main Jet

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

Hyosung Tank Removal

3. Loosen Airbox Clamps

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

Loosen Airbox Clamps

4. Remove Airbox

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

Remove Airbox

5. Exposed Bike View

Bike Exposed View

6. Loosen Rubber Intake Clamps

Unscrew the clamps holding the carburetors down.

Loosen Intake Clamps

7. Lift Carbs Slightly

Leave cables & pipes connected. Fuel will be drained next.

Lift Carbs Slightly

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

Drain Fuel

9. Remove Idle Screw Adjuster

Unscrew the steel bolt holding the idle screw (left side of engine).

Idle Screw Adjuster

10. Remove Float Bowls

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

Remove Float Bowls

11. Use Mole Grips if Needed

Bite bolt heads carefully if screws are stuck, then remove completely.

Use Mole Grips

12. Swap Main Jets

Install upgraded jets: one size up front & rear, depending on model.

Swap Main Jets

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

Install Upgraded Jets

14. Double-Check Jets

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

Double-Check Jets

15. Oil the Gaskets

Use small amount of cooking or motor oil on your fingers to coat gaskets.

Oil Gaskets

16. Reassemble Bike

Reverse steps #12 → #1. Double-check all screws & clamps.

17. Final Outcome

Final Result
Final Result 2

18. Reinstall Fuel Tank & Pipes

Check all clamps, throttle, choke line, fuel drain cable, and idle screw.

Reinstall Fuel Tank

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!

[Download] Hyosung GT125 & GT250 (Comet/R/RC) – Service Manuals & Owner’s Books

Hyosung GT125 & GT250 Series Resources

The Hyosung GT series is the most popular range in the lineup, spanning the naked GT125/250 Comet, the faired GT125R/250R, and the modern RC/P facelifts. As these bikes are very common across the UK and EU, we have preserved these official workshop manuals and owners books to ensure access for the community.

Model Identification & Technical Data

Whether you are tuning carburettors on a Gen 1 silver-engine model or diagnosing a Delphi EFI system on a newer black-engine 250cc, identifying your generation is key:

  • GT125/250 (1998–2008): Silver engine, Mikuni Carbs, “Old Shape” (Gen 1).
  • GT125/250 RC & P (2009+): Black engine, Facelifted bodywork, Carburetted or EFI.
  • EFI Models (2008+): Features an O2 sensor in the exhaust downpipe and 28hp DOHC V-Twin engines.
  • Maintenance: All models feature the iconic 75° V-Twin DOHC 8-valve architecture.

For more details, see our Pre-2009 (Gen 1) Wiki or the Post-2009 Facelift Gallery.

Carburetted (Carby) Models


⬇ OWNER’S MANUAL – GT125/250 [Gen 1]


⬇ MANUAL – GT125/250 [Facelift & Hybrid]

Electronic Fuel Injection (EFI)


⬇ EFI SERVICE MANUAL – GT250 & GV250

Need Help? If you have technical questions, please visit the Technical Help Forum.

[How to Tutorial] [Bike Mod] 650/250/125 – AiS Block Off Removal (EGR Delete)

Hyosung AIS Removal Guide: EGR Delete for GT650, GT250 & GV125

⚠ WARNING: This technical guide involves modifying the Air Injection System (AIS). Proceed at your own risk. Incorrect installation can affect engine longevity or emissions compliance. Ensure you have the correct tools before starting.

If you are looking to improve the exhaust note and throttle response of your Hyosung GT650R, GT250, or GV125 Aquila, performing an AIS (Air Injection System) block-off—often referred to by riders as an EGR Delete—is one of the most effective “free” mods available. This process is common among race teams and enthusiasts looking to strip away parasitic emissions equipment.

What is the AIS (Air Injection System)?

The AIS is designed to help motorcycles meet strict Euro emissions standards by injecting fresh air into the exhaust port. This helps burn off unburnt fuel in the header pipes. While great for the environment on paper, it often causes excessive “popping” on deceleration and adds unnecessary clutter to the engine bay. It essentially “fakes” cleaner emissions by diluting exhaust gases, acting like a parasite that steals potential engine efficiency.

Benefits of the AIS Block-Off Mod

  • Reduced Exhaust Popping: Eliminates the loud, annoying backfiring often heard with aftermarket stainless steel exhausts.
  • Improved Induction Roar: Gives the bike a deeper, more natural groan at idle and under load.
  • Smoother Mid-Range: Many riders report a noticeable improvement in the 4,000–7,000 RPM range, particularly on smaller 125cc and 250cc models.
  • Simplified Maintenance: Clears up space around the cylinder heads, making spark plug changes and valve clearances easier.

Whether you are running a stock setup or a full custom exhaust system, removing the AIS is the first step toward a true performance tune. Full credit goes to the original community author who compiled this technical manual.

Ready to start the mod? Download the step-by-step technical book below:

Community Note: This mod is highly recommended if you have recently upgraded to a Delkevic or similar aftermarket silencer.

[How to Tutorial] Clutch Plates Upgrade Replacement for Hyosung GT GV125 GT250R GT125R GV250

Learn how to change Hyosung GV/GT 125 & 250 Clutch Plates on the GV125 / 250 Engines (GT/GV)

💥 NOTE! – SOAK YOUR CLUTCH PLATES IN OIL FOR 24HRS BEFORE DOING THIS! 💥

🛠 Tools Needed

Size 8-10 “O-Spanner” Stubby Flat Screwdriver Allen Keys (if needed)
Oil Drain Pan Size 15/17/19 Sockets Pliers or MoleGrips
Rags Time Patience

Optional 19mm socket/spanner for CNC Magnetic Oil Filler Cap

🎁 Parts Replaced

Clutch Plates (fibre) Clutch Bearing Steel Clutch Discs
Oil (1.5L) JASO MA2 Grade Oil Filter & O-Rings

💪 Skill Level = Anyone can do it, but if unsure, speak to a mechanic. (Follow this guide at your own risk)

HYOSUNG GV 125 250 CLUTCH PLATES uk hysoung FORUM OWNERS CLUB

⚠ Symptoms / Troubleshooting Hints

Always replace your clutch plates when you notice the following:

  • Every 5000-6000 miles. Regardless (just like brake pads have a wear limit!)
  • When you find yourself adjusting the clutch cable frequently.
  • When gears feel notchy or hard to shift.
  • When the bike slips during gear changes.
  • Does your clutch basket “knock” during idle?
  • Does the bike “rumble” when holding the clutch lever in?
  • Acceleration is slower than before.
  • Clutch seems to “slip” on hard acceleration.

🔧 Let’s fix it….

Technical Inspection (What to look for inside)

HYOSUNG GV 125 250 CLUTCH PLATES uk hysoung FORUM OWNERS CLUB

  • Clutch Plate Thickness Limit = 2.6mm (New ones range from 3-4mm)
  • Clutch Spring Free Length Limit = 36.2mm (Normal range 3.8 to 4.6mm)
  • Deep grooves on the clutch basket? Time to replace entirely, or file gently if minimal wear.
  • Silver clutch metal discs (separators) – 1mm distortion limit.
  • Loud rattling sounds? Your clutch plates may be cracked and must be replaced immediately to avoid blocking oil passages.
  • Clutch basket center nut must remain tight with its washer intact.

🚦 Let’s begin!!!

HYOSUNG GV 125 250 CLUTCH PLATES uk hysoung FORUM OWNERS CLUB

Park your bike, get your tools out, and start with the drain pan to drain oil first…

HYOSUNG GV 125 250 CLUTCH PLATES uk hysoung FORUM OWNERS CLUB

  1. Use either 15mm, 17mm, or 19mm socket to drain the oil:
    • A. Clean the oil drain plug.
    • B. Drain all the oil completely.
    • C. Refit the plug.
    • D. Tighten to a maximum of 15 ft.lbs (20 N.M) – torque wrench recommended. If unavailable, tighten until it feels snug.
🔥 BE WARNED! – THIS IS THE “DRAIN” PLUG (DO NOT UNDO ANYTHING ELSE UNDER THE BIKE)

Drain plug warning

Ready? Get the new clutch plates and discs out…

New clutch plates ready

Remove the engine cover bolts in the blue circles. Remember where each bolt goes! Some bolts vary in size. 🔥 Be warned: leftover oil will spill out!

Oil filter warning

When you reach the oil filter rod, carefully remove it:

  1. Put two nuts on top of each other.
  2. Undo the second nut (the top one acts as a stopper).
  3. Continue unscrewing until the rod comes out entirely.

⚠ Note: Mole grips can be used with caution but may scratch the rod.

O-Rings warning

Don’t lose the O-Rings! There are 2: one for the cap, one inside the oil filter hole. Always replace O-rings & filter during oil or clutch work.

Clutch cover removal

Use a stubby screwdriver to gently loosen the clutch cover, working from the bottom and then top near the oil cooler pipes. Carefully remove the cover.

Gasket warning

🔥 NOTE: The gasket must be replaced with a new one. Old gaskets may tear.

Remove clutch bolts

Remove clutch bolts from top down gradually (25% each), then fully remove. This protects the clutch springs.

Inspect clutch hub

  • Remove the front clutch hub facia plate and inspect underlying parts.

Diagram

Inspect parts #10, #16, #17, #13 on the diagram.

Remove plates

Remove clutch plates one by one, remembering the order: First and last plates are always fibre. Steel discs go in between.

Plate order diagram

Fit back parts in this order: #13 → 17 → 16 → 9 → 10

Tightening sequence

Re-tighten clutch bolts: Start top 25%, then bottom left & right 25%, then remaining bolts to full torque. Max torque for clutch bolts = 5 ft.lbs (finger tight!).

Final steps

  • Max torque for clutch engine cover bolts: 5.5 ft.lbs (aluminium)
  • Use fresh oil, O-rings, filter, and gasket
  • Record this event in a log or back of owner’s manual to track mileage and service history

👍 Recommendation: Use high-quality 10W40 or 10W50 JASO MA2 ester oil. Avoid low-quality brands; Motul & Silkolene are best.

🏍 Ride Safe! Any questions? Leave a comment below!

[Download] GV125 & GV250 Aquila / Mirage – Service Manuals & Hyosung Owners Books

Hyosung GV125 & GV250 Aquila (Mirage) Resources

The Hyosung GV125 and GV250 Aquila (also known as the Mirage in Korea) are the backbone of the Hyosung cruiser lineup. As the first 125cc V-twin cruiser to hit the global market, these bikes have earned a massive following. Below, you will find the essential PDF owners’ books and workshop data to keep these classic cruisers on the road.

Cruiser Specifications & Engine Data

The GV series shares its 75° V-twin DOHC engine architecture with the GT125/250 sportbikes, meaning many internal components and torque settings are identical. However, the GV features a unique cruiser chassis, longer wheelbase, and classic aesthetics.

  • GV125 Carb: 124.5cc V-Twin, DOHC 8-Valve, Mikuni Carburettors.
  • GV250 Carb/EFI: 249cc V-Twin, 28hp (EFI versions) or 26hp (Carby).
  • Drive: 5-Speed gearbox with chain final drive.
  • Maintenance: Shared engine parts with the GT series Comet/GTR.

For more details, check our GV125 Carb Wiki or the GV250 (Carb/EFI) Gallery.


⬇ OWNER’S MANUAL – GV250 (Carby / 125cc compatible)

⬇ OWNER’S MANUAL – GV125 [First Gen 1998-2001]

⬇ OWNER’S MANUAL – GV125C [2002 Model Only]

Pro Tip: If you are looking for the GV250 EFI Service Manual, please head over to our GT/GV EFI Manual Topic. For troubleshooting assistance, post in the Parts Help section!