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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] GT GTR 125 250 R Comet Carby & EFi Service Manual & Hyosung Owners Books

[Download] GT GTR 125 250 R Comet Service Manual & Hyosung Owners Books

Hyosung GT 125 / 250 Series – Hyosung’s Most Popular Bike Range!

This topic is a living resource with links added and updated over time. Owners of all GT / GTR 125/250 models can follow it anytime for reference.

If you need specific help with these machines, post a New Topic in the “Technical Help” section.

Ok, here we go…

CARBY BIKES (Carburetted) – 1998 to 2009/2010

DOWNLOAD – 125/250 GT & GTR [OLD SHAPE MODELS] – 1998-2009 OWNERS MANUAL
OTHER MODELS (HYBRID BIKES) – HALF-NEW SHAPE & RC NAKEDS

DOWNLOAD – 125/250 GT & GTR – CARBY & EFI MODELS – 2009+ OWNERS MANUAL
EFI / EFi / Fi BIKES (Electronic Injectors) – 2008+

(Hint: Exhaust pipe has a sensor)


DOWNLOAD – 250 GT & GTR (& GV250) – EFi Models – 2008+ SERVICE MANUAL (Workshop Book)

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

 

⚠ WARNING: This guide involves AIS (EGR) removal. Follow at your own risk! Incorrect modifications can damage your bike or void warranties. Make sure you understand the steps fully before attempting.

Credits: I did not write this, but full credit to the lad who created this book. (BTW, follow at your own risk!)

Benefits & Proofs

Aimed at 650cc bikes but the same method also applies to smaller CC bikes.

Deleting the AIS (EGR block) from the bike results in:

  • Loud pops on the exhaust pipes
  • Deeper groans on idle (louder induction roar)
  • Improved performance from 4-7k rpm, especially for low CC bikes
  • Recommended for stainless pipes anyway
  • First step race teams take—removing parasitic parts
  • Overall marginally louder bike

What is AIS? (Often incorrectly called EGR by uneducated mechanics)

  • It makes the bike “fake” its emissions by burning cleaner air to meet Euro laws.

That’s it!

  • It has never proven to benefit bikes; it actually “steals” energy from the engine. Think of it like a “Remora” fish under a shark, migrating across the seas 😉

DOWNLOAD/VIEW – 650/250/125 – AIS Block Off (EGR Delete) PDF

[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

DOWNLOAD GV125 GV250 AQUILA 250 MIRAGE SERVICE OWNERS MANUAL PDF BOOK HYOSUNG

Hyosung’s favourite & the first 125 twin to roll out of the factory, the GV 125, then followed the GV250 Aquila!

A printed owners manual is a must-have for each rider. But hey, the internet is here. Download the PDFs below…

Don’t forget, the GV250 plus the 125 version shares the engines with GT125 & 250cc Bikes. Some data is the same.

Here they are…!


OWNERS MANUAL – GV250 Aquila – From 2003 Onwards – CARBY EDITIONS (Ok for 125cc bikes also!)

OWNERS MANUAL – GV125 Classic – 1998-2001 – FIRST GEN

OWNERS MANUAL – GV125C – 2002 Model – 2002 MODEL OWNERS BOOK

If unsure or need help with a particular problem, post in the “Parts Help” section instead…