Topic Tag


GT250 EFI DSpec Stalls at Idle after Warm Up

I own the above mentioned bike bought second hand from the original owner who had purchased it in 2013. When I bought it, it was still under warranty with about 3000km on the clock. It is important to note that this version, the DSpec, has some minor differences in the number of sensors than similar GT250’s of that period. It is a twin cylinder equipped with Delphi electronics. The bike, which is always garaged, worked flawlessly for the first couple of years but after that it had started to loose power erratically after warm up. This was happening after about 15mins of starting from cold. From time to time I was also getting error code 132 intermittently which indicated problems on cylinder one. When the O2 sensor was inspected it was found to be covered in soot. Because of this I had assumed that the problem was limited to around cylinder 1. I replaced the plugs and high voltage cables and also swapped the coils and the O2 sensors around but the problem did not seem to change or move. I also had the injectors cleaned up by an experienced bike mechanic. I was also lucky to find a nearly new ECU which I could purchase at half price, but even this did not improve the situation.

Finally I decided to replace the coils which solved the problem completely.

Unfortunately a few more years down the line I am starting to experience a similar, though not so drastic situation so far with the bike starting perfectly but after warm up it starts missing occasionally at idle and if I stay at idle long enough the engine stalls. I don’t feel any loss in power when I am revved up.

Because of this I am starting to suspect the coils again even though these are no more than 3 years old.

Any ideas?


Another new GT250 owner

I have been oddly interested in the GT250 Naked for a few years and the right one at the right price finally came my way. I bought this yesterday, and it is as close to new as you can get. 231 miles on the odometer. 👍👍 I put 115 miles on yesterday. These are very interesting bikes. That v-twin has a WIDE “power” spread. 6th gear out on the highway, roll into a town at 30mph, and you don’t have to downshift if you choose. I’m in the USA and I suspect the  fueling is very lean( for emission standards)  in the 7500rpm range as it does not like to accelerate from that rpm. It will rev through no problems otherwise. It was very clean when I picked it up, but had to ride in some rain slop for half the ride.

[Tutorial] How to Change Oil & Filter on Hyosung GV + GT125/250 +R

Time for an oil change ? …One of the crucial things to stay on top of!

This short tutorial aims to show  you how to change your oil with ease on the 125/250 twins (2001-2018) and let’s not forget the filter too!

This applies to: GV125 (01-2018) / GV250 (upto 2018) / GT & GTR 125 (03-2018) inc GT250+R too.

Personally, one of the other twins I own gets its oil changes every 1,000-1500 miles using Ester oils . I’ve been this way for almost 4-5yrs at time of this writing, the manual suggests the oil can be changed at longer intervals.  This is optimistic to say the least, so i’ll leave that there. I know too well, oil acts funny around the 1500 mark tops (gearbox, clutch, false N’s, hotter, louder).  The internet will send you down a rabbit hole,  so my opinion? Use more premium oil marked JASO MA2 , Ester is good. Change around every 1500 tops.

Moving on, let’s prepare the parts & tools first…

Oil .  Get 2x 1L Bottles  
(Bike needs 1.5L total)
Oil Filter & O-Rings 
(don’t skip filters!)
 Drain Pan/Bucket
(that can hold 3L+ of fluid.)
Torque Wrench
(too crucial, you’ll see why below⏬)
Size 15/17mm Socket  
(12 sided!)(NO SPANNER)
 Size 10mm Socket / Driver

Plus these bits also…
Pliers | Funnel | Plastic/Rubber Gloves | Clean Rags/Cloths. | Measuring Jug

1. Okay let’s go, Undo your filler cap first!

For stock caps – use a Pliers to open it (anti-clockwise).
For custom CNC Caps – We use either 19/21mm socket.

2. Go under engine & loosen the drain plug
For GTR owners, use a bar extension to aid you in undoing the drain bolt or  just take your fairings off. 
This tutorial was done on a Comet that’s all (& it needed a clean!😬)
⚠ Be slow to prevent this plug falling in to the bucket!


3. About those drain plugs…

Roads are salty, the stock plugs can’t fight off their bad rust.  A seized plug will easily round off and you’ll replace more parts in the long run (strainer cap, o ring, bolts😬).
We avoid that by using a made coloured plug that has some weather resistance to it and it’s less prone to seizures.
Plus its magnetic tips are larger to catch those metalic shavings in the oil!

You will find them sold on here….


4. Finished draining? Close the drain-hole!
– Clean up under your engine
– Grab your torque wrench and set it to 15ft lbs max.
– Tighten your drain-bolt until your wrench clicks once.


5. Go to your oil filter cover next!
– Grab 10mm socket & undo the 3 nuts that hold the small cover.
Oil WILL fall out (leftovers from filter), so place a rag/cloth under the cover and potentially a small pan too. You will see why further down.   Wear plastic/rubber gloves too.  Ditch your old filter away 🗑
Hyosung Oil Change FIlter
Hyosung Oil Change FIlter


6. Filter Cover Inspection & O Ring Change (Pt.1)

– Clean up your oil filter cover
– Check the spring is still good and not gone soft.
– Change the big o-ring   (dress it up in oil first!⛔)  
Hyosung Oil Change FIlter Hyosung Oil Change FIlter

7. O-Ring Change (Part2)

This part needs some more focused attention so we don’t damage the inside hole you’re about to see.
On the picture below,  replace the o-ring that is sitting at the back inside the filter area.
⚠ Use a small hook/claw to pry it out  , don’t force it. ⚠
Hyosung Oil Change FIlter

8. Next, your new oil filter goes in!

See the picture below? That’s exactly how it should go in.
The “hole” of your filter goes inside the engine.
Hyosung Oil Change FIlter


9. Close the filter cover!
– Once the filter is in, close the cover.
– Make sure the nuts are tightened to 6 ft lbs max!
🚦 Check the new Cover O-Ring is dressed in oil (a reminder!)
Hyosung Oil Change FIlter -


10. Done? Grab your oil (& measuring Jug!)
– Get a measuring Jug and pour inside 1500ml of oil in there (1.5L)
  Not less, not more 😉

No Measuring Jug ?  That’s fine, as long as your bottle has volume-markers on it too.

11. Get your Funnel and pour the oil in….
Self explanatory here.
⚠ It’s messy doing an oil change without one, so I dare not digress.
Hyosung Oil Change FIlter Hyosung Oil Change FIlter


12. Close the oil filler cap!
Self explanatory here.
Just make sure the hole is clean and it’s o-ring isn’t damaged.
Hyosung Oil Change FIlter


🚦 Complete. –  Idle for 60 seconds & turn it off!

– Start the bike and let it idle for less than a minute.⌚
– Switch it off.  Let your bike sit upright temporarily⚠
(NO PADDOCKS.  Both wheels on the ground)
– Check your sight-glass says “F” and oil level matches it.
–  ^ Is your oil higher than “F”mark ? => Drain some.
– ^ Is your oil lower than “L/M” mark => Topup/Refill⛔
– Finally, Check your drain plug is good & secure. 🚦


   Tidy up & Go ride. 🏁

Questions? Fire away in the comments! 🤺

Ex-Hyorider, possible future purchase..

Greetings from Michigan. I sold my ’04 GT250 a few years ago. I loved that bike, it never gave me a bit of trouble in the 2 years I had it. I may have a chance soon to pick up another 2004 GT250 for cheap. The owner says it runs good but blows oil out the tail pipe. For $250, I guess I’ll take a chance…🤞

[Tutorial] How to adjust Carb Slide Needle (Tuning) – Hyosung GV & GT 125/250

[Tutorial] How to adjust Carb Slide Needle (Tuning) - Hyosung GV & GT 125/250
How to adjust/tune the carburettor slide needle for improved riding…

⚙ What does this do?
This modification allows the bike to reach better cruising speeds , via the slide needle adjustments.  The throttle slide needle is what controls the main jet fuelling as revs go up & down while riding.  This also eliminates some of the odd flat spots that happen during the 4k-7k rpm range , commonly on the 125s.  In other words, for example if the bike starts to pull at almost full twist throttle , this mod will make it pull with less of a twist.  Your bike should run better from 1st to 5th… fuelling wise…

👇  Before we start, This tutorial will assume the following:

  • You will do this at your own risk & without rushing.
  • You have better spark plugs (ie. Platinums) or older common IX plugs.
  • Your ignition coils & HT caps are sparking good
  • Your carb has already been upjetted (tap here…)
  • You have a performance filter (K&N, BMC, etc)
    or if you have a white-stock cotton one -> cut its snorkel.

Tools needed:

  • JIS type screwdriver
    ( +philips style )
  • Large & long flat___  head screwdriver
  • Size 7mm socket
    (sometimes bolts near the front are hex shaped.)
  • Watchmaker flat screw driver
  • Clean microfibre cloth



1. Go to the right side of your bike
You will spend most of your time here. Do 1 pot each time.
Make sure you do not get distracted as we are taking out small parts!

2. Unscrew the 1st diaphragm cover 
Remove the 4 bolts that hold the diaphragm cover
⚠ You need to disconnect your throttle cable also.

3. Remove the spring & the diaphragm
Once the cover is off,  gently remove the piston slide & its spring.


4. Inspect the parts for any debris  and clean them.  
Use Wynns Carb Cleaner to clean these parts including the diaphragm cover👍
This will stop your slides sticking between revs / gears & scoring to the internal carb walls.

[Tutorial] How to adjust Carb Slide Needle (Tuning) - Hyosung GV & GT 125/250


5. Unscrew the plastic plug holding the needle
Use a long good flat blade screwdriver for this and BE GENTLE – ⚠You do not want to damage this plug!


6.  Do not forget the order of re-fitting back!
Keep this image in mind, when you refit the needle back. The order of these small bits is crucial too.
[Tutorial] How to adjust Carb Slide Needle (Tuning) - Hyosung GV & GT 125/250


7. Remove the E-Ring (Circlip) on the needle   (SLOWLY!)  
⚠ This tiny part must be removed gently with a small watchmaker screwdriver only,don’t pull it!


8A. See those 3 grooves on the needle?   Read on…  

  • A. The pointy end of needle FACES DOWN
  • B. Push down the “black spacer” a little bit.
  • C. Put the “E-RING (circlip)” in the 3rd GROOVE
  • D. Push the “black spacer” back against the E-Ring (Circlip)

[Tutorial] How to adjust Carb Slide Needle (Tuning) - Hyosung GV & GT 125/250

8B.  Here is another view for you…

You want your “E-Ring” on the 3rd notch mark like the finished example including the black spacer being pushed back to where it was.
(Lowest groove with pointy end of slide needle facing down.  The Factory set-up is usually the 2nd (middle) or top (1st) near the FLAT end of the needle.)

9A. Grab the throttle slide and look for this hole…
Make sure the hole shown in image is clear  and  proceed to  9B ….
hyosung throttle slide piston gv125 gt125r gt250r gv250


9B.  Grab your needle too and follow the image….

9C.  Slot the “Needle => first.”  & its mini-spring on top. 
⚠ Go back to #6 ^  , to remember how the needle spring goes in.
⚠ Remember what was said in 9A & 9B ^


10.  Check the needle for free movement (a little bounce back)
Follow the 2 images for instructions/check up…

10B.   Dress up the ring of the throttle slide with a temp oily coat.
It is going back to the carb, so you want to make sure the ring of the slide will sit firmly & create a good seal once fitted. This is to help prevent any unwarranted air leaks / external particle trapping.


11.  Put the slide and big spring & close it up.

– Don’t forget to connect back your throttle cable
– Ensure the 4 screws are nice and tight (finger tight)
Finished?  Go open the next cover…

12.  Repeat at the rear carb, what you did at the front…
Repeat steps 2 to 11^ on your rear carb.

13.   Finished?  🏁  Check your lines before riding…🚦
Final check ups before you start riding , so attention to the following:

  • Air box clamps are still tight – check.
  • The carb slide cover bolts are finger tight (x8)
  • Throttle cable & choke cable are checked.
  • check your air lines and tighten them (preventing any vacuum leaks)

Done! this should have been a straightforward process ,  if you are unsure of something , just comment on this topic.   Otherwise,  go ride! 🏁

FAQ’s / Troubleshooting 🎳
  • My bike runs worse after I did this mod, what gives?
    Check again the covers are tight so its not leaking air or check your intake system too (airbox area)
  • No air leaks, but it still runs worse?
    Your spark plugs may need to be looked at, if its very sooty, clean it.
    Also remember this mod expects you to have some better sparking than a stock hyosung does
  • It has new spark plugs, but still worse…?
    If you have Platinum plugs fitted or  the IX ones , but the plugs appear sooty…
    You may need to investigate your HT Cap & Ignition Coils.
    HT Cap on its own (removed) = 6k ohms on your multi meter it should say.
    Ignition Coil = 6k ohms max  when you measure coil HT WIRE and you bike FRAME. 
  • Okay, the HT cap or coils are showing higher values than 6K ohms? Any fixes?
    Replace them both.   The stock ignition sparking parts can’t be user-repaired.
    All Hyosung stock coils and their “GOLDEN” branded caps not great, another main cause of boggy runs on the 125cc bikes especially.
  • I changed the coils and HT caps already, its still worse?
    Look at your main jets are clean and also investigate your pilot jets
    The ofricies in the 125cc jets are very small! like this text!
    Tap this link (here…) to learn how to remove the carbs off the bike
  • Can I do this on a stock 125cc un-modded bike (ie. brand new machine)
    Not really, it is already running leaner, so raising the needle may help a bit but jetting and uprating the electrics a little will make this mod more effective , because raising the needle makes the mid range a bit richer. (extra fueling)

How to change Vacuum Lines on Hyosung GT125R GT250R & GT Comet (Air Hose Diagram)

Changing the vacuum Lines on carburetor 125/250 vTwins, should be fairly easy. All you need is 15-30 mins of your time, let’s start with the basics first.

  • Hyosung says if your “standard” factory lines are 2-4yrs old.   Discard them regardless. 
  • Old pipes can also rot inside , especially those chinese ones with trashy rubber
  • Larger colder air pipes can help improve the vacuum system but get strong clips!
  • Strong clips are very essential to avoid all possible air leaks.
  • This tutorial applies to all carb operated 125/250  GT & GTR Twins  (includes GV125 too)

Let’s go now…  Strip your fairings down  (GTR owners)   
(if you have a naked bike,  go to next step)
hyosung gt125r air vacuum line gt250 gv125


Below is a diagram of how the “AIR LINE” system should be connected.
 Study (look at) the diagram images below first  , before moving on to the next step….
hyosung gt125r air vacuum line gt250 gv125 hyosung gt125r air vacuum line gt250 gv125



Got your lines ready?   
On the LEFT side of your bike Grab a hose for the fuel pump & follow the next diagram pictures below:

hyosung gt125r air vacuum line gt250 gv125 hyosung gt125r air vacuum line gt250 gv125 hyosung gt125r air vacuum line gt250 gv125


* Once your rear fuel pump  is connect from the left side.
* Now, move on the the RIGHT SIDE of your bike
* Grab a second vacuum pipe and follow the next diagram images below:

hyosung gt125r air vacuum line gt250 gv125
hyosung gt125r air vacuum line gt250 gv125


That’s It – Vacuum Setup is done! 🚦
However, if you have an AIS Device (Fake emissions/EGR) then read below!

*Remember above ^ , there is a cartoon diagram of the infamous “EGR DELETE” mod, right ?
*Remove it or block anything that feeds it.
*This image is another example for you.
* Personally, it’s a useless device. 
* But, if you want it connected -> replace “blocked” with “connected”


    Alright, we are done.   Go test ride! 🏁  


I hope this fast toturial was easy to follow,  however if you get stuck or need help. Just comment on this post!  This tutorial should also help GV owners since they use the same rear pump anyway.

How to upgrade Fuel Lines & Filter on Hyosung GT125R GT250R (Carb Hose Diagram)

Hyosung Gt125r fuel lines diagram filter hose

New Fuel Lines ? Let’s cut to the chase….
(No pun intended!)

This topic intends to show you how to swap your stock lines to larger uprated braided ones.
(By the way if your STOCK/ORIGINAL fuel lines are 2-4yrs old , you must change them regardless – As per service manual)

A. *️⃣  Pros / Cons of the braided / fresh ones vs old stock lines ?

  • Larger fuel filters (3x size) benefit from a gravity assist (& a cleaner reserve). Such filters of this size last about 15-20K miles (except china)
  • Braided hoses last more years & are more heat resistant than the basic skinny lines. (40-100*c)
  • Carbs are less likely to wait for fuel delivery as bigger lines flow more fluid faster.
  • As the bike gets more fuel = It is slightly more spirited through the gears.
    (Hint = Aftermarket exhausts will have a *very slightly* deeper exhaust note)
  • If you have bigger jets installed on the bike, bigger lines help.
  • Cons: None.

Be warned. Chinese fuel lines are very cheap, but why pay twice (don’t last even 1-2yrs) or risk fuel leaks on a hot engine!?

B. 🏍 What bikes does this work on?

  • All carb models of GT125R & Carby GT250Rs
    (Carburetors = That huge silver device between the two v-twin engines)
  • Naked GT Comet Bikes
    (If unsure, just comment on this topic!)

C. 🔧 Tools required:

  • A Cutting tool … & … a flat ( – ) screwdriver.
  • Size 10mm / 12mm spanner or socket.
  • “Stubby /Shortie” ( + ) Plus screwdriver
  • A clean cloth or a towel rag.


1. Let’s begin! – Remove your fuel tank.

  • Grab a size 10/12mm spanner or socket & unscrew the bolt that is holding the tank to the frame.
  • Afterwards, disconnect the 2 pipes under your Tank.
  • Then, put the tank away elsewhere for now.
    ⚠️ Watch out for the “Fuel Level Sensor” wire too! –> BE SLOW & UNPLUG IT! ⚠️

Fuel Lines Gt125R Hyosung

fuel lines diagram gt125 250 hyosung


2. The Air Box comes off next!

  • Unscrew the bolts which are marked in the pictures below…
  • 🔩 2x bolts = airbox-to-carb metal clamps (black color)
  • 🔩 2x bolts = airbox-to-frame (Click for larger size images)

[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets [Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets


3. Your bike should be exposed like this now

fuel lines gt250r hyosung diagram


4. Change the “CARB -> TO -> PUMP ” fuel line

  • A. Remove the hose that connects the carburetor to the fuel pump (at rear)
  • B. Replace it with a NEW LINE (Swap Old Line for New Line)

– – Remember, keep the fuel lines nice & straight! – –


5. A Finished Example = Carb to Pump

fuel lines diagram gv125



6. Ditch your old filter & old lines in the bin!🚮

fuel lines gt125rc hyosung


7. Grab your new filter & cut 2 new *short* pipes!

  • Make sure you use strong clamps to prevent leaks
  • Use the cartoon image to help you orient the filter the right way

fuel lines carb fuel diagram hyosung
Fuel diagram fuel lines hyosung


8. Ready? Install the filter to the rear pump!
(Keep the fuel lines nice & short! = No kinks!)

fuel lines hyosung gv250


9.  Refit your AirBox back on the bike.

  • So go back up ^  to step #2 above  and fit your airbox back on the bike!
  • Take your time & tighten all airbox bolts to prevent AIR LEAKS !

fuel lines diagram gt125 250 hyosung


10.   Last step! Time to fit the tank now.

  • Use the images below as a guide to connect the final 2 pipes. (Fuel & Air-Vacuum lines)
  • Don’t forget to also connect your “Electrical Fuel Level Sensor”  (white plug)
  • Once the pipes & wires are connected,  secure the tank to the bike.
  • Tidy up & grab a helmet!

fuel lines diagram gt125 250 hyosung fuel lines diagram gt125 250 hyosung



Go ride the bike! 🏁

Sidenotes / Q&A 🎳 :

1. My filter has low fuel levels in it. Why?
Check that your filter is installed the right direction.

2. The filter looks good , but it is still low in fuel. Do I Top-up?
Top up your tank.  Low fuel levels make trips harder.

3.  I am getting HIGH Revs on idle, what happened ?
There is an air leak near the airbox or carburetors, so make sure all bolts are tight.
(That also includes metal clamps under the airbox too!)

4.  The Airbox is fine, but the revs are still high!
Check your choke lever  (it is next to your clutch lever) .  To turn off the CHOKE , slide it towards the front tyre.

5. The bike won’t start after changing the fuel lines….?
Check that you connected the pipes the right way & filter is the right direction.
It takes about 2 extra seconds to crank the bike after changing lines.
Here is a hint: when the bike cranks over (starting up),  you will see fuel drip in to the filter!

The end!🎲
If you have any questions  or get stuck,  just reply to this topic.   I hope this guide helps you!

Credits 📜:
Images (@Danny.M) // YouTube Video(@Luke.B)

[Tutorial] How to swap Carb Main Jets – Hyosung GV & GT 125/250

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 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.
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

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!
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

5.  Your bike should now be exposed like this…
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

6.  Next up, let’s loosen the rubber intake clamps…
On the red arrows pointed -> unscrew loose the clamps holding the carburetors down.
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets


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
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

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!)
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets


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.
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

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.  * * *
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets


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.
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets

 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
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.
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Jets



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 !
[Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor 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.
[Tutorial] Carb Main Jets Hyosung GT & GT 125/250 - How to swap carburetor Jets



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
[Tutorial] Carb Main Jets Hyosung GT & GT 125/250 - How to swap carburetor Jets [Tutorial] Carb Main Jets Hyosung GT & GT 125/250 - How to swap carburetor Jets

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 

[Tutorial] Carb Main Jets Hyosung GT & GT 125/250 - How to swap carburetor Jets

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)
[Tutorial] Carb Main Jets Hyosung GT & GT 125/250 - How to swap carburetor Jets [Tutorial] Hyosung GT & GT 125/250 - How to swap carburetor Carb Main Jets



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

[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 “populous” bike range! 

This topic will be a special since links will be added & revised over time.  So owners of all GT / GTR 125/250 machines can follow this topic anytime.

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

Ok here we go…

CARBY BIKES (Carburatted) 1998 – 2009/2010


Ei / EFi / Fi BIKES (Electronic Injectors) (Hint = It exhaust pipe has a sensor)

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


I did not write this , but give credits to the lad who made this book (below).
(BTW, follow at your own risk!)

Benefits, The Proofs, etc..:
Aimed at 650cc bikes but also same method for smaller CC bikes.   

So basically , deleting the AIS (EGR block) from the bike is immediately obvious by:

  • Popping Stops (Loud bangs on the exhaust pipes)
  • Much deeper groans on idle  (louder induction roar)
  • Bike is able to run better from 4-7k rpm (especially low CC bikes)]
  • For Stainless pipes , it’s advised to remove it anyway!
  • It’s 1st the thing race teams do, ditch parasitic parts.
  • Bike is overall louder (marginally than before)


What is AIS (Incorrectly named EGR by the uneducated mechanics)

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

Thats it!

  • It has NEVER proven to benefit bikes, actually it “steals” the energy from the bike to run it.
    It’s a like a “Remora” creature under a shark, as it migrates all over the seas 😉


[Tutorial] Hyosung GV/GT 125 & 250 Clutch Plates Replacement How To

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


🛠 Tools needed ::

Size 8-10 “O-Spanner” Stubby Flat Screwdriver Allen Keys
(if needed)
Oil DrainPan Size 15/17/19 Sockets Pliers or
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.  (Take this guide with your risk)


⚠ 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 see your self adjusting clutch line frequently.
  • when gears feel notchy / hard.
  • when bike slips up during gear changes.
  • Does your “Clutch” basket have any  “knocking” feel during idle ?
  • Does your bike “rumble” when you hold the clutch lever in?
  • not as fast as before (acceleration wise)
  • Clutch seems to “slip” on hard acceleration

🔧 Let’s fix it….

Technical Inspection  (What to look for inside…)

  • Clutch Plate Thickness Limit = 2.6mm  (New ones range from 3-4mm)
  • Clutch Spring Free Length Limit = 36.2mm  (Normal range is 3.8 to 4.6mm)
  • See any deep grooves on the Clutch Basket?
    Time to replace it entirely!  – This applies to some bikes after 13-20K miles of their life regardless.
    or if the damage is very minimal, gently file it until it’s smooth.
  • Silver Clutch Metal Discs (separators) – 1mm distortion limit.
  • Loud Rattling Sounds on the clutch cover?
    (your clutch plates *may* have cracked in pieces)
    (Must replace them ASAP as these things can block oil passages inside or worse!)
  • Clutch basket center nut must remail tight with its washer intact.

🚦 Let’s begin!!!
Park your bike , get your tools out.  Start with the drain pan…. Drain Oil 1st…



  1. Get either “15mm / 17mm / 19mm” Socket , go under the bike and drain the oil.
    A. Clean the Oil Drain Plug
    B. Once the oil is 100% drained.
    C. Fit the plug back inside.
    D. Tighten to a maximum of “15 Foot Pounds” (or 20 N.M) (Torque Wrench Value)
    If you don’t have a “Torque” wrench. Then just tighten your nut UNTIL you yourself know when it feels tight enough!



  • Ready?  Get the NEW clutch plates & discs out…


In the blue circles^, remove the engine cover bolts.
Remember where each bolt goes!Some of them vary in size.
🔥 Be warned, left over oil will spill out!



When you reach the oil filter – Be warned, that the rod goes all the way in to the engine.
So the quickest way to remove the rod safely is to:
1. Put 2 nuts on top of each other.
2. Undo the 2nd nut….    (the TOP #1 nut is a “stopper”)
3. Undo the 2nd nut until the rod comes out entirely.
🔥 NOTE – in some cases , a pair of mole grips can undo the stud with some force – but it may scratch the bar!

Don’t Lose the O-Rings for the filter cover! 
(there is 1 for the cap & 1 INSIDE the oil filter hole. = 2x rings.) (Ps. Its not blood^ but red oil! 😉 )
⚠It is always recommended to replace o-rings & filter “whenever” there is an oil change job or clutch changes.




Once all the engine clutch cover bolts are removed….
Use a stubby tool like a small screw driver…

1. Go under the bike and tap slightly the “marker”  in the blue circle (^LEFT PICTURE)
2. Once the cover splits –  Go to the TOP of the clutch cover.
3. Near the oil cooler pipes, put the screwdriver or a blunt knife to twitch the cover (towards you)
4. Now, we have some movement…
5. Remove the clutch cover!

🔥 NOTE! = The “gasket” must be removed and replaced with a new one
(Sometimes old gaskets may tear off!)



Now your clutch system is exposed.
Get size 8 O-Spanner & take the bolts out…

From top down….
Undo => Top Bolt 25% out  => Bottom Left 25% => Bottom Right 25%
We now have 3 bolts undone slightly => now undo the rest 100% (take em out)
Then remove the last 3 bolts – This method protects the clutch springs.


  • Remove: front clutch hub facia plate cover and investigate the parts below (just inspect them)
Parts => #10 , #16 , #17 , #13 on this diagram above^ (Inspect those numbered parts)


Take out your plates one-by-one … this is very important…
Remember the ORDER of each plate
‼Hint: The last plate is always “Fibre”  & the FIRST plate is always “Fibre”
‼Hint:  STEEL DISCS (Seperators) go in between.
⚠ Use the SAME ORDER to fit your new plates!  Or use the diagram below…


Then fit back the other parts in the diagram above^
These numbers are the parts in diagram above^ #13 => 17 => 16 => 9 => 10
This is the ORDER of fitting them….


It’s time to tighten it!
Remember start from the TOP 25% screwed in => then bottom left & right 25% each respectively => Then maximum (100%) on the remaining 2 => The first x3 bolts , 100% also.

This method is to apply equal pressure everywhere to protect the clutch springs.   Hyosung books also mention the criss-cross star style of fastening bolts.


A torque wrench really helps!
or else Tighten with 2 fingers (and that’s tight enough)

Last not, least – Close the clutch cover. Top up your bike with oil.  
Remember these points below…

  • MAX Torque for “clutch engine cover boltd” – 5.5 ft.lbs
    (It’s made of aluminium)
  • Use fresh Oil, O-Rings , Filter & Gasket.
  • Record this event in a book somewhere or back of owners manual – This creates a log of your work & record mileage

👍 Recommendation = Make sure your oil is marked 10w40/10w50 JASO MA2 ESTER – Nothing upsets these v twins than running junk overhyped  (yes, over-hyped) castrol oil , (Many small parts in a hot small v twin!)- That tells you all you need to know – Good oil is a MUST.  Motul & Silkolene are best.


🏍 Ride Safe!  Any questions hit the comments below!