♠️ M77
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Use e5 forever. E10 will just ruin everything later even piston rings, as it looses speed as time goes on. = Valve Clearance time & Compression test time. Can’t be avoided. I’d do this first before worrying about the carb, you want the engine proper sound first. get hard figures , don’t trust anyone who doesn’t write it down.
Change the oil. (Open your “oil filler cap” near clutch cover, if you smell fuel = change the oil to Motul 7100 , 300v , 5100 or Silkolene Comp 4 , Hyos love these oils)
Castrol ? = Run away from it.EGR Blanks are sold on this shop for the engine side.
Airbox side = block the hole going to the EGR.
EGR is useless these days for carb bikes.
Have you measured the resistance of the coils (without HT caps attached) both primary and seconday and tell us the figures.
If i think from your numbers the coils are okay, then it would be probably HT caps (if they are stock hyo ones, just replace them with NGK) , the 125s do appreciate the NGK products in this season despite weather , the ngk caps & laser plugs do help the bike start faster and spark even the slightest whiff of fuel coming from the carb
The carb now should be 7mm, i’ve not experienced a hyosung run well on 8 to 9 neither 6mm , it has to be dead on 7mm
If it is 7mm, then check the “float valve” needles actually spring back (has a spring inside) and the tips are super clean & not bent, they control how much fuel comes in the carb in tandem with the floats, some jets can’t reach all that deep in the bowl, so a certain level of fuel should always be present and henceforward the upside down nature of the carbs too (Korea did it this way as its quicker to send fuel to engine) (Downdraft / CV=Constant Velocity , speaking of which , the airbox helps)
You will have to make sure when it runs, there is no air leaks on the manifolds because “O-Rings” sit under them, if they are old or squeezed thin, just replace them. You can test this by spraying all over manifolds to see if revs change.
Hey,
When you twist the throttle and it dies, it means either:
- The “Throttle Slides” are not lifting up to release main jet fuel to combustion chamber as the bike revs up
(Sticky or torn slides even so much as a small rubber rip?) - Spray “Carb Cleaner” on the intake rubber pipes connecting the carbs to the cylinder heads = if revs change, there is a leak there.
- Airbox issue ~? (either the top one or the right side one)
If you have size “20 Pilot Jets” I have sent you , that should be inside the carb already (I assume cleaned from any packaging dust?)
How fresh is the fuel ? (Assuming it is E5 grade or better)
Next time it runs, twist the throttle very very very slowly and see if revs rise up?
Editing my quote below from previous reply as I believe we’re getting somewhere now.
- 8K miles. Good , it is young. Check your V5C logbook to make sure engine number is the same ?
It will need valve clearances doing as it’s done every 4k miles.
Tight valves = hard starts, bogging bike.
Hard to escape it. Only way to fix this is to sort clearances if possible.
Every 125cc motorcycle is like this, 650cc can get away with longer miles between valve checks. - If you are sure the carbs are clean, then ofcourse, it will be shown by how clean the pilot jets are by light shinning through pilots and the slides will lift easily with little drag.
- Big rip on airbox ?
- Any chance the “ORINGS” under the inlets have been replaced by now ? (CLICK HERE) Bad orings and bad inlets = directly affect how the bike runs and how it starts aswell, its loosing / leaking air. (Air VS Fuel (your carbs + fuel tap + pump) VS Sparks (Your coils, regulator, HT Caps, Plugs)
- How handy are you with a multi meter to test electronics on your bike (Easy stuff to do but it helps you know if the issue is fueling or air or electronics that’s holding her back)
What kind of speeds do you get ? How does it finally start (tell us what you did to get it to start eventually?) Do you use E10 ? (I reallllllllllllly hope you only use E5) Let us know!
Welcome to the site and happy holidays!
Can we ask ? (to get some understand of how it came to be)
- Total Miles on it
- When was the last time the carb had an ultrasonic clean?
- Any chance the inlet rubbers under the manifolds have cracks (Spray easy start on them to see if revs change = leaks)
- Any chance the “ORINGS” under the inlets have been replaced by now ? (CLICK HERE)
Bad orings and bad inlets = directly affect how the bike runs and how it starts aswell, its loosing / leaking air.
(Air VS Fuel (your carbs + fuel tap + pump) VS Sparks (Your coils, regulator, HT Caps, Plugs) - How handy are you with a multi meter to test electronics on your bike (Easy stuff to do but it helps you know if the issue is fueling or air or electronics that’s holding her back)
What kind of speeds do you get ? How does it finally start (tell us what you did to get it to start eventually?)
Do you use E10 ? (I reallllllllllllly hope you only use E5)
Let us know!
take a look here;
Intake Pipe O-RING (Inlet Boot Seals) x2 – GV125 GT125 RX125 GV250 GT250 Hyosung
and also
Uprated & Braided Fuel Line + Filter Kit :: Hyosung GV125 & GV250 {Carby Models}
Order anytime now ill dispatch after weds when posties open up
have a good weekend
As per simon 🙂
You likely have an air leak where the o-rings are sitting inside the manifolds , frustrating i know but i would use new o-rings everytime the carb is off the bike they are only a few quid but save a lot of stress later on.Air leaks will make the bike feel like is going up and down between revs when its low and conks out or if revs are high, it gets higher when you ride then it does drop very very slowly when in neutral.
Also check vacuum hoses , if you have EGR on the bike. Delete it off the bike and only block the lines that were feeding the EGR , less stress there If the bike doesn’t have it that’s fine, you’re in luck. Less pipes to deal with.
Though, i pray the “Pilot” jets didn’t suddenly get clogged again from debris coming from the pump/tap/ or old air filter or just generally the airboxes (2 of them) not fully secure.
Keep us posted , im sure Simon will likely beat me to it for further replies, you will be alright 🙂
Have a good weekend break!
Hey,
well done on putting her back together! –
I hope this is true below:- Both jets were clean (pilots)
- Starting jet is clean (the tall one inside the carb that can’t be removed)
- Main Jets are clean
- Every part in the carb was meticulously put together.
- Fuel pipes are connected and fuel tank tap is working
When you crank the bike, you will see the fuel drip inside the clear fuel filter as a hint the engine is now pulling fuel out of the tank
This also means the fuel pump is working and connected well.
Now, this should be a very easy process to start it (I hope!)
- Remove the “Air filter” on the RIGHT SIDE of the bike (That small baby air box that hangs on the right side of the bike)
Open it. - Get this product in your local car shop or bike shop, (or something similar to it)
- One you get it…
- Put your bike in neutral (make sure of it by checking the rear wheel spins free, as rarely but sometimes fake neutrals happen)
- Put your carb in FULL choke mode
Since you have 2 hands ofcourse,
- 1 hand twists the throttle & thumb presses start (twist throttle on and off as fast as you can)
- Hand #2 will be spraying “crazy” amounts of easy start in to the air abox hole (since you removed the filter)
Trust me, if your bike is sparking good, it will INSTANTLY start and scream to 4K revs.
- Now leave it on choke for 5mins screaming.
- Then slowly turn the choke off and watch the “IDLE SCREW” adjuster on the left side of the bike to make sure bike idles at 1500 rpm
The fuel should be E5 only please. No E10 fuel. (if i haven’t mentioned it elsewhere on the forum)
Once the bike is okay , get this bottle below (DO NOT BUY ANYTHING ELSE) … It must be precisely this one as i swear by it,
- Now, if you have HALF a tank of fuel. Do not add more fuel.
- Pour the WHOLE bottle of the Formula Gold above
Go for a short ride but dont be far from home yet, just go around the neighbourhood. See how it feels before venturing out far.
Now the reason for 1/2 tank + full bottle of formula gold, is more like an agressive mixture thats going to make its way down to the ;
- Tap (collect debris)
- Fuel filter (dump debris there)
- Travel to fuel pump , collect debris in the lines..
- to the carb
- Full Throttle will have the main jets & throttle slides fully open to dump any micro debris out of the exhaust while riding the bike.
If all is well, the fuel will go to low level after many miles of riding and the bike should be fully awake at this point 🙂
Oh yes before i forget, put the air filter back nicely once the bike starts from easy start.
Also use Formula gold once every 3-6 months to keep the carbs optimal and fuel system generally clean.
Fresh fuel lines and fresh filter helps a lot.
hope this digest of information helps 🙂
Have a good winter break!
Hey , welcome!
I would say try these first;
- Check chain slack (sometimes it affects gearing while moving) slightly or more.
- Check spark plugs (for colour and show us , and also check it says CR8E only)
- Perharps the clutch plates will need changing to better ones ?
Electrical Test;
- Take a picture of the regulator under the bike and show us even also its plugs , ill see if i can do some kind of regulator upgrade for it if its producing less power for electrics
- Test the regulator when the bike is at 5000 revs , see how much voltage the battery is making
Electrical Test 2 (advanced);
- Probe the 2 pins inside each coil and tell us ohms under 200 ohms.
- Probe 1 pin and the HT wire together and tell us ohms under 40,000 ohms.
- Unscrew HT cap gently anti clockwise and test “ONLY” the HT CAP (these cause spark issues and power problems because it is made in china) = Tell us ohms under 30,000 ohms.
After that connect HT cap to coil properly by screwing it back in to the coil before you start the bike.
- Sometimes it could be a sign of a failing coil (Happens to all Hyosungs, fortunately it is not the end of the world 🙂 …)
Other;
- Check clutch cable ,maybe its pulling the clutch a little bit ? so check cable is fully adjusted well and is not trying to engage the clutch while bike is in neutral.
I hope this helps and let us know your progress. You’ve got this! 🤞
Big mission isnt it ?
Sadly though;
- Crank won’t handle it whatsoever as its already at max capacity trying to spin 2 pistons as it is
- Carb of the 125 probably will handle about 7psi pressure of fuel or max.
- Need injectors and go all efi
- No turbo exists that we know of in UK that can spool over a single 62cc piston
- Too much drama and work for the accomplishment.
- More power , more heat , more cooling needed. the little 5-row oil cooler won’t cut it sadly pal.
Though i have to ask , why the need for 2 turbos on a 125cc? (That i would like to see and i shall hang my head in shame , i’ve thought about this for 10yrs , i just cant find a feasable way to do it without things going wrong, can’t get past the limitations of a 125cc-250cc)
Hence most turbo’d up custom hyosungs are on the GV65o bikes instead, abroad. Rare sight in UK (and if they did, they wouldn’t really brag about it lol)
(Electric taiwan turbos do not work, they rob more energy than it produces sadly)
Thank you for your advice. I will check alignment according to your method over the next few days. One point… you say slack has to be 15mm but the Hyosung manual recommends 30mm!!!
15mm when the bike is in the air (resting on paddock stands), it means the suspension has compressed, so the chain slack reduces.
When the bike has both wheels on the ground and on its side-kick stand , it should be 30mm like most motorcycles 🙂
I will say always check the slack again once its on the ground after tightening things up & dropping the bike down gently away from its paddock stands.
if i made any sense 😀
The GV sender tends to be a little optimistic sometimes , for example on my GTR it will say full then maybe 30 miles later it drops a bar.
If i am to assume i am riding super economically to get 200+ mile range MPG out of a full 17L tank. I expect 3 bars to drop from full if i use around 5L of it. I am not sure how many miles that is as its been a while since i paid attention to the odo counter lol.
The GV in this case will probably say full until its almost 3/4 left of it, then it should start to move down as the float lowers.
You can always test the fuel sensor like this:
- Full Tank = Ride 10 miles , and go home. Unhook the sensor cable. Test with your meter many OHMS you get from the SENSOR itself.
Report back here. - If you suspect the tank is half-full left
Test ohms of the sensor
Report back here.
Ie. Full was 400 ohms , almost low fuel left was 200 ohms etc… (Example here) these are not actual Hyosung figures. You will get that from your meter,
I will know if your sensor is working or your gauge is fishy or just a harness issue.
Have date with that pocket meter 🙂
Hi I’m in the UK and it’s the round fuel sender gasket I need.not the tap gasket Hyosung gt 125 comet 09. Thanks billy
i am working on this , my plan over the xmas period to mid jan is to stock a fuel level sender on the shop.
I will test it and make sure it works on my donor bikes first.Please keep an eye at the shop around Mid-Jan , if you think there is no update , just reply here and i will respond accordingly.
I am on the hunt to find senders from other bikes to fit a Hyosung GT Carby , wish me luck 😉
JT XRING isn’t too bad , so you should be okay , if in doubt you can always try any RK XRing / EK XRing as my personal preference , there is also other japanese brands like Tsubaki, Kobura. Whatever you do, avoid the likes of TVR, Choho, and them unfamilar chinese brands , they always snap!
For wheel alignment, go behind your rear tyre , kneel down … and so do it like this
- Bike needs to be on paddock stands.
- Make sure your rear axle is a little loose so the wheel moves.
- Loosen chain tensioners a little.
- Pull the rear wheel back as far as you can safely.
- Look inside the “INNER walls” OF swingarm (facing the tyre sidewalls)
- You will see each side a “drain hole” for the swingarm towards the front of the bike.
- You want your tyre to be sort of inbetween them
- ie….
[swingarm leg left] [drain hole] [tyre here] [drain hole] [swingarm leg right]
sometimes a straight long strong ruler helps check the chain is straight also.
- Tension of the chain while it is on paddock stands should be about 15mm when you push the bottom chain up^ (rough guess)
- Check wheel again and chain are straight, then tighten the rear wheel axle to max of 60 ft lbs torque (make sure to use a torque wrench)
If you brake hard the brake disc and caliper apply force on the right side of wheel, that might bring the wheel forward
if you take off hard , the chain wants to pull the wheel on the left side , and vise versa. - I think you get the idea of where i am going with all this
Did my reply help so far ?
What do you use to hold the O rings in place whilst trying to ease the intake pipes back on?
Hey, this is how Korea does things and i am very convinced they put the engine and carb together in to the chassis during production but im sure they have other ways too, until i find more tricks , i will expose it!
Anyways, the manifold orings …..
- Clean the intake pipe grooves well
- Get something similar to PUTOLINE RACE GREASE
- Dress the orings in that grease (no silicone / no instant sealants / no RTV markers)
- Put the oring inside the manifold circle groove
- Find a glass table or table that is very straight.
- Place manifold (oring facing down) on table
- Hold the manifold and press down hard (but be gentle?) so that the o-ring firmly sits inside the groove.
- Hold the pressure for maybe 30 seconds.
Next
Lift the manifold, the oring should now be firmly inside , it should not fall out anymore.
Again, get a little bit of grease, apply a very small coat on the engine cylinder heads where the manifold is going to sit. Just make sure the area is clean.
Take extreme care not to let dust, or debris inside the valves as they are directly exposed in the intake holes of the engine.
Get a very short allen key, and m6 socket cap bolt ready, as soon as you fit a manifold nicely between the carb and cylinder heads = screw in 1x bolt all the way down , while your other hand is holding the carb+airbox combination. It is tricky but you will practise this and it will be easier next time you service the bike.
in my experience (no matter how good i get at this) it will always take me 1hr to put everything back to gether nicely before i start it!
first time i tried years ago , it took me 4hrs and a lot of swearing and an almost damaged intake pipe.
You will be forgiven if it takes you time, it’s okay to be patient 🙂 = After all the real enemy is the frame spar in the middle of the chassis but i guess it was the best way they can design the chassis and cram in everything it needs to function well .
Since it is friday..:
😂🤣 I had to giggle lol
Hey , i got your email , please check it again since i responded to it 🙂
I have got your PM , and i replied to it , so check it when you can 🙂
Welcome to the site!
The Idle adjustment is on the carb itself or it will have an extended cable dangling between the coil cover and left engine crank cover.
if the revs are slow to climb down , it is commonly these (all or one of them, guaranteed)
- Carb is clogged, needs air compressor or ultrasonic cleaning
(It has many holes inside) - Jets inside the carb are also clogged
- Bad E10 fuel ? (If you have E10 fuel, drain it and use E5 forever.)
(E10 will kill the 125cc one way or another , modern cars made for E10 are still better on E5) - Inlet Manifolds have splits or leaking air thru a big crack?
(Spray carb cleaner there and if revs change, i was correct.) - AirBOX is not sitting properly
(air leaks and the carbs are CV, they need a fully sealed up intake system to function right)
____ - Inlet Manifold “O-RINGS” have caked up, allowing air in (click here for visual look)
Click here again, for a quick tutorial on how to carefully remove the airbox out of the carb
___ - Clean your spark plugs
If they are already acting funny or you ain’t sure they are not breaking down
Go halfords and get fresh CR8E
Even if the old plugs work, put them on shelf as backup.
Then start using new ones from here.
The carb mixture is good when it says “all brown” on the plug
(Post picture of plugs sideways and center and top down , so we see how rich they are)Jetting Tutorial? click here
Do each bullet point above first, before you try other parts of the bike , you want to get the above ^ out of the way first.
Then report back as you progress each stage. 🙂
Good luck!
I have another question: If someone has a disassembled 125cc engine, please let me know the length of a con-rod, center-to-center (hole) measurement in cm/mm preferred.
I have stripped engines to sell parts on my ebay shop , but i have not measured parts sizes inside because no one in UK asks me that questions , so its a long forgotten memory!
I will keep you in mind if i ever come across a wrecked engine again 🙂
- The “Throttle Slides” are not lifting up to release main jet fuel to combustion chamber as the bike revs up
