โ ๏ธ M77
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If the jets are 90 and 92.5 with slide needle on rich setting, with K&N air filter (K&N 6507) andย Lextek OP1 exhaust muffler and Black Widow Exhaust headers
The 125 tends to pull the best (acceleration wise)ย ย , this is also assume the mixture screws of carb did not get touched.The Pilots can stay size 15 forever.ย ย ย ย When you remove DB Killer, bike runs leaner and screams louder but has a slower pull through gear , the only real gain is the top speed (maybe 1-5 mph extra at best)
The OEM Hyosung stock can (muffler) is very heavy and it is quite taxing for the 125ccย (not really a problem for 25occ)ย but lightness is important to the 125cc indeed
If i use 92.5 and 95 on a customer bike, that would be maximum tube because of using NGK LASER plugs, they are a nice gold brown colour .ย Having a 250 cooler helps a lot too.ย You may be surprised how heavily used these 125s are in the UKย ย (for work ride, for pleasure, and general riding to citiesย this is everyday)
Also i have advised some owners i work with in person to stop using E10 fuel, it is rubbish.ย Nothing positive about extra ethanol mix.ย ย So we use e5 and the bikes like it.ย ย ย (E0 would be ideal lol)
DB Killer = Remove it, and hammer the tube inside of DB killer so that “hole is smaller”ย ย (about dimensions of 10 Pence UK coin)
This means less gas leave bike
Better lower end torque (very tiny boost) but DYNO will notice.Also Jetsย 90 and 92.5 are most optimal .ย ย ย 95 and 97.5 is a lot!
Then try DYNO again ๐
Hi, Just joined and have been reading through this topic since I seem to be facing a similar issue, for the second time. I own a GT250 EFI 2013 and a few years ago I started facing stalling and misfiring issues roughly after about 15 minutes that the bike is on. I spent may hours trying to diagnose the problem including the replacement of ht cables and plugs, ECU change, and injector cleaning. In the end the problem were the ignition coils. I started with replacing one which reduced the missing significantly but since I was still getting some misfiring I replaced the second one as well. There were no external signs that any of these were heating up and getting intermittent. For the past 2 years I have had no further issues but lately it seems the problem is starting to rear its head again though not as significantly. I am assuming it is the coils again but I am somewhat disappointed if the coils fail again after such a short period. I donโt make heavy use of this bike since I prefer my scooter for city riding.
Maybe you could make a new topic about this ? and perharps include more information about the history of the bike etc and what you did in the past to fix anything etc , and any modifications done to it etc… Do this all in a new topic and trust me if i don’t reply , someone else will and also simon on this forum is a good 250 wizard himself ,ย even helps to also upload any video about itย (you can upload to your instagram or youtube and set video to public, then share link here, if it helps)
Welcome to the forum and hope maybe one of us will help you fix the bike
While you wait,ย take a look at the service manual that’s on this forum and try to trick your ECU to get any stored error codes.
He also means the “black tubes” that actually connect to the spark plugs deep inside the engine but yes look for;
– Loose HT wires trying to escape the coil
– or Loose HT wire trying to escape the black tubeย (plug cap as it’s known as)
– Spark plug is tight in engine and is on the correct torqueย ย (max 15ft lbs on torque wench, engine is aluminium, so threads can still strip due to overtightness)Also grab a copy of fresh CR8E plugs (OEM) and use that forever, do not worry about getting “cold” type plugs etc, or adjusting the gaps of the spark plugs. There is other ways to help the bike stay a tiny bit cooler on blazing summer heat.
13.2BHP at wheel? If that’s the translation ?
What modifications were on the bike to achieve this ?ย This may be useful for future owners to keep an eye on this thread , it is interesting.ย I don’t know too many people in UK who have taken their bikes to DYNO and published results online (especially with everyone trying different mods on the bike)
25 km total, bike is brand new, but I am doubtful. Throttle cables have a bit of play, chain OK. Shop called it “engine break”, common with fuel injection, Hi Marcel, quarter inch. Will check what you mentioned. Need to know if faulty and/or the shop sold it faulty. Dangerous in traffic and jerky at low speeds, reg.this month, Steve., thanks
Just fixing your old reply as it was entered in the “tags” box instead of the main big white reply box , but i will get rid of it from future replies to make navigation easier.
Shop is trying to act funny by saving their own skin,ย they must comply with the warranty if they sold you the bike and it must work normally, I’ve had a brand new bike before and it never did such things and it wasn’t even a korean or jap bike.
Hello Sir
How old is the bike (total miles?)
When was its last service and specifically which oil was used on it ?
How is the brake calipers ? (2 bolts hold each caliper to the forks) (and rear holds to its carrier)
Brake cleaner inside the caliper can help and also dont forget to use some grease behind the pads to avoid pads getting stuck if they don’t slide back smoothlyHow is the chain slack ?
Sometimes a very slack chain can do this.Check the front sprocket nut isn’t loose
(Please take a pic and show us on this forum if possible)Do you also feel like there is some kind of misfiring going on ?
Or did the dealer fit wrong plugs ?
(causing them to breake down)Update us fella, i will keep an eye on this thread do not worry.
Welcome sir!
Glad you got the bike and managed to fix , i did notice the cylinder issue when i commented on the FB group (Official Club) , so that’s good.
Korea didn’t put an OEM gear indicator , but with it being 5 gears, you will definitely get used to it quickly ๐ , all my years of owning a Hyosung i never saw the need for a gear indicator dare i say!
Be sure to share pics of the bike whenever you get time , even on this link here
Happy summer riding sir , especially with the hot week this is!
I got you, give me 24hrs i will have info you need to answer your comments , and for sales on this shop, just send an email with any updated requests for parts, i can always use cheaper shipping.
If i know a source in USA, i will also link it too, if you are in a rush!
Keep an eye out for ST7 and the GV650 versions of the bikes too, to be fair , you will even come across a 125cc version than a 250cc, least with a 125cc you will have an easier time working on it, when your bike does show up again, sell it or trade it in for the GV250 of your desire
Same body = Different engines (125/250).
Then from what i have read, i say go for it and get it. If anything you will have fun restoring the Hyosung (though some days you may swear at it but efforts will pay off) , I could be biased to say get it so you can stay in this club haha.
Honestly though the GV’s most common faults tend to be the paint work on the swingarm rust and such, so cosmetic restoration is where you will spend most time unless the bike is looking very good!
Engine wise, if it sounds healthy like it’s purring like a kitten , you’re in good hands! – If it sounds like a helicopter, expect to rebuild the engine one way or another , but i hope it doesn’t come to that!
Either way, as you learn about it, the forum is here for assistance , so get it if you dare ๐ ,
The 125cc works a lot harder than let’s say a 650 Hyosung , and generally i’ve not had my hyosungs require me to do valve clearances for least 15,000 miles because
– I change the oil every 1,000 miles with strictly Motul Ester grade oil (7100 or 5100 if i can’t get 7100 quick enough)
– Change filter every oil change
– I start the bike and wait 1 minute , then slowly ride out of the property then let it idle for 10 mins.
– Ride in traffic easy then 5 mins i can use it normally.
– I keep my ear out for any strange valve chatter even the cam tensioner.
– Try not to redline it past 10k revs cause it doesn’t like 11-13k revs (red zones), it will rev and go there but you can hear the engine hates it.
– Peak power is made at 10k revs so it’s not really a concernWhat you can do to extend its life
– Change oil as per simon , as regular than what OEM manual suggests because the manual is optimistic when it comes to oil change intervals.
1500 miles seems to be a good limit in my opinion, though the manual could be in KM anyways
– Invest in a 250 oil cooler (get any from the GT250 or GV250, the 125cc will thank you a million.
– e5 fuel forever.
– I don’t know how you warm up your bike before riding to get her up to temp but maybe steal my way as inspiration/ideas if you will.0.25 seems to be the sweet mark for all exhaust valves on the 125cc , so in the future you want a shim that’s a lower number.
Eg. If my exhaust is 0.30 clearance , and i have 150 shim. I would use 140-145 shims. that should bring you back in to the 0.20s that hyosung wants for the exhaust clearances.
The rear engine runs hotter than rear but as Simon says you won’t have to worry about accelerated wear if you treat her well ๐
Yes you can but caution the mudguard doesn’t rub. Hyosung GT bikes (all of them) share the same rim. So smaller capacity bikes use 110/70 and the 650 uses 120/60 the mudguard of Hyosung GT bikes accomodate this nicely.
The cams on the rear face up but torwards each other.
But look at the edge of the cam shaft bars and make sure the dark black _________ line is on the same level as the surface of the cylinder head (where rubber gaskets sit)
Both cams must be equal ________ marked to match the surface of the engine gasket area.
Also if i remember correctly (it’s 10pm, so my brain is a little funny!) , the #2 on the sprocket is 100% facing straight up | ^ to match the cam cover holder bolt thread.
#3 on the sprocket for the intake is facing UP but it will be like this / than | straight up
Spin the crank 10x to be accurate and recheck feeler gauge for clearances.
Have you tried everything below ?
Use E5 only
Go to the battery and test these;- What volts is the battery at idle (DC)
- What volts is the battery at 5k revs (DC)
Switch the multi meter to 200 to 500v AC next, while the bike is still running, go and find the location of the stator on left side of the engine.
Follow its wire. Disconnect the yellow plug (3 pin yellow plug), now do the following and report back
- Probe the meter and put both โredโ and โ blackโ probes of the meter inside the 3 pin plug in this sequence.
- Pin 1 & Pin 2 = Volts at 5k revs?
- Pin 1 & Pin 3 = Volts at 5K revs?
- Pin3 and Pin 1 = Volts at 5K revs?
It is a three phase stator, so we check all three lines.There is no need to ground the frame or anything like that, weโre just seeing how much volt the stator gives in AC when disconnected (just the yellow plug!)
GV650, GT650, ST7 = Main major electric faults is usually the bad regulators korea ships with the bikes because Hyosung doesnโt make them, but asks another company in Korea to make stators and regulators.
Stators usually survive okay, but please check the wires are not burning hot or cooking up either, usually happens towards the regulator.
I do not mean to scare you off or anything as iโd want you to just go ride the bike but iโve had instances where iโve had to send both a new reg and new stator to folks in Scotland (itโs colder up there though???, i digress!)
Also check service history to see if anything about the injectors have been cleaned etc OR go to Halfords or EuroCarParts and only get this item below:
https://www.halfords.com/motoring/engine-oils-and-fluids/fuel-and-oil-additives/wynns-formula-gold-petrol-treatment-500ml-166754.htmlI do not recommend Redex, it isnโt great, despite its raves about it. Its effective on cars mainly.
If you have time and patience, you may strip the tank and (gently!) , remove the fuel pump and make sure its mesh filter isnโt overly clogged from years of debris or past keepers using junk supermarket fuel. We do not need to replace the pump if it is still working but less clogged mesh is good to improve its fuel flow.
A new pump is quite costly , we have to avoid replacing it. It will last as long as its taken care of
I hope i have made sense in this reply
I would 1000% avoid having pods on a hyosung. You will be in a long abyss trying to make it happy afterwards.
Sorry for the sour news , but really i have tried many setups , the carby motors run best with their airboxes on.
Why not just fit K&N 6507 Air Filter. Mind you, the Stock GT250 air filter is the same as the 650, and the carby 250 airbox has more than enough air capacity to run a 650 too.
What’s the restriction in air ? – The silencing cones that’s inside the air filter, that’s why K&N air filter is just an open hole without the noise suppressing one inside.
I don’t know about USA but here in UK, we don’t really like the sigma kits (wishy washy, and sometimes they make you make modifications to carbs we don’t like, like drilling things etc, it’s very silly).
Is there a local Mikuni dealer around you or any bike shop that deals with Mikuni carbs ? – Get 95 Main Jet Front and 97.5 rear = This is the most max setup. Going beyond 97.5 is wasted fuel really, the bike won’t see much dyno gains after that.
Take a look at these carb links for best set up for your bike
[Tutorial] How to adjust Carb Slide Needle (Tuning) – Hyosung GV & GT 125/250
and this one
[Tutorial] How to swap Carb Main Jets – Hyosung GV & GT 125/250
Also, this is hard to ignore ,but “you will notice this” – Get a FULL Size exhaust can (muffler) and make sure it has baffle.
Zero Baffle = Higher top speed (maybe 2-3mph extra) , same with the 125cc here in UK
Baffle included = More low end torqueModify baffle (make the baffle have smaller hole about the dimensions of a 10 Pence british coin) , bike has better acceleration overal.
Baffle = aka. DB Killer.
Why I said to use “Full Size” cans/mufflers, it is because “tiny” ones or “shorty” ones are pointless and extra loud with no real gains. (no backpressure for a heavy twin that needs a little thrust to set off from 0mph)
Full Size Muffler is similar to size of the stock Hyosung GT250 muffler
Hope this info helps, just my 2 cents ๐
Todays conspiracy: How-come even japanese-made feeler-gauges are made of recirculated British steel from early 70โs?
The Japs & the brits got a little cosy during war days and they both fought wars (to each other and to their enemies) expect them to know how to both work the steel/metals for whatever instruments they made. But ofcouse im going off-track on a bike forum , history is wishy washy so i dare not digress haha
Maybe a little quicker warm up and low speed torque. Nothing more.
Who doesn’t want that, every little helps!
Though back then i had a little fiddle with MS ECUs on other little bikes. But never dared to use a MegaSquirt ECU on an EFi 250 Hyosung (650 yes because very very few have gone turbo on their 650s so they do all sorts to the electrics!) But on a 250 factory bike, hmmm i would imagine i’d only use the MS ECU to custom tune the fuel setup if i used bigger injectors and modified the hell out of the intake (air filter) and exhaust pipes
Send me an email via the site contact form if you require me to try source the parts for you and thank you for the pictures.
You have the Sentec ECU (2008 only era ECU) , very impossible to replace, so please i’d advise making sure no electrics cook it!
