Need help with hyosung GT125 P
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Hi all
I jave a 2017 hyosung gt125 p, been stood for over 3 years, no mot only 6k on the clock, the bike didn’t start at first so I got a new battery new NGK Iridium spark plugs ans did a little service oil filter fuel filter oil ( flushed the old oil out with 5L of new oil, 10mm fuel pipe from vac pump to carb and bigger fuel filter an fuel lines to 8mm, even after all this the bike didn’t start at first used abit of easy start and boom its started BUT its idling high at 3k rpm and if I don’t keep the revs up it just dies, I e taken it all apart and given the carb a good clean and put it back together but it still doesn’t not start without the easy start, any advice and help would be much appreciated.
Many thanks
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Welcome to the forum!
Sorry to hear about that but we have faith as the engine is indeed alive 🙂
If you have an Amazon account, get USPRO compression tester or get a mobile mechanic to come over with his compression tester (don’t matter if its for a car, it does the same thing)
Make sure the bike is 10mins hot, then take the spark plugs out , both of them.
Put compression tester in one of the cylinders , eg. Cyl 1 is front engine spark plug hole. Put it there and hard.
Use a “METAL” rod with a curvy rubber tip , dont use a flexi hose because they are known to give false positives sometimes.
Twist throttle to full throttle, then hold it there. Crank the bike for 5 seconds ish max.
Get readings and report back .
We tend to always check engine compression of bikes before address other parts such as carb, coils, electrics and all that comes with it.
The engine is the main key to check to see if its actually healthy
Also, it helps to check valve clearances (if you can or mobile mech?) = Tight or loose clearances make it hard to run right no matter how tuned up the bike is.
It helps to keep fuel lines around 7-9mm max. 10mm is a little overkill to be fair and vacuum lines should be around 5-7 max.
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Sometimes , with the 125cc bikes, half its starting issues or idling issues are a result of the inlet manifolds , it is of the utmost importance to take them off (becareful, rusty bolts!) and visually inspect them.
Under the inlet pipes are rubber orings that are terrible because they don’t last long and usually is recommended to replace them eventually.
You will be surprised how much help these little things can do to a bike that’s got a vacuum loss due to caked up old rings or it’s struggling to start.Intake Pipe O-RING (Inlet Boot Seals) x2 – GV125 GT125 RX125 GV250 GT250 Hyosung
Please becareful with intake pipes, the 2014-2019 bikes have intake pipes that are much much soft and prone to splits than the old school bricks of the 2003 bikes (trusted inlets, they are thicker and rubber is better quality). General consensus is that new shape (2014-2019) bikes were a little rushed than the old school 2003-2010 era bikes with grey engines.
It may seems like a little more maintenance is needed on factory new shape bikes but overall they are still wonderful when everything runs right 🙂
I hope i have made some sense but report back as you go along , i will keep an eye on this topic , you got this 🙂
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Thank you so much for the insight ! Honestly didn’t even think about replacing the O rings I’ll get on that ! On my next day off. I’ll check on how to check for valve clearance and report back with my findings, in regards to the rubber intake manifolds I’ve noticed that theu are pretty worn even tho the bike is only a 2017 woth very few times they look trashed I will try to upload some pictures once I figure out how and a video, once again many thanks
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Since you are in Yorkshire, you aint too far from us if push comes to shove and i’ll sort the motor out but for now lets see what your own fine hands can do, i trust that you will get a breakthrough!
As for picture uploads , I am currently fixing this as i did something silly under the hood when updating site code (developing/custom programming isn’t always fun but i’ll get to that very asap this weekend , that’s on top of fixing bikes everyday lol)
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Hi sorry for the late update so managed to change the O rings but the rubber/plastic manifolds are abit worn out so might look to change them, had to change the vacuum pump just stopped working and found that 1 of the slifers was stuck so had to get that sorted, so the bike starts with easy start abit of trouble and it’s idling high and then cuts out now I have got a 6mm fuel filter which I’m looking to change to 8mm and I thi k the battery is not getting charged so the magneto or voltage regulator is up next as the battery is new and I’ve changed it up for 3 days
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I am happy you made some progress with it my fellow yorkshire man.
Do you have carb cleaner still left ? – Look at the tutorial on this forum for “Carb jet” , just follow the instructions on how best to fit the carb and airbox together etc . Then put the air filter back in.
Set the idle screw high (to about 2k revs) , then remove choke and accelerator cables off the carb while it runs.
Now reduce the idle screw to 1500 rpm.
Does the RPM needle bounce up and down (like its starving for fuel)
or
using your hand , twist the throttle at the side of the carb , and see if the revs go down fast on the dash, if the revs a very slow to climb down to 1500 rpm and settle in.
You may be looking at sooty plugs or plugs breaking down (inconsistent spark power)
or
Take your carb cleaner out as i said earlier in this reply , spray the manifolds, spray under the manifold gaps where orings sit , spray under the airbox where carb swallows the box
Do 1 step at a time and make sure to really listen for any engine sound changes , usually if you spray somewhere the revs go up or bike shuts off = indicating that particular spot you sprayed is leaking air (& subsequent vaccum loss to draw fuel out of the carb)
Since its a CV carb = it implies constant velocity (Constant Velocity) , it is not able to tolerate even a minor air leak or an airbox missing, it just runs rough.
The inlet manifolds will still last long as you will make sure they do not split up , and keep using E5 fuel forever. E10 will rot it.
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Slow progress unfortunately haha, right so yes it does exactly as you stated the revs do bounce they go up to 3k then down to about 1.5 then up again and down, then cuts out or when I rev it, slow response revs go high and down very very slowly, I have changed the spark plugs 3 times they keep getting scooty extremely scooty, will try with the carb cleaner and let you know how ot goes, once again thank you
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The vacuum-piston seems stuck open on one carb….? Dirt? Broken spring?
Old and grumpy,(?), have fairly wide engine & tuning experience, "restoring/resto-mod" on cars & mc, worked as electronic tech. (retired).
GT125R - sold. New (used): 2012-GT250R-efi (sold)+ 2017-GV-125C (sold) - got my old 2018-GT-125R back... 🙂
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Hi Yes it was 1 of the issues, only realised once I had tekn the air box off, I then open the carb up fully and sent it for a ultra sonic wash and put it back together, since then the sliders/ plunger has been okay, still looking for air leaks but don’t understand why my spark plugs are scooty?!?
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You should check float-valve / float-level. easy with some transparent hose. Should be similar to this… just slightly open the drain-screw, till it stabilize, then compare carbs, front/rear.
Old and grumpy,(?), have fairly wide engine & tuning experience, "restoring/resto-mod" on cars & mc, worked as electronic tech. (retired).
GT125R - sold. New (used): 2012-GT250R-efi (sold)+ 2017-GV-125C (sold) - got my old 2018-GT-125R back... 🙂
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hi Marcel sorry for the late update just been busy with work no time to even fix up the bike managed to change the O rings and sort out the carb and chage the 
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