help with a hyosung 125

damz · World Chat Champion Joined: 15 Jul 2007 Karma :

my friend called me round to his to help sort his bike out and needing some help. He told me that when he was riding the bike it lost lots of power and at full throttle he was only doing 30mph and wouldnt rev above 7k rpm. he eventually got home and called me up.

went round to check it over and it wouldnt even start, at first i thought it may have been a fuse but they were fine. Next i checked the spark plugs and this is where it gets interesting. If i removed the spark plug at the front of the bike it would start but run pretty rough as it was only on one cylinder. If i put the plug back in it wouldnt start. i removed the plug at the rear while leaving the one at the front in the engine and it wouldnt start. I drained the carb that ran into the front cylinder but it didnt seem to help.

my current theory is that the front cylinder is fucked. any idea what it could be?
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Current Bikes RVF400 - 94
Cars: RX7 FD, Eunos Roadster
Former Bikes: FZR 250R 89, ZX7R 98

Dr. Quack · Could Be A Chat Bot Joined: 17 Mar 2011 Karma :

My neighbour had a Chinese bike, his brake lights only worked when his head light was turned on, then the frame snapped on him.
Could be alternator related, (Just wondering if they have points and not pick ups too which are a nightmare)

Anyway it could also be a dodgy earth because these bikes rust away within no time.

Rogerborg · nimbA Joined: 26 Oct 2010 Karma :

Hyosung is Korean, using technology cribbed from Suzuki. Rolling Eyes

The plug could be shorting. Swap the plugs over and see if the behaviour changes with them.

Are you getting compression on the front cylinder? Just stick your thumb over the plug hole to feel for it.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

P.addy · *Formerly known as* P. Joined: 14 Feb 2008 Karma :

Check front cylinder compression, you are fairly local I think?? I have a tester sitting in the garage, unsure if its the correct size though Neutral

Problem being the plugs are buried in the actual head...cant really get finger in there.

Take the tank off, the rear plug is under there.

Try swapping the whole spark plug/ht lead/coil pack(?)

Livefast123 · Nearly there... Joined: 14 Mar 2010 Karma :

damz · World Chat Champion Joined: 15 Jul 2007 Karma :

hi, we bought a new plug at halfords for the front cylinder and it didnt make a difference. You can feel air being pushed out of the cylinder. When you have the spark plug in the cable and touch metal with it a spark happens so it is sparking but it just doesnt like the front cylinder.

paddy im now in glasgow, havent updated my location yet.
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Current Bikes RVF400 - 94
Cars: RX7 FD, Eunos Roadster
Former Bikes: FZR 250R 89, ZX7R 98

Livefast123 · Nearly there... Joined: 14 Mar 2010 Karma :

Try swapping the rear coil for the front one, it may be that the coil has enough output to spark under no load but once in the cylinder under pressure it just dies. At least you can eiliminate the plugs and the coil like this.

If this doesn't work then a compression test is a must (although being cold is not ideal) just to see what kind of compression you have. Obviously if very low or none then you have an issue.....from memory I think Hyosung state it should be between 142 - 168 Psi.

Other issues could be the cam chain tensioner, did your mate say it was noisy before loosing power? What year is it and whats the mileage on it?
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Current ride - Yamaha MT-07

adamck · Crazy Courier Joined: 15 Feb 2005 Karma :

Its abit of a pain in the arse but... try removing the main jets and cleaning them out, a quick blast with an air flow is best, but blowing will do or maybe brake cleaner.

pics from the last time i did this... (with a Hyosung GT125R)
https://i52.photobucket.com/albums/g12/adamck/Picture001.jpg

https://i52.photobucket.com/albums/g12/adamck/IMG_0278.jpg

Ive had issues with crap in the jets before and it was stopping the fuel to the front cylinder.

The only other similar issue ive had in the past was due to an aftermarket petrol cap, the tank cap was sealed without any valve to let the tank breathe (well there was a valve but it was under the seal making it useless...), this stopped the fuel getting to the carb and the front cylinder was starved.
When i opened the petrol cap, it fired up and ran, with it closed it ran until starved again!

Other than that i would check all the pipes are connected and not split etc...
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P.addy · *Formerly known as* P. Joined: 14 Feb 2008 Karma :

Fairies attack · Banned Joined: 21 May 2011 Karma :

As mentioned it could be the petrol cap.
Run the engine up and wait until it starts to bog, turn it off and open the cap, if it makes a sucking noise then the tank isn't venting properly.

They do have a small problem with the alternator on the old bikes, which can easily be fixed by reading here:
https://korider.com/index.php?topic=14684.0

Also there was a website where you could put the vin number in and get it checked by Hyo (pre 05 bikes) as they were susceptible to valve failure from the guides hardening and then the valves and causing a lovely mess of the engine.