Looses power and cuts out at high speed
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Hi everyone. New member on here. Loads of good information. My story is I’ve bought my first hyosung gt125rc few weeks ago. Had few rides on it around town all good. Today when out from Derby to Skegness for a coffee all good but on the way back started loosing power in 5th then cuts out completely. Restarts only with full choke on. Struggles to pick up revs but after few seconds all good and rides as it should with full power. Then same again. Happened few times. Suspected fueling issue. Toped up full tank all good for another 15 miles then same loss of power at top speed then cuts out. Restarted OK and made it back home no issues. Any opinions? Thank you in advance 2016 carburated model. Rc project straight through exhaust. Upgraded chain etc. No idea if carbs upgraded etc.
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Hi everyone. New member on here. Loads of good information. My story is I’ve bought my first hyosung gt125rc few weeks ago. Had few rides on it around town all good. Today when out from Derby to Skegness for a coffee all good but on the way back started loosing power in 5th then cuts out completely. Restarts only with full choke on. Struggles to pick up revs but after few seconds all good and rides as it should with full power. Then same again. Happened few times. Suspected fueling issue. Toped up full tank all good for another 15 miles then same loss of power at top speed then cuts out. Restarted OK and made it back home no issues. Any opinions? Thank you in advance 2016 carburated model. Rc project straight through exhaust. Upgraded chain etc. No idea if carbs upgraded etc.
Ive read through almost every post on this forum and seen about changing the oil cooler off a 250 as the are bigger and should cool the engine a little better might be something to try but I’m sure marcel can diagnose it with a few questions for you
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How is the bike behaving now ? The UK temps have calmed down a little bit
And what work has been doing by you (or anyone else) to improve it from “standard” to “better than before” ?
I’d say the 250 cooler is a BIG BIG must have on these 125s, they will really thank you and strictly ESTER oil (That is Motul ESTER or Silkolene ESTER)
Only 10w40 or 10w50. I use 10w50 all year on all my bikes.
20w is a no. 15w is a no. those 2 figures are too thick, it could wear down the bike even more as it is the only 125 on our soil with crazy amount of bearings inside the motor & 2 angry pistons lol.
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Hi thank you for reply. Still the same. Haven’t cut out but was not doing full speed lately just city riding. Starts quite poorly when cold takes few times and needs some throttle with choke on full. Warm start not bad. No history on the bike so no idea what has been done to it tbh. Visually has only sc project straight through exhaust. That’s all I know about it. I have ordered upgraded fuel filter with lines from your side as well as new filter and iridium spark plugs. Will update the post when all that is fitted.
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If possible please, can you ride to any bike shop or car shop. Tell them exactly this (takes 10mins while you are stood there)
- Make sure engine is warm (if owner has arrived, you may test right away)
- Remove both front and rear spark plugs
- Get a compression tester Gauge and use a “STRAIGHT METAL PIPE” end (as flexi hose will be inaccurate, no buts)
- Hold the throttle on FULL OPEN position (FULL Throttle)
- Crank the bike for 5 seconds max.
- Record compression of the rear cylinder.
- Repeat for front cylinder head.
- Fit everything back & return customer bike
Come back to this forum and tell us how much compression bike has.
Keep in mind
this assumes the “Solenoid” is new and healthy or it will spin the starter motor slow, giving false readings
this assumes the “starter motor” is replaced , if it has issues of spinning slow all the time (it must crank fast)
this assumes the “battery is charged”
If in doubt, please ask bike shop or halfords for a free battery life test (not volts, but it’s cranking power) using their “YUASA LOAD GAUGE PROBE”
Keep us posted!
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Hi again I will test the compression. Your upgrade filter arrived and was fitted yesterday. Bike started fine. Filter filled up fit fuel etc. Whent for a quick ride no more loss of power high end rides good but after about a mile started loosing all power and cut out. Fuel filter is empty and not getting fuel. Thank you.
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- Replaced my spark plugs with new iridium ones but didn’t have the chance to test ride it. Getting fuel now but still in 4th or 5th gear at higher speed when coming off throttle and going back on the bike feels erratic. Sputters for few seconds then picks back up as normal. Replaced spark plugs with new iridium ones. Picture of old spark plug. Is it good colour?
- Replaced my spark plugs with new iridium ones but didn’t have the chance to test ride it. Getting fuel now but still in 4th or 5th gear at higher speed when coming off throttle and going back on the bike feels erratic. Sputters for few seconds then picks back up as normal. Replaced spark plugs with new iridium ones. Picture of old spark plug. Is it good colour?
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I would not use IX plugs because Hyosung’s hate them. The best plugs for them are normal CR8E or “LASER” type spark plugs.
The reason why you are also conking out on higher gear / higher revs is because the fuel tap is on it’s way out.
I would replace the pump also if it acts suspicious but 10000000% replace the tap.
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Hi again. As per your advice measured the compression it’s 12bar on each cilinder. Took of carburetor took it apart and cleaned everything. Put it back together now it starts but won’t rev up. Dies as soon as any throttle applied. Checked for vacuum leaks with brake cleaner all seems good. New spark plugs. Upgraded fuel filter from your site. Hoses not bent fuel coming through. Fuel pump has pressure when pipe removed shoot petrol out. Complete lost at this stage.
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Hi again. As per your advice measured the compression it’s 12bar on each cilinder. Took of carburetor took it apart and cleaned everything. Put it back together now it starts but won’t rev up. Dies as soon as any throttle applied. Checked for vacuum leaks with brake cleaner all seems good. New spark plugs. Upgraded fuel filter from your site. Hoses not bent fuel coming through. Fuel pump has pressure when pipe removed shoot petrol out. Complete lost at this stage.
Applying revs and bike conks out and dies usually (always) means:
- Pilot Jets and Main Jets are clogged again (the holes are almost microscopic)
- See tutorial section of this site and follow every carb tuning guide to help the bike if you can.
- Fuel has gone stale
- E10 ? (if yes, drain it all and put E5 back in)
- Slides (diaphragms) for carb are jammed or sticky
- Carb needs complete disassembly , and put in an ultrasonic bath (it has many & many holes inside) it will love you for it. Trust me on that.
- Carb Floats are bent => revs applied => pilots can’t reach to draw more fuel and bike conks out & dies
Put it this way, as you allow more air in, it should smoothly rev up or we have fuel woes again and likely electric woes (entire sparking system including stator)
But electrics will be the last war
12bar ? is that on hot and as per my instructions ? How sure was the mechanic or you watched him ?
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I will try to take it apart again. Nice bike don’t want to give up on it. Measured per your instructions with hard pipe not the hose on hot engine. Told me compression is good. Bike ran OK before carb was taken apart just bogging at higher rmp so must be something in the carburettor. Thank you for the input Marcel I will try and clean it all again.
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