Unusually low top speed....

[mat91]

hey guys I have a hyosung gt125r which I recently changed the engine with one off a gt125 but for some reason it doesnt go past 9,000 rpm in 4th gear like its ran out of steam can anybody help me with this please? cheers mat
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hyosung rider

[N cee thirty]

same engine

what speed's it get to
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'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

Jewlio Iglesias wrote: I actually did vote BNP once

[mat91]

getting 52mph at 9,000 rpm and before the bike used to get 75mph on redline in 5th gear at 10,500 rpm
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hyosung rider

[N cee thirty]

different front sprocket ?
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'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

Jewlio Iglesias wrote: I actually did vote BNP once

[mat91]

same as what came off my last engine
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hyosung rider

[Frost]

Might have different internal gearing requiring different sprockets for the same result?

[Fairies attack]

Would help, what year the engine is.

Is it carb or EFI?

[Pete.]

Something is wrong with the fuelling or the replacement engine is knackered.

Could also be the timing is set up different for each, requiring the correct CDi.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[mat91]

Engine is a 55 plate and is a Carb, my original was a 56 Carb but was thinking if the valve clearances Would affect the engine this much?
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hyosung rider

[mat91]

Btw the compression is 120 psi in both cylinders, when I compression tested the front the back started the bike on its own so plenty of compression in the engine
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hyosung rider

[Kickstart]

Hi

Quick look and on stock gearing the GT should do 50.7 @9k in 4th and 70.7 at 10.5k in 5th. As such seems to match up with what you are getting / got before so the gearing seems to be the same between the 2 engines.

Does suggest that something else is wrong, but there are loads of things it could be. 120psi sound a bit low for compression on a 4 stroke, but being the same on both cylinders sounds unusual.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[P.]

Where are you based in the UK?

[mat91]

Hey Keith cheers for the info I have a 15 tooth on the front and 54 tooth. On back but this was the same on my last engine, even tho the carbs are the same off the original engine could it be a fuel/ air problem you think? Maybe I need to tweek the mixing screw a bit?? Cheers mat
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hyosung rider

[mat91]

I'm in long Eaton not far from Nottingham and derby
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hyosung rider

[P.]

Your gearing should be 14 and 52... Think thats what mine was before we swapped them.

Was going to say if you was local, i'd come have a look, but thats missions

[Kickstart]

[mat91]

Cheers Keith anyway I can up the compression? Maybe valve clearances? And where bouts are you paddy?
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hyosung rider

[Kickstart]

Hi

Possible valve clearances could affect it, but I would be a tad surprised if it affected both cylinder equally. Also to lower compression the valve clearances would need to be tight, which would burn out the valves. And burnt valve are a far more likely cause of low compression. Damaged rings would be another likely cause.

Valve timing could possibly do it, but not sure how the cam drive is set up on the Hyosung engine so whether it could easily be out on both cylinders at once. Let alone if being that far out would have resulted in valves and pistons meeting.

However double check the compression (and see if someone else can confirm what it should be) as yours seems very low and strange for both cylinder to be equally low. You should be able to trigger the starter relay to crank the engine over (full throttle for a compression check) without the ignition on to prevent it starting on one cylinder.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[mat91]

Right what I shall do after work is check valve clearances and spark plug colour and adjust my mixing screw to alter colour of my spark plugs and see if they increase anything
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hyosung rider

[Kickstart]

Hi

While easy things to double check, I wouldn't hold out much hope.

Not sure if the Hyosung has CV carbs but if so the plug colour will be fairly meaningless. Way too hard to control things to get a consitant amount of fueling for long to do a meaningful plug chop.

At the revs / speed you are at you should be on the main jet, or at least at a point where it has a major influence. The mixture screw will have a negligible effect on the fueling there. Suppose putting it in real world terms, imagine the mixture screw as a salt shaker, capable of making a plate of food salty. If you now use it on a giant cooking pot of food with enough there to server a regiment the effect will be negligible.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[mat91]

Hey guys I have found out that the engine is restricted does anyone know where they restrict these?
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hyosung rider

[zombie_dave]

your engine is a 125 four stroke, it isnt going to be restricted................

only 2smokes and bigger bikes* get restricted (*e.g if you have a restricted licence)
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ZombiE_DavE

[Kickstart]

[Livefast123]

The Hyosung service manual states that the compression should be between 156 - 185 Psi with the service limit running at 142 Psi so if you are reading 120 Psi then you have got some major ring wear or another problem with the valves

Are you sure that you did the compression test right with the engine up to working temperature, if you did it cold then the reading will be well down?

Hyosung engines are not restricted, have you checked the obvious stuff like fuel filter, spark plugs etc and the exhaust is not partially blocked. Something to note is that airfilters are very expensive if you buy them in the UK so many don't bother changing them. I'd check it as it only takes a few minutes and if blocked would make the bike run stupidly rich which would give poor top speed.

Check the throttle is properly adjusted as they have a stupidly long action throttle and it may not be opening properly.

Might be an idea to post on https://ukhyo-ownersclub.co.uk/ or Korider as many more Hyo 125 riders hang about there.
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Current ride - Yamaha MT-07

[mat91]

I changed cdi as it gave power problems before but will check valve clearances and look at piston rings see what they look like but they said they only got 60 out of it in the bike it was from but does more than that it mine
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hyosung rider