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Author Topic: Cloud's Hyosung 125 Ultimate Tuning Guide Light  (Read 5041 times)

Cloud

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Cloud's Hyosung 125 Ultimate Tuning Guide Light
« on: October 21, 2014, 10:44:56 PM »

A basic stripped-out version of the tuning guide I'm working on.

This is for power tuning only. Nothing about spring rates or light-weighting in here.

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0 - F O R E W O R D

There's a whole lot on the internet about tuning 125cc motorbikes, and no matter what 125 bike it is, there's always a resounding consensus that a better way to go faster on a bike is to get a faster bike. This may well be the case, and for the money potentially a more sensible option. However, there are many reasons why one would rather keep their GT125, reasons I will not go into. But many, like myself, are happy with the machine, but sometimes would like a little more from the engine, and would like to know what is actually possible.

This is a technical guide, written by a former motorsport engineering technician. Prices, retailers, availability, and mechanic services, all vary according to your location, so for simplicity I have left these out and focussed on parts, materials, and their effects. Additionally, this is not the absolute limit of what can be achieved with the GT125 engine, it can be tuned further, to the detriment of reliability. This guide is an idea of how to get the best from the engine, if not the most.

All of the information is based on real tuning done to a GT125R. Tuning can also be applied to the GV125.


1 - C A R B S

All GT125s had Mikuni carbs as standard. For tuning, they are more versatile, and I have found it's best to stick with them for both simplicity and for results. Carb jet main settings are individual to your exhaust, fuel type, altitude, weather conditions, and how much of the changes in this guide you implement. Even a rough guide could potentially be grossy incorrect. Float height should be set at the standard 7mm.
I drilled a hole in my throttle bodies, both above and below the butterfly valves, and ran a simple braided hose between the two, 1mm internal diameter. This allows a constant additional flow of air, as though jamming the throttle a little open. Additional petrol flow is created, to balance the fuel mixture, by modifying the idle jets to flow a little higher; the next-size-up is often too much of an increase. Throttle control is regained by dropping the jet needle down a notch. Throttle response is increased by stiffening the spring for the throttle slide, and drilling an additional hole through the slide valve, to allow it to move more freely.


2 - A I R   I N T A K E

The CFM of the standard air filter is more than enough for even a high-tuned 125cc engine. The vent port on the top provides minimal restriction. De-snorkelling involves drilling additional holes into the top of the air filter unit, to allow it to take more air in. However, to maximise the air feed, the air filter can be removed, and the hole blocked off entirely. Large holes can be drilled on the front side of the air box, and hoses attached, roughly 30mm diameter plastic hose is fine, and these hoses can be run through the fairing to the vents on the bottom of the GTR's nose-cone, or either side of the horn on the GT Comet. These will push cold air into the air box, and the greater the speed the bike travels at, the greater the pressure will become inside the air box, and will effect a very mild boost at higher speeds. Filtration of the air is done by two cone filters fitted directly to the top of the carbs, still within the air box. The original airbox to carb rubbers must be in place or the additional pressure from the increased air flow will be lost.


3 - C Y L I N D E R   H E A D

The cylinder head ports on the GT125 are smaller than that of the GT250, but the meat of the metal is there to bore out the ports to the same size, which is roughly 28mm. This will allow full air-flow into, and out of, the ports. The internals of the ports are very well engineered and need only minimal gas-flowing. While this is being done it is a good idea to also change the valve stem seals, clean the valves, and re-seat them. To close the valves, titanium valve springs react faster, which is essential when combined with additional cam lift, to prevent the pistons colliding with the valves at high revs. Titanium valve springs are a custom-manufactured part. The cam profiles can be altered slightly, for higher lift and longer duration. The intake cam profiles can be given a higher lift at peak. The exhaust cam can be given an extended lift by roughly 0.6 degrees with a sudden drop, which will allow more exhaust gases to be vented before the piston comes up for TDC.

4 - C A M S H A F T   D R I V E

The GT250 uses a wider camshaft drive chain than the GT125, which is an important upgrade considering the additional cam loads from higher lifts and stiffer springs. To accommodate the wider chain, GT250 chain tensioner guides should be fitted. GT250 camshaft sprockets can also be used on the camshafts. The crankshaft sprockets don't need to be changed to run the wider chain.

5 - I G N I T I O N

Suzuki SV650 coils produce a massive whack in comparison with the standard coils. They're easy to fit and compatible with the Hyosung system. Performance HT leads are good to carry the discharge to the sprak plugs. NGK's CR8EIX iridium plugs provide a clean sharp spark.

6 - C R A N K S H A F T

Custom forged pistons made from aluminium alloy are lighter than the standard cast pistons. This allows less rotational energy to be lost when the pistons change direction at BDC and TDC, and more power to be transferred into drive. The crankshaft has to be balanced to account for the weight loss on the small-end. This weight loss also increases throttle response, the balancing allows for higher revs, less vibration, and weight loss of the machine overall.

7 - C O O L I N G

The additional performance of the engine creates additional heat. A matte black engine will radiate heat better than any other colour. The main change though is the oil cooler. In order to keep the engine operating at optimum temperature, the GT250 oil cooler radiator can be used. It is larger and provide better cooling for both low-speed and serious abuse.

8 - E X H A U S T

At high levels of engine tune, high flow levels are appropriate. Having free flow through the exhausts doesn't provide any more power at lower levels of tune, it merely moves the power band further up the rev range, leaving less power down at low revs and pushing some of the power beyond the redline, so power is lost at both ends. To limit flow restriction, a 2:2 system, with a pipe per cylinder, is best. There are many 2:1 systems that also minimise restriction, and for a small capacity engine, even at high speeds any system beyond standard should be perfectly adequate.

9 - H O S E S

In the same way that braided brake hoses make the structure of the hose more rigid for consistent fluid conductivity, braided hose for both the vac pipes and fuel lines keeps intake flow as consistent as possible.

10 - B A T T E R Y   C O N D E N S E R

Much is said about the regulator/rectifier on the Hyosung motorcycles. A simple modification to stabilise the battery and reduce fluctuation in the power supply. These can be bought or made cheaply, connected to the battery terminals, and forgotten about. This will give a more stable current to the coils and result in stronger spark and more torque.
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Khaosie34

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #1 on: October 21, 2014, 11:07:42 PM »

I assume the SV coils and the battery condenser go hand in hand?

How much of a difference did the SV coils make?

And would you mind explaining what a battery condenser is and where/how to get one? A quick Google search didn't give many answers.
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GT125R Red/Black
Full Scorpion system
Iridium plugs
Desnorkled air filter
C.D.I. Mod
Uprated HT leads
Braided lines
Upjetted
LED running lights
"Pazzo" levers
HID

Cloud

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #2 on: October 22, 2014, 12:45:29 AM »


The SV coils are ace for starting. I still haven't bothered mending my choke, because it fires first crank even after a few days of lay-up.

Condenser is really basic. You can make them yourself after a quick trip to Maplin.

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Khaosie34

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #3 on: October 22, 2014, 09:50:28 AM »

Could you show me how you've done your condenser?
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GT125R Red/Black
Full Scorpion system
Iridium plugs
Desnorkled air filter
C.D.I. Mod
Uprated HT leads
Braided lines
Upjetted
LED running lights
"Pazzo" levers
HID

eddieeardley1974

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #4 on: October 22, 2014, 12:27:48 PM »

brilliant will be trying these out cheers cloud :)
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kylestuff115

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #5 on: October 22, 2014, 05:04:02 PM »

The ram air style intake is an interesting idea... Might add that to the list of winter projects.
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It's not about what you ride; it's how you ride it.

Cloud

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #6 on: October 23, 2014, 01:18:10 PM »

100+mph top speed confirmed by independent third-party.
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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #7 on: October 23, 2014, 01:39:13 PM »

My winter consists of making my own wiring loom out of a much higher quality cable, as my bike will now limit my revs to around 6k if you're in the cold with your headlight on. Fun.
I've got a Honda Revere to ride around on for the winter, so I can spend some proper time on the GTR. A new loom, combined with the air ram method, sv coils and ht leads should make it more fun.

Then I'll do a full rebuild on the old engine. :P Teach me your ways, mighty Cloud.
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stuzo

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #8 on: October 23, 2014, 07:25:31 PM »

Nice write up cloud deff going to try these tips out
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Cloud

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Re: Cloud's Hyosung 125 Ultimate Tuning Guide Light
« Reply #9 on: November 19, 2014, 01:03:52 AM »


Having custom alloy pistons made isn't cheap. But it's cheaper than having the crank shaft professionally re-balanced.
My engine is made of money.

The condenser is really basic but makes such a difference to everything electrical. Lights react faster, the dashboard display gets a more stable current, the coils gather charge faster, and it will help extend battery life as well. Not bad for something so simple. I know the regulator does this job already, but a direct piggyback on the battery makes the job more complete.

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