Honda CG125 Basic Fault Finding
The Honda CG125 is pretty tough, strong and reliable even with severe neglect.
The basic fault finding below will fix almost all the common faults your likely to get with the bike.
Just like the Servicing, its very easy for anyone without any skills to do and its much cheaper than a mechanic.
A worn out Battery is the most common fault, its also one of the easiest faults to find and fix.
If the bike has been stored for a long time see Get bike working after storing for months in Servicing page.
Front Disc Brake
If the brake looses pressure or becomes spongy, check fluid level, see Front Disc Brake in Servicing page (also part 12 of fluid changing).
Brake fluid will become spongy over time, replace it at least every 2 years, see Front Disc Brake in Servicing page.
If using the brake sounds like metal grinding against metal, look to see if the pads are worn out, replace immediately if they are or disc will be damaged.
Replacing brake pads is simple, search the internet for information.
According to the Honda CG125 specification its a,
240mm hydraulic disc with single-piston calliper and sintered metal pads.
The original pads are sintered type and made by a good quality manufacturer called Nissin.
You can buy the original pads from your Honda dealer and they cost around the same as other brands.
The part numbers for other brands are EBC FA54, GOLDfren 144, Dunlopad DP107, Ferodo FDB250, SBS 536, Vesrah VD120.
The part numbers do not include the pad material type, the two main types are sintered and organic, some makes only make an organic for that part number.
Sintered are much better at braking in the wet and braking power is less affected by temperature.
Organic has more feel and often generates less heat.
For everyday road use Sintered is normally recommended and fitted by manufacturers on the front.
Each pad material can be designed for different uses, you want one designed for road use or sports not off road.
If the brake seizes on, you need to clean the brake, again its simple, search the internet for information about cleaning (its a single piston calliper).
Petrol
Petrol ages, if its old petrol it can cause trouble, there are also bad batches from time to time, try to dilute it with fresh petrol or drain and replace it.
Oil
Check the oil level (see Oil in Servicing page), if its burning oil the level will drop, you need to see a mechanic if it's substantial.
Make sure you change the oil at the correct mileage or before (see Oil in Servicing page).
Spark Plug
Check the spark plug (see Spark plug in Servicing page),
look at the colour, this tells you how the engine is running, see the Links page or search the internet for a colour chart.
If its running rich (black sooty deposits = Carbon Fouled) clean the air filter.
If its running lean check the Exhaust and Silencer (see Exhaust and Silencer section below), it could also be a fuel starvation (blocked) fault in the petrol line or carb.
When choke is on the mixture will be rich.
Its always a good idea to change the spark plug and see if it fixes the fault.
Air filter
Clean it (see Air Filter in Servicing page), it's always a good idea to clean the filter and see if it fixes the fault, this would cause the engine to run rich
Exhaust and Silencer
If the Exhaust or Silencer has rusted all the way through it will make the engine run lean or rich (probably lean).
You must replace it, be careful with non Honda parts unless it says it will not change the mixture of the engine (rich / lean).
If you fit a non Honda part you may have to alter parts inside the carb to get the mixture correct at all throttle positions.
Make sure any parts are designed for your version of Honda CG125.
Clutch or Gearbox
Clutch or Gearbox faults can often be due to Clutch free play, Chain Tension, Chain Oiling or Idle speed (check all of these in the Servicing page).
Clutch Cable
Clutch cables break on any motorcycle, often without warning and no way of predicting it, see Clutch cable in Servicing page on how to service and replace it.
Valve Clearance
Check the valve clearance (see Valve Clearance in Servicing page).
If the valve clearance is wrong, some of the problems are, engine performance / power may reduce, if its very wrong valve or engine damage may occur.
If the valve clearance is to big, you may hear excessive noise from the valves.
If the valve clearance is to small, you may hear no noise from on of the valves, this is very bad for the engine.
The noise test should not be relied on, you should only rely on the Valve Clearance in the Servicing page.
Ignition Timing
If you have a Honda CG125 before 1995 (before S & T models) it could still have the old Points system.
See Ignition Timing in the Servicing page.
Just like the Valve Clearance, the Points need to be checked and adjusted.
The Points effect the timing of when the spark plug fires. It also varies the timing dependant on engine Revs.
If the timing is wrong the power will reduce from the engine.
Air Suction Valve
It’s a Secondary Air Induction (SAI) valve system, not an Exhaust Gas Recirculation (EGR) valve system.
The Air Suction Valve is only fitted in countries that require it to reduce emissions, the UK and most of Europe did when the front brake went from Drum to Disc.
The Air suction valve is on the left hand side of the bike just under the petrol tank.
It has pipes connecting it to the top of the engine, the air filter and the metal pipe connecting the carb to the engine (intake port).
Air is sucked from the Air Filter in to the Exhaust port
which is on top of the engine, this enables the excess fuel in the exhaust gas to burn
in the exhaust.
The Air Suction Valve is actually 2 valves, both valves must be open for air to flow from air filter to exhaust port.
One opens when there is negative pressure on the exhaust port at the top of the engine (so when fumes are sent out the exhaust it sucks air from the air filter with it).
The other valve is open all the time except when there is negative or low pressure at
the intake port (metal pipe connecting the carb to the engine).
Negative pressure is generated when closing the throttle and decelerating, the
valve must close to prevent after burning.
Low pressure for example is generated at high engine revs, the valve must close
to prevent a loss in performance due to Exhaust Gas Recirculation.
I'm guessing a fault creating Exhaust Gas Recirculation
does not sound good if the exhaust gas ends up in parts not designed for the
heat or fumes (air suction valve).
It looks like you disable the valve by disconnecting the pipe going to the top of the engine.
You then bung up the pipe and tie it tight to stop air flowing through the pipe.
The UK MOT still may not check the exhaust emissions for motorcycles (check first), cars have been checked for many years.
I'm not sure, but its probable that disabling the valve will not improve engine performance unless the valve is faulty,
people who guess it would may be thinking of Exhaust Gas Recirculation (EGR) valve system which does.
Someone told me their Air Suction Valve stuck open, the bike juddered at around 5500 rpm and could not get past 6000 rpm in any gear, see MPH to RPM page.
If you disable the Air Suction Valve I am not sure if it would change the air mixture (lean or rich), it might at all or some throttle positions?
It could well be that it does not since you have to disable the valve (closed it) when you do an emissions check.
You can find out by checking the spark plug colour
http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm
http://hondanighthawks.net/carb14.htm
How to charge a battery and detect if it is worn out
If a light bulb fails, you must fix it as soon as possible, you would fail a police check and a MOT in the UK.
Never ride in the dark without a working headlight.
If the bike is very old the bulbs will be 6 V (volts), if its a modern bike it will be 12 V (volts), bulb and battery will be marked 6 V or 12 V.
If you accidentally put a 6 V bulb in a 12 V bike the bulb will probably just burn out immediately (bright flash of light then never works again).
If you accidentally put a 12 V bulb in a 6 V bike the bulbs light output will be half what it should be.
Headlight, if the dipped headlight blows, use the non dipped headlight instead, you can point the headlight down to stop blinding other vehicles.
Legally you must still replace the bulb as soon as possible (see How to replace the Headlight Bulb in the Servicing page).
Rear Brake light, if it does not come on when rear brake is applied, there's an adjustment wire, it's vertical and has a spring connecting to the pedal.
Push the brake pedal down and up and find the cable and spring, there's an adjustment screw in the middle of it.
Left and Right Turn Indicators, there's a fuse (see Fuses section below) and a box that makes the indicators flash.
The box could be around the battery or might be the black box where the Idle Speed Screw is (see Servicing page), don't touch the idle speed screw.
The box will be a standard flasher, you could probably buy a universal one, maybe even a car one (if you have a 12 volt system, look at battery to find out).
But I would see how much a genuine Honda one is.
Check the fuses, they are around the battery (behind bikes left side panel), see if any are blown, there should be spares with them.
Front Disc Brake model has
20A Fuse, this powers the electric engine starter motor and is also the master fuse.
5A Fuse, this powers the left and right turn indicators.
15A Fuse, this powers everything else including the electric starter motor button.
If moisture gets in to any electrical connections or wires (usually were the connectors touch the wire) in the bike, they can corrode (rust),
this increases the resistance for electricity (drops the voltage) and can even break the connection.
If you have this problem, you need to find it, check any connections you can find and also look at the wire where it connects to the connector.
If you know how to solder, cutting off the connector and any corroded wire and soldering instead is much better, it is sealed from moisture and dirt.
Of course if you are bad at soldering, you can create what's known as a dry joint, this will corrode internally and break the connection.
For people who do not no how to solder, you can do the following.
If the fault is with the wire, you will have to cut off the connector,
cut off all the corroded part of the wire (usually that's only the bit without plastic around it), take a bit of the plastic off the wire,
then try to reuse the connector or buy a new one.
If the fault is a corroded connector, you can use a normal can of WD40 (available at most motor shops, DIY and even supermarkets).
But only use WD40 if the connector is corroded, WD40 cleans, removes water and eats corrosion (over time), but leaves a deposit that attracts dirt.
This deposit may help the connection to begin with, but later dirt could brake the connection,
you can then put more WD40 on and if the corrosion is gone, wipe off the WD40 as much as possible.
Low battery voltage effects all the electrics, including the spark plug.
A test of low voltage is to switch the ignition key on, with the engine off try the horn, if the tone is much lower than normal, you have low voltage.
You can also do this test with the lights, if they are dimmer than usual with the engine off, you have low voltage.
There are several reasons for low battery voltage.
1. It's simply worn out, this is by far the most likely.
2. If it's a maintenance battery, see Battery in the Servicing page.
3. You have left it for months and it has gone flat.
4. You have been doing loads of very short journeys and it has not been able to recharge enough (not so likely if you use a kickstart),
5. The engines battery charger has stopped working or the fuse / wire connecting it to the battery has failed.
6. You have an alarm fitted and it has drained the battery.
7. You have left the lights on (if you model has that ability).
Testing the motorcycles battery charger is probably easiest by simply replacing the battery, if the new battery suffers low voltage with use,
it must be the motorcycles battery charger.
You can test the battery with an automatic battery charger, see the section below called How to charge a battery and detect if it is worn out.
You can test the battery, but you could also just replace it.
How to buy and fit a new Battery
The front disc brake model Honda CG125 is supplied with a Yuasa maintenance free battery.
It's probably for the electric starter motor, since it has twice the current to start the engine, recovers and recharges faster than a conventional maintenance battery.
It also self discharges nearly 3 times slower than a maintenance battery (so bike can be stood for longer without battery needing to be charged).
The downside of the maintenance free battery is it costs 3 times more than a maintenance battery but Yuasa claims it will last 3 times longer so costs the same.
I am not sure if the older Honda CG125 electric start bikes were supplied with a Yuasa maintenance free battery or not (read the paragraph above about it).
I have a strong feeling they were since electric start needs a massive amount of current from the battery to start the engine.
A Yuasa conventional maintenance battery would need to be twice the size(capacity = eg. 8 Ah) to deliver the same current as a maintenance free battery(eg. 4 Ah)
I am pretty sure the kickstart only Honda CG125 have always had the maintenance type battery since you need far less current to start the bike.
Some maintenance free batteries need a bike designed to work with them, other makes of maintenance free batteries may have a different amount of current output.
When buying a new battery, you need the details of the old one.
If you find a model number on the battery, you may be able to buy the same battery or find a conversion chart for other makes.
Front disc brake model has a Yuasa YTX5L-BS (12V 4Ah maintenance free)
Front drum brake model with electric start probably has a Yuasa YTX5L-BS (12V 4Ah maintenance free)
Front drum brake model with kickstart only and 12 Volts probably has a Yuasa YB2.5L-C (12V 2.5Ah extra power maintenance)
Front drum brake model with kickstart only and 6 Volts probably has a Yuasa 6N6-3B (6V 6Ah conventional maintenance)
The voltage should be on the battery and on every light bulb, it will be 12V or 6V. A 12V battery has 6 compartments (6 filling holes), 6V only has 3.
If the model number is no good you will have to find the following on the battery.
The most important thing is the voltage and capacity (front disc brake model is 12V and 4Ah), do not change the voltage or capacity.
There have been many different versions of Honda CG125 over the years and the voltage and capacity required has changed several times.
A common mistake is to fit a higher capacity battery than original, this can overload the bikes electrical system and cause many faults.
The next thing you need is the physical size of the battery, height, width, depth to make sure it fits.
Then you need to no which way round the + and - connections are.
If its a maintenance battery, you need to make sure the plastic breather tube is in the correct place or its supplied with a longer one that will fit.
Maintenance Free batteries have no plastic breather tubes since they are sealed.
Of course the battery compartment of the bike might allow a different size of battery and plastic breather tube (if the battery has one).
There are different battery terminals requiring different connectors (some times a new battery comes with new connectors to put on the bikes wires).
A new battery will come with an instruction leaflet, if you pour the acid in to the battery read it,
it will tell you to stand the battery for at least 30 minutes after pouring the acid in (before charging).
It may tell you to tap the battery to get air bubbles out and pour more acid in if the level drops on maintenance type batteries.
A new battery is semi charged when assembled (probably 80% charged),
for maximum performance and life it should be fully charged with a charger before putting it in the bike.
A shop or you can charge it, but it takes time.
After charging for the first time, if it's a maintenance battery the fluid levels must be checked (ideally pour acid in if needed), see Battery in the Servicing page.
When fitting a maintenance battery, you must connect the plastic breather tube, it needs to point down and away from the bike,
this is usually in the middle of the bike, just in front of the rear wheel.
This tube may have corrosive gasses and liquids pouring out, so do not let it point at anything other than the ground.
The battery terminals and any exposed (bare metal, not plastic) metal wires connecting to them,
need to be protected from the corrosive gasses from the battery, this might not apply with maintenance free batteries,
but it definitely does for maintenance batteries.
I put any type of grease over them to protect them, you can buy special grease for this, but there's no need.
Make sure you do not get grease between the battery terminals and the connectors, unless the grease is designed to allow voltage to pass through it.
How to charge a battery and detect if it is worn out
Do not use a non automatic battery charger unless its designed for motorcycle use, else it will charge the battery far to fast and damage it.
Maintenance free (sealed) batteries can easily be damaged if overcharged so an automatic (smart) battery charger is recommended because it will not overcharge.
If you have a battery charger that's designed only for motorcycle use (not car and bike), you can skip this paragraph.
You may be able to use a car automatic battery charger, it may even say suitable for motorcycles or has a motorcycle switch.
If an automatic(smart) battery charger charges the battery with a constant voltage during the entire charging sequence, you can use it on any motorcycle battery.
Do not confuse it with a charger that uses both constant voltage and constant current in the charge sequence.
Constant voltage means the battery controls the current so it does not matter how small the battery is (motorcycles are very small), it will not damage it.
If the battery charger uses constant current in any of the charging sequence you need to be very careful to make sure it does not damage a motorcycle battery.
A motorcycle battery is identical to a car battery except for its capacity (size), cars might be 35 to 70 Ah and motorcycles 2.5 to 25 Ah.
Some car battery chargers that claim to also work with motorbikes claim 2 A charge current, many (including motorcycle only chargers) are 0.8 or 0.6 A (600 mA).
Ideal charge current is 10% of its capacity. The Honda CG125 battery is one of the smallest motorcycle batteries made, since its engine size is small.
Honda CG125 front disc brake model battery is 4 Ah so it ideally wants 0.4 A charge (your very unlikely to find a charger with such a low output).
Often you can not find out if an automatic battery charger has constant current in the charging sequence so my advice in these cases is to,
check the charger outputs or has a switch to output 0.6 or 0.8 A (600 or 800 mA) or it claims to work with your battery capacity (size).
To charge a battery, you must disconnect at least one of the battery terminals from the bike.
This is to stop the battery charger damaging the bike's electrical system.
The negative battery terminal is often hooked up to the bikes metal frame, so its best to remove the positive battery terminal (Red wire),
since if the battery charger accidentally touches the metal frame,
it would send the battery charger voltage through the bikes electrical system if only the negative terminal was disconnect.
An automatic battery charger will tell you when the battery is fully charged, a faulty or worn out battery will probably never reach fully charged.
If you have a very old Honda CG125, your battery may be 6 V (volts) instead of the normal 12 V,
if its 6 V you need a battery charger which is or can switch to 6 V.
If you have a 6 V battery, all of the bikes electrical system and everything that connects to it will be 6 V,
do not change it for a 12 V battery since it would destroy everything.
All batteries will have a label on them stating if they are 6 V or 12 V, if the label has worn off, all the light bulbs will also state if they are 12 V or 6 V.
6 V maintenance batteries have only 3 compartments to top up with water (distilled), 12 V maintenance batteries have 6.