125cc Motorcycle Owners Club

Honda CBF125, CBR125 and Yamaha YBR125

Basic Fault Finding

If your motorcycle has been involved in an accident,
please see Bike Damage after an Accident page.

A worn out Battery is the most common fault,
it's also one of the easiest faults to find and fix (see Battery section below in Electrical Faults).


Mechanical faults often affect the fuel to air mixture entering the combustion chamber (engine).
If the air is restricted you will get a rich mixture,
if the petrol is restricted you will get a lean mixture,
if you get a hole (could look like a crack or slit)
in a pipe or rubber which results in air from outside being sucked in you will also get a lean mixture.
If the mixture is to rich or lean it will ether result in bad engine performance (and other side effects)
or if really bad it will stop the engine from working.

A spark plug colour chart will show you how the engine has been running, normal, rich, lean or other problems.
Look at the Normal, Overheated, Carbon Fouled pictures in the link below.
For more Spark Plug information, see Spark Plug section below.


See Ethanol in Petrol Warning
Also see Petrol

Petrol ages, if it's 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.

Some petrol brands and versions have more inbuilt cleaners in than others.
Shell V Power, BP Ultimate, Tesco 99 are more expensive than normal Unleaded Petrol,
but they have extra cleaners in them.
If the engine is able to work, using petrol with extra cleaners in over time may slowly clean the engine.


Check the oil level (see Engine Oil page), if it's 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 Engine Oil page).

Spark Plug

Check the spark plug (see owners manual),
look at the colour, this tells you how the engine is running, see Basics section above for a link to a colour chart.
If it's running rich (black sooty deposits = Carbon Fouled) clean the air filter.
If it's 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.

It's always a good idea to change the spark plug and see if it fixes the fault.

Air filter

Clean it (see owners manual), 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.
If there are holes in the air filter or if the engine is run without an air filter,
the engine would run lean or not at all. It's very bad for the engine to run without filtered air.
See Basics section above for more information and links about rich or lean running.

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-original parts unless it says it will not change the mixture of the engine (rich / lean).

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 owner's manual).

If the clutch has done a very high mileage or has been severely mistreated over its life and so needs to be replaced,
I very highly recommend you put the EBC brand of clutch plates and clutch springs in (see Clutch in General page).

Clutch Cable

Clutch cables break on any motorcycle, often without warning and with no way of predicting it,
see Clutch Cable on how to service and replace it.

Valve Clearance

Check the valve clearance (see Valve Clearance page).
If the valve clearance is wrong, some of the problems are, engine performance / power may reduce,
it can make the engine run rich or lean, reduce engine compression,
make it hard to start or if it's very wrong valve or engine damage may occur.

If the valve clearance is too big, you may hear excessive noise from the valves.
If the valve clearance is too small, you may hear no noise from one 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 page.

Front Disc Brake

If the brake loses pressure or becomes spongy, check fluid level,
see owner's manual (also part 12 of Front Disc Brake fluid changing)
Brake fluid will become spongy over time, replace it at least every 2 years,
see Front Disc Brake fluid changing in Brakes page.

If you hear metal grinding against metal when you use the brake,
see Front Disc Brake for new brake pad recommendations.

If the brake seizes on, you need to clean the brake, again it's simple,
search the internet for information about cleaning.

Light Bulbs

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.

Rear Brake light,
if it does not come on when rear brake is applied,
there's an adjustment wire (see owner's manual),

Fuses, Connectors, Wires

Check the fuses, (see owner's manual), see if any are blown, there should be spares with them.

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
then soldering instead is much better, since solder seals the join 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 know how to solder, you can do the following.

If the fault is with the wire, you will have to cut off the connector,
then cut off all the corroded part of the wire (usually that's only the bit without plastic around it),
then 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 break the connection,
you can then put more WD40 on and if the corrosion is gone, wipe off the WD40 as much as possible.

On old bikes the electrical wires can become stiff and brittle.
There are several electrical wires that go to the headlight and the front indicators.
Every time you turn the handlebars
these wires are stretched and bent around the part of the bikes metal frame (chassis) that the handlebars attach to.
Eventually these wires can break and even short out, blowing the fuse.
The battery's - (negative = maybe brown or black wire) connector is wired to the bikes metal frame,
the headlight and front indicator wires are likely to have worn the paint off the bikes metal frame
and so any + (positive) wires can short out if their insulation is worn away
and they touch any unpainted part of the bikes metal frame.

Some electrical devices on the bike do not have a - (negative wire)
instead they connect to the bikes metal frame to get the - voltage.
When corrosion / rust builds up between them it can reduce the voltage or even break the connection.


Low battery voltage affects all the electrics, including the spark plug.
To test for low voltage, switch the ignition key on, while the engine is 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.
Of course a standard cheap voltage meter (multimeter) is far easier to use and far more accurate.

If the engine will not start due to low battery voltage and you need it started soon,
try switching off the ignition for a minute or two.
If that fails to work you could try bump starting the motorcycle (may or may not work).
I have never bump started (push started) a motorcycle but I believe the technique is the same for all bikes.
Sit on the bike (I do not recommend you try this off the bike, you could drop it),
switch the ignition on, put the bike in to 2nd gear and keep the clutch pulled in,
then move the bike forwards with your feet as fast as possible,
the faster the bike is moving the more likely the engine will start and stay running so try to go downhill
or ideally get a person to push the middle of your back at least at jogging or ideally running speed,
then let the clutch out smoothly and quickly, when the engine starts pull in the clutch.
If that fails to work you may have to open the throttle a bit just before or when you release the clutch,
if that fails you may have to try the bike in 3rd gear.
To get the bike in to 3rd gear you may need the bike to be moving at the time.
Someone made a video of how to bump start a motorcycle

An electric start bike needs a massive amount of power (voltage and current)
to start the engine (battery needs to be healthy and fully charged or nearly fully charged).
A failing battery might seem to have enough voltage but when the electric start is used
the voltage can drop significantly resulting in the engine not firing up.
The electric start may turn the engine over (might be slower than normal),
but then the battery may not have enough voltage left to ignite the petrol at the same time.
It may not even have enough voltage to turn the engine over and you only hear a click instead.
If you have a kick-start as well, try it since it needs far less power than an electric start.
As soon as you take your finger off the electric start button the battery may return to a good voltage.
My motorcycle automatically cuts off the front headlight briefly when the electric start is used
(this is a deliberate design feature and not a fault).
Some electric starter motors have failed
(they just go click and do not turn over the engine even though the battery is fine),
I believe they fail due to black dust building up inside,
all you have to do is remove the starter motor and clean its insides.
Someone made a video Part 1  Part 2  Part 3
Some people have got the starter motor going by hitting it with a hammer instead.
There is also the possibility that the engine just needs to turn over once or twice
(with ignition off, put the bike in to 2nd gear and turn the back wheel by hand in the forwards direction, not reverse).

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, top it up (see owner's manual).
3. You have left it for months and so it has gone flat (all bike batteries loose charge over time so charge it).
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 kick-start).
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 your 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.

If you have a voltage meter you can do some basic tests.
When the engine has been off for at least 1-2 hours with ignition key turned off,
100% Charged 12.60 to 12.8v
75% Charged 12.4v
50% Charged 12.1v
25% Charged 11.9v
0% Charged less than 11.8v

When the engine has been off for at least 1-2 hours,
Switch ignition key on and switch headlight on (engine must not be running),
you need at least 11.5v.

When the engine is running the bikes inbuilt battery charger should raise the voltage well above 13v
especially when the engine revs are well above idling.
You need at least 13.2v to start recharging a 12v battery,
only when the voltage gets close to 14.0v will it be charging well.
Above 14.5v a maintenance free battery will be damaged (voltage can go even higher if the bikes charging system is faulty).
A maintenance type battery can handle up to 18v,
bikes designed to use this sort of battery can often have a charging system that supplies up to 18v.
A failing battery can drag the bikes inbuilt battery chargers voltage down
(also a flat battery will drag the voltage down far more than a full battery).
But also a failing bikes inbuilt charger can drag the voltage down,
increase it or even fail completely and so not charge the battery at all.

How to buy and fit a new Battery

When buying a new battery, you need the details of the old one (also see owner's manual).

The CBF125 original battery was a maintenance free Yuasa YTX7L-BS (12v, 6Ah) and was rated at 85 CCA.

The CBR125 original battery was a maintenance free
Yuasa YTZ7S (12v, 6Ah, 130 CCA) for the fuel injected CBR125 (no choke lever or petrol tap).
Yuasa YTZ6 (12v, 5Ah, 90 CCA) for the carburettor CBR125 (has a choke lever and a petrol tap).
The YTZ6 is very hard to obtain but it is the same physical size as the YTZ7S, so the YTZ7S can be used instead.

The YBR125 original battery was a
Yuasa YTX7L-BS (maintenance free, 12v, 6Ah, 85 CCA) for the fuel injected YBR125 (no choke lever or petrol tap).
Yuasa YB5L-B (maintenance, 12v, 5Ah, 60 CCA) for the carburettor YBR125 (has a choke lever and a petrol tap).

From 2010 the Yuasa YTX7L-BS battery has been upgraded to 100 CCA
the CBF125 was first manufactured in 2008 and the YBR125 fuel injected in 2007.

You can upgrade the Yuasa YTX7L-BS to a Yuasa YTZ7S, which is still 6Ah but has 130 CCA.
The YTZ7S has 26mm less height and 1mm less length and width than a YTX7L-BS so would need some padding.
Honda dealers may be far cheaper than others for this battery since they may have a special contract through Honda UK.

After a lot of research, it appears the only brands of battery's which are widely known and have a decent reputation,
are Yuasa, GS, Varta, Motobatt and Exide. GS is in fact part of Yuasa the company name is GS Yuasa.

I highly recommend you do not install a Gel or Lithium motorcycle battery,
since they are probably not compatible with the motorcycle's electrics as well as many other problems.

An electric start needs a large amount of current (measured in CCA) to start the engine.
So the CCA (cold crank amps) mentioned above is very important, especially in winter.
Some battery brands on the market claim to be the equivalent of the Yuasa YTX7L-BS (or YTZ7),
but are only 75 CCA. These battery's may be around half the price of a 85 (or 130) CCA Battery,
but they are not the same battery technology. The TX (or TZ) part of the model number stands for VRLA,
or a more exact name is AGM. The AGM battery technology is much more advanced.
It results in a much higher CCA, recharges much quicker and lasts many more years.
It can also be mounted at a tilt of up to 90 degrees,
which means it is the only type that can be fitted to the CBF125 and CBR125.
If the manufacturer of the motorcycle fits a AGM battery as standard,
it is a very bad idea to try and fit a non AGM battery,
unless you have an alternative to using the electric start, especially in winter.
The TZ is smaller and produces higher CCA than the TX.

The CCA of a battery is sometimes very hard to find. It is not normally printed on the battery.
But it should be in the battery manufacturers catalogue.

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
(see How to charge a battery and detect its worn out below).
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 you should pour battery acid in if needed, but only use acid after charging for the very first time with a new battery.
After that you should not use acid.

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.
You can put any type of grease over them to protect them, you can also 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

Using the correct battery charger is crucial to prevent the battery from being overcharged and permanently damaged.
Most motorcycle and car batteries are now Maintenance Free type (you must only use the type your vehicle is designed for),
this means you cannot top them up with water (distilled or ionised).
Overcharging boils the water in the battery which then escapes,
as the water level drops in the battery the performance will drop until the battery becomes useless.
Overcharging happens when the voltage goes above 14.5V.
Overcharging will also happen if the voltage is above 13.9V for excessive lengths of time with a fully charged battery.
That's why most modern battery chargers
will ether switch off or drop the voltage below 14.0V to stop the battery from being overcharged.

You must also be careful about the charging current,
to much current will overheat the metal plates inside the battery,
which will permanently twist and distort them (severely damaging the battery).
A battery is meant to be charged with a current that's 10% of its capacity (size),
for example a 6 Ah battery would need 0.6A of current.
You can charge at a higher rate but it will harm the battery,
20% is acceptable, 30% is the absolute limit before real damage is expected.

You also have to be careful with modern quality battery chargers
which automatically tests the battery to see if it's refusing to charge.
If it thinks it's refusing to charge it will try to force (recover) it with up to 20V for a short time.
It is crucial if you have one of these chargers that it's designed to work with your battery capacity (size),
the chargers user manual should specify which battery capacity's it will work with.
If your battery is below the minimum capacity,
the charger will probably think it's refusing to charge
and so will try to force it with up to 20V every time you put it on charge (which is very bad for the battery).

There are many battery chargers available.
Some have a switch or mode for motorcycles and another for cars (since their batteries are larger).
Others do not and are designed for ether car or motorcycle batteries only.
It's crucial to choose a battery charger that claims to work with your size of battery.
It's also important to check it charges at a maximum of 14.5V (14.4V and 14.3V are also ok).
As well as the charging current
(if the charger has a car / motorcycle switch or mode it will supply a higher current in car mode than motorcycle).

I have seen a car / motorcycle charger that supplies 2A current in motorcycle mode (that's far too much).
Another car / motorcycle charger that only supplies 0.8A current in motorcycle mode (that's ok),
but claims to only work with 7Ah batteries and above,
since it has a force (recover) automatic mode I would not dare use it on batteries below 7Ah.
I have even seen some battery chargers that are so basic they have no fixed voltage.

Oxford Battery Chargers (reading reviews on Amazon, it appears this brand has reliability problems)

Well known quality Battery Chargers in the motorcycle and car world.

Optimate is the original modern battery charger brand in the motorcycle world.
They allow a few companies to put their own brand on their chargers like Honda and Triumph motorcycles.
But you will always see the Optimate brand on them as well.
Optimate 1 has 0.9A current output, Optimate 4 has 0.8A current output.
Optimate 2 has 0.8A current output, Optimate 5 has 2.8A current output (do not use on motorcycle battery's).
Optimate 3 has 0.6A current output, Optiamte 6 has 5A current output (do not use on motorcycle battery's).

Ctek is the original modern battery charger brand in the car world.
They allow their battery chargers to be rebranded with other peoples brands.
But they are identical in every way to a Ctek charger, the only difference is the label on the top.
Yamaha motorcycles put their brand on them and many car manufacturers and others do as well.
Their cheapest model should have 0.8A current output.
All other models are at least 3.6A (which is far to high for motorcycle battery's),
but if they have a motorcycle mode it will switch the charger down to 0.8A.

Yuasa Battery Chargers (900mA = 0.9A current output).
Yuasa YCX range are rebadged Ctek.
But they also make their own battery chargers that are much cheaper than Optimate or Ctek.
They are much cheaper because they do not have all the advanced recovery modes for severely neglected battery's.
But they are still very good reliable battery chargers
and may even charge healthy battery's more fully than Ctek or Optimate.
Yuasa Yu-Power 12v 900mA Motorcycle Battery Charger model number YPC09A12MC, is widely available (ebay etc..).
In some country's there is a Yuasa Smart Shot 900 model instead.

Honda CBF125 and nearly all CBR125 and Yamaha YBR125 use a Maintenance Free 6 Ah battery
so the ideal charging current is 0.6A = 600mA (that's 10% of the capacity).

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 it's 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.

Some modern battery chargers like Ctek, Optimate and Yuasa,
may claim you can use them without disconnecting the battery from the bike's electrical system.
That may be true. Many people use them like that and have no problems.
The battery chargers unlike others, have special components inside to smooth out the power.
If anything goes wrong with these special components or you have a mains surge (e.g. lightning strike),
there is still a possibility your bikes electrical system may be damaged.
The only reason I can see for charging a battery like that is if an alarm is fitted to the bike.
An alarm can dramatically shorten the life of a battery unless a charger is switched on all time.

Some modern battery chargers claim you should leave them switched on all the time,
for weeks and months in order to increase the battery life.
Other people would argue doing that will decrease the battery life (unless you have an alarm fitted).
One thing is for sure, if anything goes wrong with the battery charger (due to a fault or lightning strike),
it can easily kill the battery and can even cause a battery fire or acid to pour out of the battery.
The chances are that will not happen, but you will be wearing out the battery charger.

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.