If you still have excessive noise from the top of the engine after adjusting the valves in the sections below,
your CAM Chain is probably in need of adjustment or repair.
Do not ignore it, the CAM Chain can easily damage your engine if ignored (see a mechanic if you're not sure about what to do).
New valves need to be checked regularly until they bed in (run in),
when you check the valves and find they do not need adjustment,
you do not need to check them so often in the future.
To begin with I would check them more often than the manufacturer states (see owner's manual).
At least check them when the valves have done around 1500 miles.
It's not recommended to ignore checking the valve clearance but if you do,
listen (helmet off) to the valves at the top of the engine when it's running.
If they are excessively noisy (clatter, clatter) the clearance is too big and needs adjustment.
If one of the valves goes quiet its clearance is too small and must be adjusted immediately to prevent damage.
Performance drop, bad idling, problems starting or compression drop could mean the valve clearance is too big or small,
check immediately to prevent damage.
Valve clearances normally increase due to mechanical wear, but can sometimes decrease.
If the valve clearance is too big the only damage you are likely to have done is increased the wear rate of the valve.
Valves wear out (normally takes a very long time and high mileage) and can be replaced,
when the gap is to large they wear out far quicker
How to adjust the Valve Clearance
The pictures below are from a Honda CG125,
but they are good enough to work on the CBF125, CBR125 and YBR125.
Of course there are some slight differences.
CBF125 you can find better pictures in the Owner's Manual in the Valve Clearance section.
You can also have a look at this persons webpage about CBF125 Valve Clearance
but I still advise you to look and read all of my advice below.
CBR125 you can also use the CBF125 pictures but not the figures for the Valves.
1. Leave bike overnight to cool down, this is essential since the valve clearance changes with a hot engine.
2. If you really cannot do this step 2 then try step 3 instead.
Remove the two round covers
on the left hand side of the engine case (see picture above).
(If they will not undo, warm the bike up and try again, once they are off leave the bike overnight to cool down again).
The large one covers the nut for turning over the engine.
The small one covers the marks for valve clearance and timing.
Turn engine over anticlockwise with a socket
CBF125 and CBR125 until a small on its side T appears near the outside edge of the disc (don't mistake F for T).
YBR125 until a small vertical line appears near the outside edge of the disc (don't line up with the H).
If you do not have a socket
you can use rear wheel + 5th gear instead (wheel needs to be turning to change between gears)
(Never turn the rear wheel in the direction that would make the bike go backwards when the bikes in gear,
that would turn the engine over in reverse which it is not designed to do)
You can remove the spark plug to make turning over the engine easier but it's not really needed.
Use a torch and magnifying glass since the mark is very small and stamped in to the metal,
do not use a house light bulb since it's too powerful and will hide it with glare.
The black outside edge in the drawing above could be curved away from you making it invisible under the wrong light
(grey area is not curved so will be visible).
Direct sunlight or strong daylight could also make it impossible to see
(take bike in to a building with low or no daylight or at least find as much shade as possible)
CBF125 and CBR125 Line the mark (vertical line) below (maybe next to) the T up with the grove at the top of the hole
YBR125 Line the mark (vertical line) up with the grove at the top of the hole
(only turn the socket anticlockwise since engines don't like going in reverse)
(if your eyesight is too bad ask someone else).
3. Skip all of this step 3 if you managed step 2.
This step is not as good as step 2 so only do this step as a last resort.
You need to get the piston to the highest point in the engine; this is called Top Dead Centre (TDC).
Never turn the rear wheel in the direction that would make the bike go backwards when the bikes in gear,
that would turn the engine over in reverse.
Put the bike in to 5th gear (the back wheel needs to be turning to change between gears).
Remove the spark plug and put something like a thin screw driver down the spark plug hole.
Turn the back wheel and watch the piston move up and down
(make sure that whatever you put in the hole does not get trapped or damage anything or fall in to the engine).
When the piston moves upwards it will push whatever you put in the hole upwards,
after the piston reaches Top Dead Centre (TDC) it will go downwards.
It's essential to get the piston to the highest point in the engine to get the correct valve clearance.
4. Remove the valve cover
CBF125 and CBR125 the valve cover, covers the entire top of the engine.
YBR125 there are 2 small valve covers on top of the engine.
One covers the Exhaust valve, the other the Intake Valve.
Someone did email me about the valve cover screw threads being weak and he stripped them.
A Honda dealer mechanic told me he has had no problems, but when you tighten, use very little force.
He said you only need to slightly compress the rubber, it will then shrink and expand to maintain an air tight seal.
So be careful or ask a Honda mechanic for advice or try to remember how tight it was when you removed the valve cover.
5. Check that you can rock the valves, if you cannot,
you are on the wrong engine stroke, simply repeat step 2.
If you cannot work out if the valves are rocking, they will only rock if there's some valve clearance.
If you're on the wrong stroke there's no valve clearance (see picture below).
Valve Clearance (Exhaust valve)
Intake Valve Feeler Gauge
6. There are only 2 valves.
A Feeler Gauge is available from most car motor shops;
a feeler gauge is just a set of metal plates of various thicknesses.
The valve nearest the exhaust pipe is the exhaust valve.
And the intake valve is the one furthest away from the exhaust.
The valve clearance is the gap between the round metal rods (it's easy to assume it's a single rod since the gap is so small).
If the valve clearance is correct, you will feel very slight resistance from the feeler gauge when moving it through the gap.
If you find the valves need to be adjusted,
it's worth trying to turn the engine over several times (repeat step 2 twice then check and repeat if necessary)
to see if it is then correct, there's always a slight variation every time you turn the engine over.
To adjust a valve, loosen the nut on the valve you wish to adjust,
use pliers (or hand) to turn the top of the screw thread (or obtain the proper tool)
until the clearance is at the correct amount, then re tighten the nut.
Recheck the valve clearance after re tightening the nut, if it has gone out, loosen the nut again and readjust.
If the valve clearance changes every time you tighten the nut, you may have to hold the screw when tightening the nut
or set the clearance slightly wrong so when the nut is tightened the clearance becomes correct.
The Valve Clearances need to be set to
Intake Valve 0.08 mm, Exhaust Valve 0.12 mm
Intake Valve 0.06 mm, Exhaust Valve 0.27 mm
Intake Valve 0.08 - 0.12 mm, Exhaust Valve 0.10 - 0.14 mm
7. Reassemble everything in reverse order.