2021 YAMAHA MT-125 engine

Periodic maintenance an d a djustment
6-35
6
EAU86350
Trou bleshootin g chart
Check the fuel level in
the fuel tank.1. Fuel
There is enough fuel.
There is no fuel.
Check the battery.
Supply fuel.
The engine does not start.
Check the battery.
Try starting the engine.4. Compression
There is compression.
There is no compression.
The engine does not start.
Have a Yamaha dealer check the vehicle.Have a Yamaha dealer check the vehicle.
Remove the spark plug
and check the electrodes.3. Ignition
Wipe off with a dry cloth and correct the
spark plug gap, or replace the spark plug(s). Have a Yamaha dealer check the vehicle.
The engine does not start.
Check the compression.
Operate the electric starter.2. Battery
The engine turns over
quickly.
The engine turns over
slowly.
The battery is good.
DryWet
Try starting the engine.
Check the battery lead connections,
and charge the battery if necessary.
The engine does not start.
Check the ignition.
UB6GE1E0.book Page 35 Thursday, July 30, 2020 1:08 PM

Periodic maintenance an d a djustment
6-36
6
EAU86420
En gine overheatin g
WARNING
EWAT1041
Do not remove the ra diator cap when the en gine an d ra diator are hot. Scal din g hot flui d an d steam may be
b lown out un der pressure, which coul d cause serious injury. Be sure to wait until the en gine has coole d.
 Place a thick ra g, like a towel, over the ra diator cap, an d then slowly rotate the cap counterclockwise to the
d etent to allow any resi dual pressure to escape. When the hissin g soun d has stoppe d, press d own on the cap
while turnin g it counterclockwise, an d then remove the cap.TIPIf coolant is not available, tap water can be temporarily used instead, provided that it is changed to the recommended cool-
ant as soon as possible.
Wait until the
engine has cooled.
Check the coolant level in the
reservoir and radiator.
The coolant level
is OK.The coolant level is low.
Check the cooling system
for leakage.
Have a Yamaha dealer checkand repair the cooling system.Add coolant. (See TIP.)
Start the engine. If the engine overheats again,
have a
Yamaha dealer check
and repair the cooling system.
There is
leakage.
There is
no leakage.
UB6GE1E0.book Page 36 Thursday, July 30, 2020 1:08 PM

Motorcycle care and stora ge
7-3
7
will
become slippery, which
coul d cause loss of control.
Thorou ghly clean the surfaces
of these parts before operatin g
the vehicle.
[EWA20650]
5. Treat rubber, vinyl, and unpainted plastic parts with a suitable care
product.
6. Touch up minor paint damage caused by stones, etc.
7. Wax all painted surfaces using a non-abrasive wax or use a detail
spray for motorcycles.
8. When finished cleaning, start the engine and let it idle for several
minutes to help dry any remaining
moisture.
9. If the headlight lens has fogged up, start the engine and turn on
the headlight to help remove the
moisture.
10. Let the vehicle dry completely be- fore storing or covering it.NOTICE
ECA26320
Do not apply wax to ru bber or
unpainte d plastic parts. 
Do not use a brasive polishin g
compoun ds as they will wear
away the paint.
 Apply sprays an d wax sparin gly.
Wipe off excess afterwar ds.
WARNING
EWA20660
Contaminants left on the brakes or
tires can cause loss of control.  Make sure there is no lu bricant
or wax on the brakes or tires.
 If necessary, wash the tires with
warm water an d a mil d d eter-
g ent.
 If necessary, clean the b rake
d iscs an d pa ds with b rake
cleaner or acetone.
 Before ri din g at hi gher spee ds,
test the vehicle’s brakin g per-
formance an d cornerin g b ehav-
ior.
EAU83472
Stora geAlways store the vehicle in a cool, dry
place. If necessary, protect it against
dust with a porous cover. Be sure the
engine and the exhaust system are
cool before covering the vehicle. If the
vehicle often sits for weeks at a time
between uses, the use of a quality fuel
stabilizer is recommended after each
fill-up.NOTICE
ECA21170
 Storin g the vehicle in a poorly
ventilate d room or coverin g it
with a tarp, while it is still wet,
will allow water an d humi dity to
seep in an d cause rust.
 To prevent corrosion, avoi d
d amp cellars, stab les (because
of the presence of ammonia)
an d areas where stron g chemi-
cals are stored .Long term stora ge
Before storing the vehicle long term (60
days or more):
UB6GE1E0.book Page 3 Thursday, July 30, 2020 1:08 PM

Motorcycle care and stora ge
7-4
7 1. Make all necessary repairs and
perform any outstanding mainte-
nance.
2. Follow all instructions in the Care section of this chapter.
3. Fill up the fuel tank, adding fuel stabilizer according to product in-
structions. Run the engine for 5
minutes to distribute treated fuel
through the fuel system.
4. For vehicles equipped with a fuel cock: Turn the fuel cock lever to
the off position.
5. For vehicles with a carburetor: To prevent fuel deposits from build-
ing up, drain the fuel in the carbu-
retor float chamber into a clean
container. Retighten the drain bolt
and pour the fuel back into the fuel
tank.
6. Use a quality engine fogging oil according to product instructions
to protect internal engine compo-
nents from corrosion. If engine
fogging oil is not available, per-
form the following steps for each
cylinder:a. Remove the spark plug cap and spark plug. b. Pour a teaspoonful of engine
oil into the spark plug bore.
c. Install the spark plug cap onto the spark plug, and then place
the spark plug on the cylinder
head so that the electrodes are
grounded. (This will limit spark-
ing during the next step.)
d. Turn the engine over several times with the starter. (This will
coat the cylinder wall with oil.)
WARNING! To prevent d am-
a g e or injury from sparkin g,
make sure to groun d the
spark plu g electro des while
turnin g the en gine over.
[EWA10952]
e. Remove the spark plug cap
from the spark plug, and then
install the spark plug and the
spark plug cap.
7. Lubricate all control cables, piv- ots, levers and pedals, as well as
the sidestand and centerstand (if
equipped).
8. Check and correct the tire air pressure, and then lift the vehicle
so that all wheels are off the
ground. Otherwise, turn the wheels a little once a month in or-
der to prevent the tires from be-
coming degraded in one spot.
9. Cover the muffler outlet with a plastic bag to prevent moisture
from entering it.
10. Remove the battery and fully charge it, or attach a maintenance
charger to keep the battery opti-
mally charged. NOTICE: Confirm
that the battery an d its char ger
are compati ble. Do not char ge a
VRLA battery with a convention-
al char ger.
[ECA26330]
TIP If the battery will be removed,
charge it once a month and store
it in a temperate location between
0-30 °C (32-90 °F).
 See page 6-30 for more informa-
tion on charging and storing the
battery.
UB6GE1E0.book Page 4 Thursday, July 30, 2020 1:08 PM

Specifications
8-1
8
Dimensions:Overall length:1960 mm (77.2 in)
Overall width:
800 mm (31.5 in)
Overall height: 1065 mm (41.9 in)
Seat height: 810 mm (31.9 in)
Wheelbase:
1325 mm (52.2 in)
Ground clearance: 160 mm (6.30 in)
Minimum turning radius: 2.9 m (9.51 ft)Wei ght:Curb weight:
142 kg (313 lb)Engine:Combustion cycle:
4-stroke
Cooling system: Liquid cooled
Valve train: SOHC
Number of cylinders:
Single cylinder
Displacement: 124 cm³
Bore × stroke:
52.0 × 58.6 mm (2.05 × 2.31 in)
Starting system:
Electric starter
Engine oil:Recommended brand:
SAE viscosity grades:
10W-40
Recommended engine oil grade: API service SG type or higher, JASO
standard MA
Engine oil quantity: Oil change:0.85 L (0.90 US qt, 0.75 Imp.qt)
With oil filter removal: 0.95 L (1.00 US qt, 0.84 Imp.qt)Coolant quantity:Coolant reservoir (up to the maximum level
mark):
0.15 L (0.16 US qt, 0.13 Imp.qt)
Radiator (including all routes): 0.49 L (0.52 US qt, 0.43 Imp.qt)Fuel:Recommended fuel:
Unleaded gasoline (E10 acceptable)
Octane number (RON): 95
Fuel tank capacity: 10 L (2.6 US gal, 2.2 Imp.gal)
Fuel reserve amount:
3.0 L (0.79 US gal, 0.66 Imp.gal)
Fuel injection:Throttle body:ID mark:BK71 00Drivetrain:Gear ratio:
1st: 2.833 (34/12)
2nd:
1.875 (30/16)
3rd: 1.364 (30/22)
4th: 1.143 (24/21)
5th:
0.957 (22/23)
6th: 0.840 (21/25)Front tire:Type:
Tubeless
Size: 100/80-17M/C 52S
Manufacturer/model: MICHELIN PILOT STREETRear tire:Type:Tubeless
Size: 140/70-17M/C 66S
Manufacturer/model:
MICHELIN PILOT STREET
UB6GE1E0.book Page 1 Thursday, July 30, 2020 1:08 PM

Consumer information
9-1
9
EAU53562
Id entification num bersRecord the vehicle identification num-
ber, engine serial number, and the
model label information in the spaces
provided below. These identification
numbers are needed when registering
the vehicle with the authorities in your
area and when ordering spare parts
from a Yamaha dealer.
VEHICLE IDENTIFICATION NUMBER:
ENGINE SERIAL NUMBER:
MODEL LABEL INFORMATION:
EAU26401
Vehicle i dentification num ber
The vehicle identification number is
stamped into the steering head pipe.
Record this number in the space pro-
vided.TIPThe vehicle identification number is
used to identify your motorcycle and
may be used to register your motorcy-
cle with the licensing authority in your
area.
EAU26442
En gine serial num ber
The engine serial number is stamped
into the crankcase.
EAU26481
Mo del la bel
1. Vehicle identification number
1
1. Engine serial number
1. Model label
1
1
UB6GE1E0.book Page 1 Thursday, July 30, 2020 1:08 PM

Consumer information
9-2
9The model label is affixed to the frame
under the seat. (See page 3-19.) Re-
cord the information on this label in the
space provided. This information will
be needed when ordering spare parts
from a Yamaha dealer.
EAU69910
Dia
gnostic connectorThe diagnostic connector is located as
shown.
EAU85300
Vehicle data recor din gThis model’s ECU stores certain vehi-
cle data to assist in the diagnosis of
malfunctions and for research, statisti-
cal analysis and development purpos-
es.
Although the sensors and recorded
data will vary by model, the main data
points are:
 Vehicle status and engine perfor-
mance data
 Fuel-injection and emission-relat-
ed data
This data will be uploaded only when a
special Yamaha diagnostic tool is at-
tached to the vehicle, such as when
maintenance checks or service proce-
dures are performed.
Vehicle data uploaded will be handled
appropriately according to the follow-
ing Privacy Policy.
Privacy Policy
1. Diagnostic connector
1
https://www.yamaha-motor.eu/eu/
privacy/privacy-policy.aspx
UB6GE1E0.book Page 2 Thursday, July 30, 2020 1:08 PM

10-1
10
IndexAABS ....................................................... 3-15
ABS warning light ................................... 3-3
Air filter element and check hose,
replacing and cleaning ....................... 6-14BBattery .................................................. 6-30
Brake and clutch levers, checking and lubricating ........................................... 6-27
Brake and shift pedals, checking and lubricating ........................................... 6-27
Brake fluid, changing ............................ 6-24
Brake fluid level, checking .................... 6-22
Brake lever ............................................ 3-14
Brake light switches.............................. 6-21
Brake pedal........................................... 3-14
Brake pedal free play, adjusting ........... 6-21CCables, checking and lubricating ......... 6-26
Canister................................................... 6-9
Care ........................................................ 7-1
Catalytic converter ................................ 3-19
Clutch lever ........................................... 3-13
Clutch lever free play, adjusting ........... 6-19
Coolant ................................................. 6-13
Coolant temperature warning light ......... 3-2
Cowlings, removing and installing .......... 6-8DData recording, vehicle ........................... 9-2
Diagnostic connector.............................. 9-2
Dimmer switch ...................................... 3-13
Drive chain, cleaning and lubricating .... 6-25
Drive chain slack................................... 6-24
EEngine break-in ...................................... 5-1
Engine idling speed .............................. 6-15
Engine oil and oil filter element ............ 6-10
Engine overheating ............................... 6-36
Engine serial number .............................. 9-1FFront and rear brake pads, checking ... 6-22
Front brake lever free play, checking ... 6-20
Front fork, checking ............................. 6-29
Fuel ....................................................... 3-16
Fuel consumption, tips for reducing ...... 5-4
Fuel tank breather hose and overflow hose ................................................... 3-18
Fuel tank cap ........................................ 3-16
Fuses, replacing ................................... 6-31HHandlebar switches .............................. 3-12
Helmet holder ....................................... 3-20
High beam indicator light ....................... 3-2
Horn switch .......................................... 3-13IIdentification numbers ............................ 9-1
Ignition circuit cut-off system ............... 3-21
Indicator lights and warning lights ......... 3-2LLicense plate light bulb, replacing........ 6-33MMain switch/steering lock....................... 3-1
Maintenance and lubrication, periodic ... 6-4
Maintenance, emission control system.................................................. 6-3
Malfunction indicator light (MIL) ............. 3-3
Matte color, caution ............................... 7-1 Model label .............................................9-1
Multi-function meter unit .........................3-4
NNeutral indicator light ..............................3-2PParking ....................................................5-4
Part locations ..........................................2-1
Pass switch ...........................................3-13RRear suspension, lubricating ................6-28SSafety information ...................................1-1
Seat .......................................................3-19
Shifting ....................................................5-3
Shift light .................................................3-3
Shift pedal .............................................3-14
Sidestand ..............................................3-20
Sidestand, checking and lubricating.....6-28
Spark plug, checking ..............................6-8
Specifications .........................................8-1
Start/Engine stop switch .......................3-13
Starting the engine ..................................5-2
Steering, checking ................................6-29
Storage ...................................................7-3
Supporting the motorcycle ...................6-34TTail/brake light ......................................6-32
Throttle grip and cable, checking and
lubricating ...........................................6-26
Throttle grip free play, adjusting ...........6-16
Tires ......................................................6-17
Tool kit ....................................................6-2
Troubleshooting ....................................6-34
Troubleshooting chart ...........................6-35
UB6GE1E0.book Page 1 Thursday, July 30, 2020 1:08 PM