engine YAMAHA YFM90R 2021 Owners Manual

Your ATV was delivered with an adjustable speed limiter. Yamaha recommends that all beginners start off
to limit the amount of speed available while they learn. The adjusting screw may be gradually turned out
as the beginner becomes more familiar with operating the ATV. Parents should decide when to adjust the
ATV for more power as their youngster’s riding skills improve. Once the ATV rider can operate with skill at
the top speed permitted by adjusting the vehicle speed limiter alone, the engine speed limiter plate can be
removed. Since removal of this limiter will result in a significant increase in power, turn the vehicle speed
limiter adjusting screw completely back in again; adjust it out in stages as done previously.
If your child is a beginner or is inexperienced, he or she should take a training course.UBD365E0.book Page 2 Monday, January 27, 2020 9:05 AM

EBU17421
TABLE OF CONTENTS
VEHICLE IDENTIFICATION ............................ 1-1Identification numbers ................................ 1-1
LOCATION OF THE WARNING AND
SPECIFICATION LABELS .............................. 2-1
SAFETY INFORMATION ................................ 3-1
DESCRIPTION................................................ 4-1 Left view ...................................................... 4-1
Right view.................................................... 4-1
Controls and instruments............................ 4-2
INSTRUMENT AND CONTROL
FUNCTIONS ................................................... 5-1 Main switch ................................................. 5-1
Indicator lights............................................. 5-1
Handlebar switches .................................... 5-2
Throttle lever ............................................... 5-2
Speed limiter ............................................... 5-3
Front brake lever ......................................... 5-5
Rear brake lever .......................................... 5-5
Parking brake .............................................. 5-5
Drive select lever ......................................... 5-6 Kickstarter .................................................. 5-7
Fuel tank cap .............................................. 5-8
Fuel ............................................................. 5-9
Fuel cock .................................................. 5-10
Starter (choke) ......................................... 5-12
Seat .......................................................... 5-13
Front shock absorbers ............................. 5-13
Rear shock absorber ................................ 5-15
PRE-OPERATION CHECKS .......................... 6-1 Fuel ............................................................. 6-3
Engine oil .................................................... 6-3
Transmission .............................................. 6-3
Front and rear brakes ................................. 6-3
Throttle lever ............................................... 6-3
Drive chain .................................................. 6-3
Tires ............................................................ 6-3
Chassis fasteners ....................................... 6-6
Instruments and switches........................... 6-6
OPERATION .................................................. 7-1 Starting a cold engine................................. 7-1
Starting a warm engine............................... 7-3UBD365E0.book Page 1 Monday, January 27, 2020 9:05 AM

Operating the drive select lever and driving in reverse ....................................... 7-3
Engine break-in ........................................... 7-4
Parking ........................................................ 7-5
Parking on a slope ...................................... 7-5
Accessories and loading ............................. 7-6
RIDING YOUR ATV ........................................ 8-1 GETTING TO KNOW YOUR ATV ................ 8-2
RIDE WITH CARE AND GOOD JUDGMENT .............................................. 8-2
BE CAREFUL WHERE YOU RIDE ............... 8-9
TURNING YOUR ATV................................ 8-12
CLIMBING UPHILL ................................... 8-13
RIDING DOWNHILL .................................. 8-16
CROSSING A SLOPE................................ 8-17
CROSSING THROUGH SHALLOW WATER .................................................... 8-18
RIDING OVER ROUGH TERRAIN ............. 8-20
SLIDING AND SKIDDING .......................... 8-20
WHAT TO DO IF... ..................................... 8-21
PERIODIC MAINTENANCE AND
ADJUSTMENT................................................ 9-1 Owner’s Manual and tool kit ....................... 9-2
Periodic maintenance chart: Emission control system .......................................... 9-3 Periodic maintenance chart: General
maintenance and lubrication .................... 9-5
Checking the spark plug............................. 9-9
Engine oil .................................................. 9-10
Why Yamalube.......................................... 9-13
Transmission oil ........................................ 9-14
Cleaning the air filter element ................... 9-15
Cleaning the spark arrester ...................... 9-18
Adjusting the carburetor ........................... 9-19
Adjusting the engine idling speed ............ 9-19
Adjusting the throttle lever free play ......... 9-20
Valve clearance......................................... 9-21
Brakes....................................................... 9-21
Checking the front and rear brake shoes ...................................................... 9-21
Adjusting the front brake lever free play... 9-23
Adjusting the rear brake lever free play .... 9-24
Drive chain slack....................................... 9-25
Lubricating the drive chain ....................... 9-27
Checking and lubricating the cables ........ 9-28
Checking and lubricating the front and rear brake levers ..................................... 9-28
Checking the wheel hub bearings ............ 9-29
Lubricating the front arm pivots ............... 9-29
Lubricating the front knuckle pivots ......... 9-30
Lubricating the steering shaft ................... 9-30
Battery ...................................................... 9-31
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3-4
3
Never maintain an ATV without proper knowl-
edge. Contact an authorized ATV dealer to in-
form you on basic ATV maintenance. Certain
maintenance can only be carried out by certified
staff.
In the event of an accident 1. If serious injuries are incurred, you may be safer staying next to your vehicle; it may
shield you from oncoming riders. Depending
on your injuries and position in the trail; you
must make a judgment call as to whether you
should move to a position off the trail. If you
have sustained head, neck, or back injuries,
or cannot feel your limbs, you should not
move. Keep your helmet on and remain mo-
tionless. Lie down next to your vehicle and
wait for help.
2. If less serious injuries are incurred and you can walk, then move to a position off the trail.
Check yourself for injuries and apply first aid
as needed.
3. If your injuries are light, when safe to do so, move your vehicle off the trail to avoid colli-
sions with oncoming riders. Check for injuries
and apply first aid as necessary. If you can
physically operate your vehicle, inspect your vehicle. If the vehicle is in safe operating con-
dition and you can safely operate it, restart it
and ride gently back to camp or other known
location where you can receive medical at-
tention. If necessary, contact your riding party
or local authorities to let them know where
you are and what has happened.
In the event of a breakdown 1. If your vehicle will not restart or is not in safe operating condition, turn off the main switch
and engine stop switch. If the breakdown oc-
curs at twilight or night, leave the main switch
on so that your lights may warn other riders of
your stopped vehicle.
2. Get out of danger. Check for oncoming vehi- cles and when safe, push your vehicle to the
side of the trail or even off the main trail to
avoid any possible collisions with oncoming
riders. If you cannot move your vehicle by
yourself, when safe, walk to the nearest van-
tage point and signal an oncoming rider to
help you push your vehicle to a safe place off
the trail.
3. Inspect your vehicle for any immediate haz- ardous conditions. The most obvious hazard-
ous conditions are leaking fuel and
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3-6
3gerous loss of lights or engine power. Also, the
battery tends to discharge more quickly if elec-
trical accessories are added.
WARNING
EWB00073Avoid Carbon Monoxide Poisoning
All engine exhaust contains carbon monoxide,
a deadly gas. Breathing carbon monoxide can
cause headaches, dizziness, drowsiness, nau-
sea, confusion, and eventually death.
Carbon Monoxide is a colorless, odorless,
tasteless gas which may be present even if you
do not see or smell any engine exhaust. Deadly
levels of carbon monoxide can collect rapidly
and you can quickly be overcome and unable
to save yourself. Also, deadly levels of carbon
monoxide can linger for hours or days in en-
closed or poorly ventilated areas. If you experi-
ence any symptoms of carbon monoxide
poisoning, leave the area immediately, get
fresh air, and SEEK MEDICAL TREATMENT.

Do not run engine indoors. Even if you try to
ventilate engine exhaust with fans or open
windows and doors, carbon monoxide can
rapidly reach dangerous levels. 
Do not run engine in poorly ventilated or par-
tially enclosed areas such as barns, garages,
or carports.
 Do not run engine outdoors where engine
exhaust can be drawn into a building through
openings such as windows and doors.

WARNING
EWB02592When transporting the ATV in another vehicle,
be sure it is kept upright and that the fuel cock
is in the “OFF” position. Otherwise, fuel may
leak out of the carburetor or fuel tank.
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4-1
4
EBU17681
DESCRIPTION
EBU17691Left view
EBU17701Right view
1. Fuel tank cap
2. Fuel cock
3. Air filter
4. Fuse
5. Battery
6. Rear shock absorber
7. Kickstarter lever cap
1 2 3 4,576
1. Spark arrester
2. Reflector
3. Tool kit
4. Front shock absorber
5. Spark plug
6. Engine oil filler cap
7. Transmission oil filler cap123 4
76 5
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5-1
5
EBU17739
INSTRUMENT AND CONTROL FUNCTIONS
EBU33850Main switchThe positions of the main switch are as follows:
ON
All electrical systems are supplied with power, and
the engine can be started. The key cannot be re-
moved.
OFF
All electrical systems are off. The key can be re-
moved.
EBU17794Indicator lightsEBU17861Neutral indicator light “ ”
This indicator light comes on when the transmis-
sion is in the neutral position.EBU17831Reverse indicator light “ ”
This indicator light comes on when the transmis-
sion is in the reverse position.
ON
OFF
1. Reverse indicator light

5-2
5
EBU18068Handlebar switchesEBU18081Engine stop switch “ / ”
Set this switch to “ ” before starting the engine.
The engine stop switch controls the ignition and
stops the engine when it is running. Use this
switch to stop the engine in an emergency situa-
tion. The engine will not start or run when this
switch is set to “ ”.
EBU18102Start switch “ ”
Push this switch to crank the engine with the start-
er. See the starting instructions on page 7-1 prior
to starting the engine.EBU18286Throttle leverPush the throttle lever to increase engine speed.
Spring tension returns the lever to the rest position
when released. Always check that the throttle le-
ver returns normally before starting the engine.
1. Engine stop switch “ / ”
2. Start switch “ ”
21
1. Throttle lever
1
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5-3
5
EBU33802Speed limiterYour ATV was delivered with two speed limiter de-
vices. A vehicle speed limiter adjusting screw and
a removable engine speed limiter plate. Yamaha
recommends that all beginning riders start off with
the vehicle speed limiter adjusting screw fully
turned in and the engine speed limiter plate in-
stalled to limit the amount of speed available while
they learn.
The vehicle speed limiter keeps the throttle from
fully opening, even when the throttle lever is
pushed to the maximum. The adjusting screw may
be gradually turned out as the beginner becomes
more familiar with operating the ATV. Parents
should decide when to adjust the ATV for more
power as their youngster’s riding skills improve.
Once the ATV rider can operate with skill at the top
speed permitted by adjusting the vehicle speed
limiter alone, the engine speed limiter plate can be
removed. Since removal of this plate will result in
a significant increase in power, turn the vehicle
speed limiter adjusting screw completely back in
again and adjust it out in stages as done previous-
ly.Vehicle speed limiter
1. Loosen the locknut.
2. To increase the maximum engine power available and the maximum speed of the ATV,
turn the adjusting screw in direction (a). To
decrease the maximum engine power avail-
able and the maximum speed of the ATV, turn
the adjusting screw in direction (b). Do not
turn the adjusting screw out more than 14.5
mm (0.57 in) or the throttle cable could be
damaged. Always make sure the throttle lever
free play is adjusted to 1.5–4.0 mm (0.06–
0.16 in). (See page 9-20.) WARNING! Im-
proper adjustment of the speed limiter and
throttle could cause throttle cable damage
or improper throttle operation. You could
lose control, resulting in an accident.
[EWB00242]
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5-4
5
3. Tighten the locknut.
Engine speed limiter plate
When the engine speed limiter plate is installed,
engine speed is limited to 4900 r/min. When this
plate is removed, top engine speed (8500 r/min) is
available.
Remove the plate as follows. 1. Remove the seat. (See page 5-13.)
2. Loosen the screws, and then remove the plate as shown. 3. Tighten the screws.
4. Install the seat.
TIPStore the engine speed limiter plate with the Own-
er’s Manual so that it is readily available whenever
you want to reduce top engine speed.
1. Locknut
2. Adjusting screw
3. No more than 14.5 mm (0.57 in)
1
3
2
(b)
(a)
1. Screw
2. Engine speed limiter plate
12
UBD365E0.book Page 4 Monday, January 27, 2020 9:05 AM