engine YAMAHA YFM90R 2021 Owner's Guide

8-17
8When riding your ATV downhill, shift your weight
as far to the rear and uphill side of the ATV as pos-
sible. Move back on the seat and sit with your
arms straight. Engine compression will do most of
the braking for you.
Use caution while descending a hill with loose or
slippery surfaces. Braking ability and traction may
be adversely affected by these surfaces. Improper
braking may also cause a loss of traction. Gradu-
ally apply only the rear brake.
Whenever possible, ride your ATV straight down-
hill. Avoid sharp angles which could allow the ATV
to tip or roll over. Carefully choose your path and
ride no faster than you will be able to react to ob-
stacles which may appear.
CROSSING A SLOPE
WARNING
EWB01633Improperly crossing hills or turning on hills
could cause loss of control or cause the ATV to
overturn.
Always follow proper procedures as de-
scribed in the Owner’s Manual.
 Avoid hills with excessively slippery or loose
surfaces.
 Avoid crossing the side of a steep hill.
 Shift your weight to the uphill side of the ATV.
OK
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9
EBU28789
PERIODIC MAINTENANCE AND ADJUSTMENT
EBU21677Periodic inspection, adjustment, and lubrication
will keep your vehicle in the safest and most effi-
cient condition possible. Safety is an obligation of
the vehicle owner and operator. The most impor-
tant points of vehicle inspection, adjustment, and
lubrication are explained on the following pages.
WARNING
EWB01842Failure to properly maintain the vehicle or per-
forming maintenance activities incorrectly may
increase your risk of injury or death during ser-
vice or while using the vehicle. If you are not fa-
miliar with vehicle service, have a Yamaha
dealer perform the service.
WARNING
EWB02563Turn off the engine when performing mainte-
nance unless otherwise specified.
A running engine has moving parts that can
catch on body parts or clothing and electri-
cal parts that can cause shocks or fires. 
Running the engine while servicing can lead
to eye injury, burns, fire, or carbon monoxide
poisoning - possibly leading to death. See
page 3-6 for more information about carbon
monoxide.

WARNING
EWB02691Brake discs, calipers, drums, and linings can
become very hot during use. To avoid possible
burns, let brake components cool before
touching them. The intervals given in the periodic maintenance
charts should be considered as a general guide
under normal riding conditions. However, DE-
PENDING ON THE WEATHER, TERRAIN, GEO-
GRAPHICAL LOCATION, AND INDIVIDUAL USE,
THE MAINTENANCE INTERVALS MAY NEED TO
BE SHORTENED.
UBD365E0.book Page 1 Monday, January 27, 2020 9:05 AM

9-3
9
EBU21747Periodic maintenance chart: Emission control systemTIPFor ATVs not equipped with an odometer or an hour meter, follow the month maintenance intervals in-
stead.
 For odometer or hour meter-equipped vehicles, follow the month maintenance interval if the ATV isn’t
ridden for the stated mileage or engine operating hours.
 Items marked with an asterisk should be performed by a Yamaha dealer as they require special tools,
data and technical skills. NO. ITEM CHECK OR MAINTENANCE
JOB INITIAL
EVERY
Whichev-
er comes first month136612
km (mi) 320
(200) 1300
(800) 2500
(1600) 2500
(1600) 5000
(3200)
hours 20 80 160 160 320
1 *Fuel line • Check fuel hoses for cracks or other damage, and
replace if necessary. √√√
2 Spark plug • Check condition and clean, regap, or replace if
necessary. √√√√√
3 *Va l ve s • Check valve clearance and adjust if necessary. √ √√√
4 *Carburetor • Check starter (choke) operation and correct if
necessary.
• Check engine idling speed and adjust if neces- sary. √√√√
5 *Crankcase breath-
er system • Check breather hose for cracks or other damage,
and replace if necessary. √√√
UBD365E0.book Page 3 Monday, January 27, 2020 9:05 AM

9-5
9
EBU2186EPeriodic maintenance chart: General maintenance and lubricationTIPFor ATVs not equipped with an odometer or an hour meter, follow the month maintenance intervals in-
stead.
 For odometer or hour meter-equipped vehicles, follow the month maintenance interval if the ATV isn’t
ridden for the stated mileage or engine operating hours.
 Items marked with an asterisk should be performed by a Yamaha dealer as they require special tools,
data and technical skills. NO. ITEM CHECK OR MAINTENANCE
JOB INITIAL
EVERY
Whichev-
er comes first month136612
km (mi) 320
(200) 1300
(800) 2500
(1600) 2500
(1600) 5000
(3200)
hours 20 80 160 160 320
1 Air filter element • Clean and replace if necessary. Every 20–40 hours (more often in wet or
dusty areas)
2 Air filter check hose • Clean. √√√√√
3 *Front brake • Check operation and correct if necessary.
• Check brake lever free play and adjust if neces-
sary. √√√√√
• Replace brake shoes. Whenever worn to the limit
4 *Rear brake • Check operation and correct if necessary.
• Check brake lever free play and adjust if neces-
sary. √√√√√
• Replace brake shoes. Whenever worn to the limit
UBD365E0.book Page 5 Monday, January 27, 2020 9:05 AM

9-7
9
15*Steering shaft • Lubricate with lithium-soap-based grease. √√√
16 *Steering system • Check operation and repair or replace if dam-
aged.
• Check toe-in and adjust if necessary. √√√√√
17 Engine oil • Change.
• Check ATV for oil leakage, and correct if neces-
sary. √ √√√
18 Engine oil strainer • Clean. √√√
19 Transmission oil • Change.
• Check ATV for oil leakage, and correct if neces-
sary. √√
20 *Moving parts and
cables • Lubricate.
√√√√
21 *Throttle lever • Check operation.
• Check throttle lever free play, and adjust if neces-
sary.
• Lubricate cable and lever housing. √√√√√
22 *Brake switch • Check operation and correct if necessary. √√√√√
23 *Switches • Check operation and correct if necessary. √√√√√
NO.
ITEM CHECK OR MAINTENANCE
JOB INITIAL
EVERY
Whichev-
er comes first month136612
km (mi) 320
(200) 1300
(800) 2500
(1600) 2500
(1600) 5000
(3200)
hours 20 80 160 160 320UBD365E0.book Page 7 Monday, January 27, 2020 9:05 AM

9-9
9
EBU23216Checking the spark plugThe spark plug is an important engine component,
which is easy to check. Since heat and deposits
will cause any spark plug to slowly erode, the
spark plug should be removed and checked in ac-
cordance with the periodic maintenance chart. In
addition, the condition of the spark plug can reveal
the condition of the engine.
To remove the spark plug1. Remove the spark plug cap.
2. Remove the spark plug as shown, with the spark plug wrench included in the tool kit. To check the spark plug
1. Check that the porcelain insulator around the center electrode of the spark plug is a medi-
um-to-light tan (the ideal color when the ATV
is ridden normally).
TIPIf the spark plug shows a distinctly different color,
the engine could be operating improperly. Do not
attempt to diagnose such problems yourself. In-
stead, have a Yamaha dealer check the ATV.
1. Spark plug cap
1
1. Spark plug wrench
1
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9
2. Check the spark plug for electrode erosion
and excessive carbon or other deposits, and
replace it if necessary.
3. Measure the spark plug gap with a wire thick- ness gauge and, if necessary, adjust the gap
to specification. To install the spark plug
1. Clean the surface of the spark plug gasket and its mating surface, and then wipe off any
grime from the spark plug threads.
2. Install the spark plug with the spark plug wrench, and then tighten it to the specified
torque.
TIPIf a torque wrench is not available when installing
the spark plug, a good estimate of the correct
torque is 1/4–1/2 turn past finger tight. However,
the spark plug should be tightened to the speci-
fied torque as soon as possible. 3. Install the spark plug cap.EBU37001Engine oilThe engine oil level should be checked before
each ride. In addition, the oil must be changed at
the intervals specified in the periodic maintenance
chart.
Specified spark plug:
NGK/CR6HSA
1. Spark plug gapSpark plug gap:0.6–0.7 mm (0.024–0.028 in)
Tightening torque:Spark plug:12 N·m (1.2 kgf·m, 8.9 lb·ft)
UBD365E0.book Page 10 Monday, January 27, 2020 9:05 AM

9-11
9To check the engine oil level
1. Park the ATV on a level surface.
2. Wait for the engine to cool.
TIPIf the engine was started before checking the oil
level, wait 10 minutes. This will allow oil to settle
into the crankcase for an accurate reading. 3. Remove the engine oil filler cap and wipe the dipstick with a clean rag.
4. Insert the dipstick into the filler hole, screw it in, and then remove it again to check the oil
level. 5. If the engine oil is at or below the minimum
level mark, add sufficient quantity of the rec-
ommended engine oil. NOTICE: Be sure the
engine oil is at the correct level, otherwise
engine damage may result.
[ECB00852]
6. Check the O-ring for damage, and then insert the dipstick and tighten the engine oil filler
cap.
To change the engine oil 1. Park the ATV on a level surface.
1. Engine oil filler cap
1
1. Engine oil filler cap
2. O-ring
3. Engine oil dipstick
4. Maximum level mark
5. Minimum level mark
1
4
5 3 2
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9-12
9
2. Start the engine, warm it up for several minu-
tes, and then turn it off.
3. Place an oil pan under the engine to collect the used oil.
4. Remove the engine oil filler cap, and then re- move the engine oil drain bolt to drain the oil
from the crankcase. NOTICE: When remov-
ing the engine oil drain bolt, the compres-
sion spring, oil strainer and O-ring will fall
out. Take care not to lose these parts.
[ECB00312]
5. Clean the oil strainer with solvent.
6. Discard the old engine oil drain bolt O-ring, and then apply a thin coat of clean engine oil
to the new O-ring.
7. Install the oil strainer, the compression spring, the new O-ring and the engine oil
drain bolt. Tighten the engine oil drain bolt to
the specified torque. NOTICE: Before in-
stalling the engine oil drain bolt, be sure to
install the O-ring, compression spring and
oil strainer.
[ECB00322]
8. Refill with the specified amount of the recom- mended engine oil.
1. Engine oil drain bolt
2. O-ring
3. Compression spring
4. Oil strainer
4321
Tightening torque:Engine oil drain bolt:15 N

9-13
9
TIPBe sure to wipe off spilled oil on any parts after the
engine and exhaust system have cooled down. NOTICEECB00331Do not use oils with a diesel specification of
“CD” or oils of a higher quality than speci-
fied. In addition, do not use oils labeled “EN-
ERGY CONSERVING II” or higher.
 Make sure that no foreign material enters the
crankcase. 9. Check the engine oil filler cap O-ring for dam-
age, and then install and tighten the engine oil
filler cap.
10. Start the engine, and then let it idle for several minutes while checking it for oil leakage. If oil
is leaking, immediately turn the engine off and
check for the cause.
11. Turn the engine off, wait at least ten minutes, and then check the oil level and correct it if
necessary.
EBU38620Why YamalubeYAMALUBE oil is a Genuine YAMAHA Part born ofthe engineers’ passion and belief that engine oil is
an important liquid engine component. We form
teams of specialists in the fields of mechanical en-
gineering, chemistry, electronics and track test-
ing, and have them develop the engine together
with the oil it will use. Yamalube oils take full ad-
vantage of the base oil’s qualities and blend in the
ideal balance of additives to make sure the final oil
clears our performance standards. Thus,
Yamalube mineral, semi synthetic and synthetic
oils have their own distinct characters and value.
Yamaha’s experience gained over many years of research and development into oil since the
1960’s helps make Yamalube the best choice for
your Yamaha engine.
UBD365E0.book Page 13 Monday, January 27, 2020 9:05 AM