engine YAMAHA YZ250LC 2002 Workshop Manual

3-38
ELECTRICAL/SPARK PLUG INSPECTION
INSP
ADJ
EC370000
ELECTRICAL
EC371001
SPARK PLUG INSPECTION
1. Remove:
9Spark plug
2. Inspect:
9Electrode 1
Wear/Damage Replace.
9Insulator color 2
Normal condition is a medium to light tan
color.
Distinctly different color Check the
engine condition.
NOTE:
When the engine runs for many hours at low
speeds, the spark plug insulator will become
sooty, even if the engine and carburetor are in
good operating condition.
3. Measure:
9Plug gap a
Use a wire gauge or thickness gauge.
Out of specification Regap.
4. Clean the plug with a spark plug cleaner if
necessary.
5. Tighten:
9Spark plug
NOTE:
9Before installing a spark plug, clean the
gasket surface and plug surface.
9Finger-tighten athe spark plug before
torquing to specification b.
Spark plug gap:
0.5~0.6 mm (0.020~0.024 in)
Standard spark plug:
BR8EG (NGK)
20 Nm (2.0 m•kg, 14 ft•lb)
5NX-9-30-3B 6/5/01 4:06 PM Page 44

4-1
SEAT, FUEL TANK AND SIDE COVERSENG
4
EC400000
ENGINE
EC4R0000
SEAT, FUEL TANK AND SIDE COVERS
Extent of removal Order Part name Q’ty Remarks
SEAT, FUEL TANK AND SIDE
COVERS REMOVAL
Preparation for Turn the fuel cock to “OFF”.
removal Disconnect the fuel hose.
1 Seat 1
2 Air scoop (left and right) 2
3 Fitting band 1 Remove on fuel tank side.
4 Bolt (fuel tank) 2
5 Fuel tank 1
6 Side cover (left) 1
Refer to “REMOVAL POINTS”.
7 Side cover (right) 1
8 Number plate 1
1
4
Extent of removal:1Seat removal2Fuel tank removal
3Side covers removal4Number plate removal
2


 

3
7 Nm (0.7 m•kg, 5.1 ft•lb)
4 Nm (0.4 m•kg, 2.9 ft•lb)
4 Nm (0.4 m•kg, 2.9 ft•lb)
7 Nm (0.7 m•kg, 5.1 ft•lb)
7 Nm (0.7 m•kg, 5.1 ft•lb)
7 Nm (0.7 m•kg, 5.1 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
7 Nm (0.7 m•kg, 5.1 ft•lb)
19 Nm (1.9 m•kg, 13 ft•lb)
19 Nm (1.9 m•kg, 13 ft•lb)
5NX-9-30-4A 6/5/01 4:12 PM Page 2

4-5
ENGRADIATOR
EC456000
HANDLING NOTE
w
Do not remove the radiator cap when the
engine and radiator are hot. Scalding hot
fluid and steam may be blown out under
pressure, which could cause serious
injury.
When the engine has cooled, open the
radiator cap by the following procedure:
Place a thick rag, like a towel, over the
radiator cap, slowly rotate the cap counter-
clockwise to the detent. This procedure
allows any residual pressure to escape.
When the hissing sound has stopped,
press down on the cap while turning coun-
terclockwise and remove it.
EC454000
INSPECTION
EC444100
Radiator
1. Inspect:
9Radiator core 1
Obstruction Blow out with compressed air
through rear of the radiator.
Bent fin Repair/replace.
5NX-9-30-4A 6/5/01 4:12 PM Page 10

4-9
ENGCARBURETOR AND REED VALVE
EC466020
HANDLING NOTE
cC
Do not loosen the screws {TPS (throttle
position sensor)} except when changing
the TPS (throttle position sensor) due to
failure because it will cause a drop in
engine performance.
EC463000
REMOVAL POINTS
EC463110
Throttle valve
1. Remove:
9Throttle valve 1
9Ring 2
9Spring (throttle valve) 3
9Mixing chamber top 4
9Throttle cable 5
NOTE:
While compressing the spring (throttle valve),
disconnect the throttle cable.
EC463401
Pilot air screw
1. Remove:
9Pilot air screw 1
NOTE:
To optimize the fuel flow at a smaller throttle
opening, each machine’s pilot air screw has
been individually set at the factory. Before
removing the pilot air screw, turn it in fully and
count the number of turns. Record this number
as the factory-set number of turns out.
5NX-9-30-4A 6/5/01 4:12 PM Page 18

3. Install:
9Pilot air screw 1
9Throttle stop screw 2
4. Install:
9Starter plunger 1
4-14
ENGCARBURETOR AND REED VALVE
Pilot air screw:
1 turn out
*1-3/8 turns out
(for reference only)
Note the following installation points:
9Turn in the pilot air screw until it is lightly
seated.
9Turn out the pilot air screw by the number
of turns recorded before removing.
5. Install:
9Pilot jet 1
9Main jet 2
2. Install:
9O-ring 1
9Solenoid valve 2
9Clamp 3
9Screw (clamp) 4
To carburetor 5.
cC
9
Before installing the solenoid valve, blow
air on the solenoid valve and its installing
location on the carburetor in order to
remove any foreign particles such as chips
etc.
9Apply the engine oil on the solenoid valve
thread.
*For EUROPE
5NX-9-30-4A 6/5/01 4:12 PM Page 28

4-18
CYLINDER HEAD, CYLINDER AND PISTONENG
EC470000
CYLINDER HEAD, CYLINDER AND PISTONEC478000
CYLINDER HEAD AND CYLINDER
Extent of removal Order Part name Q’ty Remarks
CYLINDER HEAD AND
CYLINDER REMOVAL
Preparation for Seat and fuel tank Refer to “SEAT, FUEL TANK AND
removal SIDE COVERS” section.
Exhaust pipe and silencer Refer to ”EXHAUST PIPE AND
SILENCER” section.
Radiator hose 1 Disconnect at cylinder head side.
Carburetor Refer to “CARBURETOR AND REED
VALVE” section.
1 Spark plug 1
2 Engine bracket 2
3 Nut (cylinder head) 6
Loosen each nut 1/4 turn, and remove
them after all nuts are loosened.
4 Cylinder head 1
5 Power valve housing 1
6 Push rod 1 Refer to “REMOVAL POINTS”.
7 Nut (cylinder) 4
8 Cylinder 1
Extent of removal:1Cylinder head removal2Cylinder removal
1
2
34 Nm (3.4 m•kg, 24 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
20 Nm (2.0 m•kg, 14 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
25 Nm (2.5 m•kg, 18 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
35 Nm (3.5 m•kg, 25 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
5NX-9-30-4A 6/5/01 4:12 PM Page 36

4-29
CYLINDER HEAD, CYLINDER AND PISTONENG
7. Install:
9O-ring 1
9Side holder 2
9Screw (side holder) 3
9YPVS breather hose 4
NOTE:
9Apply the lithium soap base grease on the O-
rings.
9Install the side holder with its projection a
facing upward.
8. Install:
9Gasket (power valve cover) 1
9Power valve cover 2
9Screw (power valve cover) 3
NOTE:
9Install the gasket with its cut afacing down-
ward and the seal print side toward the power
valve cover.
9Install the power valve cover so that the arrow
mark bfaces upward.
EC475233
Piston ring and piston
1. Install:
9Piston ring 1
NOTE:
9Take care not to scratch the piston or damage
the piston ring.
9Align the piston ring gap with the pin 2.
9After installing the piston ring, check the
smooth movement of it.
2. Install:
9Gasket (cylinder) 1
9Small end bearing 2
9Dowel pin 3
NOTE:
9Apply the engine oil onto the bearing (crank-
shaft and connecting rod) and connecting rod
big end washers.
9Install the gasket with the seal print side
toward the crankcase.
4 Nm (0.4 m•kg, 2.9 ft•lb)
4 Nm (0.4 m•kg, 2.9 ft•lb)
5NX-9-30-4B 6/7/01 1:56 PM Page 4

4-30
CYLINDER HEAD, CYLINDER AND PISTONENG
EC475382
Cylinder head and cylinder
1. Apply:
9Engine oil
To piston 1piston ring 2and cylinder sur-
face.
2. Install:
9Cylinder 1
cC
Make sure the piston rings are properly
positioned. Install the cylinder with one
hand while compressing the piston rings
with the other hand.
NOTE:
After installing, check the smooth movement of
the piston.
3. Install:
9Nut (cylinder) 1
NOTE:
Tighten the nuts in stage, using a crisscross pat-
tern.
3. Install:
9Piston 1
9Piston pin 2
9Piston pin clip 3
NOTE:
9The arrow aon the piston dome must point to
exhaust side.
9Before installing the piston pin clip, cover the
crankcase with a clean rag to prevent the pis-
ton pin clip from falling into the crankcase cavi-
ty.
cC
Do not allow the clip open ends to meet the
piston pin slot b b
.
35 Nm (3.5 m•kg, 25 ft•lb)
5NX-9-30-4B 6/7/01 1:56 PM Page 6

4-32
CYLINDER HEAD, CYLINDER AND PISTONENG
9. Install:
9Spark plug 1
9Spark plug cap 2
8. Install:
9Engine bracket 1
9Bolt (engine bracket) 2
9Engine mounting bolt (upper) 3
20 Nm (2.0 m•kg, 14 ft•lb)
34 Nm (3.4 m•kg, 24 ft•lb)
69 Nm (6.9 m•kg, 50 ft•lb)
5NX-9-30-4B 6/7/01 1:56 PM Page 10

4-33
CLUTCH AND PRIMARY DRIVEN GEARENG
Extent of removal Order Part name Q’ty Remarks
CLUTCH PLATE AND
FRICTION PLATE REMOVAL
Preparation for Drain the transmission oil.
Refer to “TRANSMISSION OIL REPLACE-
removalMENT“ section in the CHAPTER 3.
Bolt (brake pedal) Shift the brake pedal downward.
Rotor and stator Refer to
“CDI MAGNETO“section.
Clutch cable Disconnect at engine side.
1 Clutch cover 1
2 Screw (clutch spring) 6
3 Clutch spring 6
4 Pressure plate 1
5 Friction plate 8
6 Clutch plate 7
Extent of removal:1Clutch plate and friction plate removal
EC490000
CLUTCH AND PRIMARY DRIVEN GEAREC498000
CLUTCH PLATE AND FRICTION PLATE
1
10 Nm (1.0 m•kg, 7.2 ft•lb)
10 Nm (1.0 m•kg, 7.2 ft•lb)
5NX-9-30-4B 6/7/01 1:56 PM Page 12