air filter YAMAHA YZ125LC 2012 User Guide

4-9
CARBURETOR AND REED VALVE
HANDLING NOTE
Do not disassemble the venturi
block "1" and main nozzle "2" be-
cause it will cause a drop in carbu-
retor performance.
REMOVING THE THROTTLE
VALVE
1. Remove:
• Throttle valve "1"
• Ring "2"
• Spring (throttle valve) "3"
• Mixing chamber top "4"
• Throttle cable "5"
While compressing the spring (throt-
tle valve), disconnect the throttle ca-
ble.
CHECKING THE CARBURETOR
1. Inspect:
• Carburetor body
Contamination→Clean.
When cleaning the main air pas-
sage "a", do not blow air at the fil-
ter side because it will clog the
passage with mud or sand.
• Use a petroleum based solvent for
cleaning. Blow out all passages and
jets with compressed air.
• Never use a wire.
• When cleaning the main air pas-
sage, blow air through it while cov-
ering the nozzle with a clean rag.
2. Inspect:
•Main jet "1"
• Main jet holder "2"
• Pilot jet "3"
Contamination→Clean.
• Use a petroleum based solvent for
cleaning. Blow out all passages and
jets with compressed air.
• Never use a wire.
CHECKING THE NEEDLE VALVE
1. Inspect:
• Needle valve "1"
• Valve seat "2"
Grooved wear "a" →Replace.
Dust "b" →Clean.
Always replace the needle valve and
valve seat as a set.
CHECKING THE THROTTLE
VALVE
1. Check:
• Free movement
Stick→Repair or replace.
Insert the throttle valve "1" into the
carburetor body, and check for free
movement.
CHECKING THE JET NEEDLE
1. Inspect:
• Jet needle "1"
Bends/wear→Replace.
• Clip groove
Free play exists/wear→Replace.
• Clip position
CHECKING THE FILTER
1. Inspect:
•Filter "1"
Damage→Replace.
Inspect the filter as it is assembled to
the carburetor. Do not remove the fil-
ter except when replacing it.
MEASURING AND ADJUSTING
THE FUEL LEVEL
1. Measure:
• Fuel level "a"
Out of specification→Adjust.
Fuel level Measurement and ad-
justment steps:
a. Remove the drain plug.
b. Connect the fuel level gauge
adapter "2" and fuel level gauge
"1" to the float chamber.
c. Hold the fuel level gauge vertical-
ly next to the float chamber mat-
ing surface.
Standard clip position:
No.3 Groove
Fuel level:
9.5–10.5 mm (0.37–0.41
in)
Above the float cham-
ber mating surface
Fuel level gauge adapter:
YM-01470/90890-01470
Fuel level gauge:
YM-1312-A/90890-
01312

4-11
CARBURETOR AND REED VALVE
• After installing the needle valve to
the float, install them to the carbure-
tor.
• Check the float for smooth move-
ment.
5. Install:
• Float chamber "1"
• Plate "2"
• Screw (float chamber) "3"
6. Install:
• Jet needle "1"
• Needle holder "2"
To throttle valve "3".
7. Install:
• Throttle cable "1"
• Locknut "2"
• Mixing chamber top "3"
• Spring (throttle valve) "4"
• Ring "5"
• Throttle valve "6"
• While compressing the spring, con-
nect the throttle cable.
• Align the projection "a" on the ring
with the groove "b" in the needle
holder "7".
8. Install:
• Mixing chamber top "1"
• Screw (mixing chamber top) "2"
To carburetor "3".
After installing, check the throttle grip
for smooth movement.
9. Install:
• Air vent hose "1"
• Overflow hose "2"
•Clamp "3"
Pass the air vent hose at the rear (on
the air cleaner side) of the throttle ca-
ble "4".
INSTALLING THE CARBURETOR
1. Install:
• Carburetor "1"
Install the projection between the car-
buretor joint slots.
2. Tighten:
• Bolt (air filter joint) "1"
• Bolt (carburetor joint) "2"
3. Clamp:
• Air vent hose "1"
• Overflow hose "2"
Refer to "CABLE ROUTING DIA-
GRAM" section in the CHAPTER
2.
4. Adjust:
• Idle speed
Refer to "ADJUSTING THE EN-
GINE IDLING SPEED" section in
the CHAPTER 3. Locknut:
4 Nm (0.4 m•kg, 2.9
ft•lb)
Bolt (air filter joint):
2 Nm (0.2 m•kg, 1.4
ft•lb)
Bolt (carburetor joint):
2 Nm (0.2 m•kg, 1.4
ft•lb)

5-38
REAR SHOCK ABSORBER
REAR SHOCK ABSORBER
REMOVING THE REAR SHOCK ABSORBER
Order Part name Q'ty Remarks
Hold the machine by placing the suitable stand
under the engine.Refer to "HANDLING NOTE".
Seat Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section in the CHAPTER 4.
Silencer Refer to "EXHAUST PIPE AND SILENCER"
section in the CHAPTER 4.
1 Clamp (air filter joint) 1 Only loosening.
2Rear frame 1
3 Bolt (rear shock absorber-relay arm) 1 Hold the swingarm.
4 Bolt (rear shock absorber-frame) 1
5 Rear shock absorber 1
6 Locknut 1 Only loosening.
7 Adjuster 1 Only loosening.
8 Spring seat 2
9 Lower spring guide 1
10 Upper spring guide 1
11 Spring (rear shock absorber) 1
53 Nm (5.3 m kg, 38 ft lb)
56 Nm (5.6 m kg, 40 ft lb)
2 Nm (0.2 m kg, 1.4 ft lb)
32 Nm (3.2 m kg, 23 ft lb)
29 Nm (2.9 m kg, 21 ft lb)

5-42
REAR SHOCK ABSORBER
6. Install:
• Rear frame "1"
• Bolt [rear frame (upper)] "2"
• Bolt [rear frame (lower)] "3"
7. Tighten:
• Bolt (air filter joint) "1"Bolt [rear frame (upper)]:
32 Nm (3.2 m•kg, 23
ft•lb)
Bolt [rear frame (lower)]:
29 Nm (2.9 m•kg, 21
ft•lb)
Bolt (air filter joint):
2 Nm (0.2 m•kg, 1.4
ft•lb)

7-4
ENGINE
ROAD CONDITION AND EXAMPLES OF CARBURETOR SETTING
A. For USA and CDN
B. Except for USA and CDN
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOMGeneral condition Sandy condition
Under 10°C
(50°F)15–25°C (59–
77°F)Over 30°C
(86°F)Under 10°C
(50°F)15–25°C (59–
77°F)Over 30°C
(86°F
(Winter) (Spring, Au-
tumn)(Summer) (Winter) (Spring, Au-
tumn)(Summer)
Main jet #440 #430 #420 #460 #450 #440
Jet needle 6BFY44-74-3 6BFY43-74-3 6BFY44-74-2 6BFY43-74-4 6BFY44-74-3 6BFY43-74-3
Pilot jet A #42.5 #40 #40 #42.5 #40 #40
B #47.5 #45 #42.5 #47.5 #45 #42.5
Pilot air screw 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4
Diameter of straight portion
ø2.72 mm
(0.1071 in)ø2.73 mm
(0.1075 in)ø2.74 mm
(0.1079 in)ø2.75 mm
(0.1083 in)ø2.76 mm
(0.1087 in)
Rich 1 richer 6BFY43-72-4 6BFY43-73-4 6BFY43-74-4 6BFY43-75-4 6BFY43-76-4
0.5 richer6BFY44-72-3 6BFY44-73-3 6BFY44-74-3 6BFY44-75-3 6BFY44-76-3
6BFY42-72-4 6BFY42-73-4 6BFY42-74-4 6BFY42-75-4 6BFY42-76-4
STD 6BFY43-72-3 6BFY43-73-3 6BFY43-74-3 6BFY43-75-3 6BFY43-76-3
0.5 leaner6BFY44-72-2 6BFY44-73-2 6BFY44-74-2 6BFY44-75-2 6BFY44-76-2
6BFY42-72-3 6BFY42-73-3 6BFY42-74-3 6BFY42-75-3 6BFY42-76-3
Lean 1 leaner 6BFY43-72-2 6BFY43-73-2 6BFY43-74-2 6BFY43-75-2 6BFY43-76-2
Symptom Setting Checking
At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixtureIncrease main jet calibration no. (Gradual-
ly)Discoloration of spark plug→If tan color, it
is in good condition.
If cannot be corrected:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
At full throttle
Stop of speed pick-up
Slow speed pick-up
Slow response
Sooty spark plug
↓
Rich mixtureDecrease main jet calibration no. (Gradual-
ly)
*In case of racing slight enrichment of mix-
ture reduces engine trouble.Discoloration of spark plug→If tan color, it
is in good condition.
If not effect:
Clogged air filter
Fuel overflow from carburetor
Clogged main air passage or clogged fil-
ter

7-5
ENGINE
This should be taken simply for an example. It is necessary to set the carburetor while checking the operating conditions
of the engine and discoloration of spark plugs. Normally, carburetor setting is made by means of the main jet, jet needle
clip position (including one with 0.5 difference), pilot jet and pilot air screw. If the result of setting is still unsatisfactory, it is
advisable to change the diameter of the straight portion the jet needle.
Lean mixture Lower jet needle clip position. (1 groove
down)
Clip position indicates the position of jet
needle groove, to which the clip is fitted.
The position is numbered from the top.
If a change in the clip position (1 groove) is
effective, try another jet needle that pro-
vides a difference of 0.5 in the clip position. Rich mixture Raise jet needle clip position. (1 groove up)
1/4–3/4 throttle
*Hard breathing
Lack of speedLower jet needle clip position. (1 groove
down)
1/4–1/2 throttle
Slow speed pick-up
White smoke
Poor accelerationRaise jet needle clip position. (1 groove up)
0–1/4 throttle
*Hard breathing
Speed downUse jet needle having a smaller diameter. Number of turns-back→Correct properly
Overflow from carburetor
0–1/4 throttle
Poor acceleration
White smokeUse jet needle with a larger diameter.
Unstable at low speeds
Pinking noiseLower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at extremely low
speedReduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above procedures.Dragging brake
Overflow from carburetor
Poor response in the range of
low to intermediate speedsRaise jet needle clip position.
If no effect, reverse the above procedures.
Poor response when throttle is
opened quicklyCheck overall settings.
Use main jet having lower calibration no.
Raise jet needle clip position. (1 groove up)
If no effect, reverse the above procedures.Check air filter for fouling.
Poor engine operation Turn in pilot air screw. Check throttle valve operation.
* In case of hard breathing, check the air vent hose for clogging.Symptom Setting CheckingGroove 1
Groove 2
Groove 3
Groove 4
Groove 5Clip
Jet needle
Leaner
(Standard)
Richer