2010 YAMAHA YZ125LC fuel

7-2
ENGINE
ADJUSTING THE PILOT JET
The richness of air-fuel mixture with
the throttle fully closed to 1/2 open
can be set by changing the pilot jet
"1". It is changed when adjustment
cannot be made by the pilot air screw
alone.
ADJUSTING THE JET NEEDLE
GROOVE POSITION
Should the engine be hard to run
smoothly at intermediate speeds, the
jet needle "1" must be adjusted. If the
mixture is too rich or too lean at inter-
mediate speed operation, irregular
engine operation and poor accelera-
tion will result. Whether or not the
richness of the mixture is proper is
hard to be determined by means of
the spark plug and therefore, it should
be judged from your feeling of actual
engine operation.
1. Too rich at intermediate speeds
• Rough engine operation is felt
and the engine will not pick up
speed smoothly.
In this case, step up the jet needle
clip by one groove or 0.5 groove
and move down the needle to
lean out the mixture.
2. Too lean at intermediate speeds
• The engine breathes hard and will
not pick up speed quickly.
In this case, step down the jet
needle clip by one groove or 0.5
groove and move up the needle to
enrich the mixture.ADJUSTING THE JET NEEDLE
(For USA and CDN)
On the carburetors used in the
YZ125, the main nozzle is a non dis-
assembly type, so it can not be re-
placed. Therefore, carburetor setting
requires the change of the jet needle.
1. The jet needle setting parts, hav-
ing the same taper angle, are
available in different straight por-
tion diameters and in different ta-
per starting positions.
In the case of the same number of clip
position, changing from 6BFY42-74
to 6BFY43-74 has the same effect as
a rising of 0.5-clip position. And in the
case of the same number of clip posi-
tion, changing from 6BFY42-74 to
6BFY44-74 has the same effect as a
rising of 1-clip position.
A. Difference in straight portion
dia.
B. Difference in clip position
a. Reference needle
b. 0.5 richer
c. 1 richer
ADJUSTING THE JET NEEDLE
(For EUROPE, AUS, NZ, and ZA)
On the carburetors used in the
YZ125, the main nozzle is a non dis-
assembly type, so it can not be re-
placed. Therefore, carburetor setting
requires the change of the jet needle.
1. The jet needle setting parts, hav-
ing the same taper angle, are
available in different straight por-
tion diameters and in different ta-
per starting positions.In the case of the same number of clip
position, changing from 6BFY43-74
to 6BFY42-74 has the same effect as
a lowering of 0.5-clip position. And in
the case of the same number of clip
position, changing from 6BFY43-74
to 6BFY44-74 has the same effect as
a rising of 0.5-clip position.
A. Difference in straight portion
dia.
B. Difference in clip position
a. Reference needle
b. 0.5 richer
c. 0.5 leaner
RELATIONSHIP WITH THROTTLE
OPENING
The flow of the fuel through the car-
buretor main system is controlled by
the main jet and then, it is further reg-
ulated by the area between the main
nozzle and the jet needle. On the re-
lationship between the fuel flow and
the throttle opening, the fuel flow re-
lates to the straight portion of the jet
needle at full closed–1/8 throttle, to
the 1st tapered portion at 1/4 throttle,
to the second tapered portion at 1/2
throttle, to the third tapered portion at
3/4 throttle and to the fourth tapered
portion at full open.
Therefore, the fuel flow is balanced at
each stage of throttle opening by a
combination of the jet needle diame-
ter and clip position. Standard pilot jet#40
*#45
*Except for USA and CDN
Standard clip
positionNo.3 groove
Standard jet
needle6BFY42-74
Standard jet
needle6BFY43-74
6BFY42-73
6BFY42-74
6BFY42-75
6BFY42-74
6BFY43-74
6BFY44-74
6BFY43-73
6BFY43-74
6BFY43-75
6BFY44-74
6BFY43-74
6BFY42-74
7

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 (For USA and CDN)
SPECIFICATIONS OF JET NEEDLE (For EUROPE, AUS, NZ, and ZA)
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 A #420 #410 #410 #440 #430 #430
B #440 #430 #420 #460 #450 #440
Jet needle A 6BFY43-74-3 6BFY42-74-3 6BFY42-74-3 6BFY43-74-4 6BFY42-74-4 6BFY43-74-3
B 6BFY44-74-3 6BFY43-74-3 6BFY44-74-26BFY43-74-4 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 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
0.5 richer 6BFY43-72-3 6BFY43-73-3 6BFY43-74-3 6BFY43-75-3 6BFY43-76-3
STD 6BFY42-72-3 6BFY42-73-3 6BFY42-74-3 6BFY42-75-3 6BFY42-76-3
0.5 leaner 6BFY43-72-2 6BFY43-73-2 6BFY43-74-2 6BFY43-75-2 6BFY43-76-2
1 leaner6BFY44-72-1 6BFY44-73-1 6BFY44-74-1 6BFY44-75-1 6BFY44-76-1
Lean 6BFY42-72-2 6BFY42-73-2 6BFY42-74-2 6BFY42-75-2 6BFY42-76-2
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

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.
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
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 Checking
Groove 1
Groove 2
Groove 3
Groove 4
Groove 5Clip
Jet needle
Leaner
(Standard)
Richer