Yamaha yz250 2015, Owner'S Manual

Page: 73 / 174

4-9
CARBURETOR AND REED VALVE
HANDLING NOTE
Do not loosen the screw (throttle
position sensor) "1" except when
changing the throttle position sen-
sor due to failure because it will
cause a drop in engine perfor-
mance.
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.
REMOVING THE PILOT AIR
SCREW
1. Remove:
• Pilot air screw "1"
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.
CHECKING THE CARBURETOR
1. Inspect:
• Carburetor body
Contamination
→
Clean.
• Use a petroleum based solvent for
cleaning. Blow out all passages and
jets with compressed air.
• Never use a wire.
2. Inspect:
• Main jet "1"
• Pilot jet "2"
• Power 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.
CHECKING THE THROTTLE
VALVE
1. Check:
• Free movement
Stick
→
Repair or replace.
Insert the throttle valve "2" into the
carburetor body while pulling up the
lever "1", and check for free move-
ment.
CHECKING THE JET NEEDLE
1. Inspect:
• Jet needle "1"
Bends/wear
→
Replace.
• Clip groove
Free play exists/wear
→
Replace.
• Clip position
MEASURING AND ADJUSTING
THE FLOAT HEIGHT
1. Measure:
• Float height "a"
Out of specification
→
Adjust.
Measurement and adjustment
steps:
a. Hold the carburetor in an upside
down position.
b. Measure the distance between
the mating surface of the float
chamber and top of the float using
a vernier calipers.
The float arm should be resting on the
needle valve, but not compressing
the needle valve.
c. If the float height is not within
specification, inspect the valve
seat and needle valve.
d. If either is worn, replace them
both.
e. If both are fine, adjust the float
height by bending the float tab "b"
on the float.
f. Recheck the float height.
Standard clip position:
No.2 Groove
*No.3 Groove
* Except for USA and CDN
Float height:
5.5–7.5 mm (0.22–0.30
in)