Yamaha YZF-R125 Service Manual: Checking the valves and valve guides
The following procedure applies to all of the valves and valve guides.
1. Measure:
- Valve-stem-to-valve-guide clearance
Out of specification
Replace the
valve guide.
Valve-stem-to-valve-guide clearance =
Valve guide inside diameter "a" -
Valve stem diameter "b" 
2. Replace:
- Valve guide
NOTE:
To ease valve guide removal and installation, and to maintain the correct fit, heat the cylinder head to 100 C (212 F) in an oven.
a. Remove the valve guide with the valve guide remover "1".
b. Install the new valve guide with the valve
guide installer "2" and valve guide remover
"1". 
a. Valve guide position
c. After installing the valve guide, bore the valve guide with the valve guide reamer "3" to obtain the proper valve-stem-to-valve-guide clearance.
NOTE:
After replacing the valve guide, reface the valve
seat. 
3. Eliminate:
- Carbon deposits (from the valve face and valve seat)
4. Check:
- Valve face
Pitting/wear
Grind the valve
face. - Valve stem end
Mushroom shape or diameter larger than the
body of the valve stem
Replace
the valve.
5. Measure:
- Valve margin thickness D "a"
Out of specification Replace
the valve.
6. Measure:
- Valve stem runout
Out of specification Replace
the valve.
NOTE:
- When installing a new valve, always replace the valve guide.
- If the valve is removed or replaced, always replace
the valve stem seal.
Removing the valves
The following procedure applies to all of the
valves and related components.
NOTE:
Before removing the internal parts of the cylinder
head (e.g., valves, valve springs, valve seats),
make sure th ...
Checking the valve seats
The following procedure applies to all of the
valves and valve seats.
1. Eliminate:
Carbon deposits
(from the valve face and valve seat)
2. Check:
Valve seat
Pitting/wear Replace the ...
Other materials:
Ecu self-diagnostic function
The ECU is equipped with a self-diagnostic function in order to ensure that
the fuel injection system is
operating normally. If this function detects a malfunction in the system, it
immediately operates the engine
under substitute characteristics and illuminates the engine trouble warning
li ...
Checking the cylinder head breather hose
1. Remove:
Left side panel
Refer to "GENERAL CHASSIS" on page 4-1.
2. Check:
Cylinder head breather hose "1"
Cracks/damage → Replace.
Loose connection → Connect properly.
CAUTION:Make sure the cylinder head
breather hose is
routed correctly.
...
Coolant
The coolant level should be checked
before each ride. In addition, the coolant
must be changed at the intervals
specified in the periodic maintenance
and lubrication chart.
To check the coolant level
Place the vehicle on a level surface
and hold it in an upright position.
TIP
...