Yamaha YZF-R125 Service Manual: Poor medium-and-high-speed performance
Refer to "STARTING FAILURES" on page 9-1.
Engine
1. Air filter
- Clogged air filter element
Fuel system
1. Fuel pump
- Faulty fuel pump
Faulty gear shifting
Shifting is difficult
Refer to "Clutch drags".
Incorrect engine idling speed
Engine
1. Cylinder and cylinder head
Incorrect valve clearance
Damaged valve train components
2. Air filter
Clogged air filter element
Fuel system
1. Throttle body
Damaged or loo ...
Shift pedal does not move
Shift shaft
Improperly adjusted shift rod
Bent shift shaft
Shift drum and shift forks
Foreign object in a shift drum groove
Seized shift fork
Bent shift fork guide bar
Transmission ...
Other materials:
Checking the speed sensor
1. Check:
Speed sensor output voltage
Out of specification → Replace.
a. Connect the pocket tester (DC 20 V) to the
speed sensor coupler (wire harness end) as
shown.
Positive tester probe
gray "1"
Negative tester probe
gray/black "2"
b. Set the main switch to "ON".
c. ...
Faulty clutch
Clutch slips
1. Clutch
Improperly assembled clutch
Improperly adjusted clutch cable
Loose or fatigued clutch spring
Worn friction plate
Worn clutch plate
2. Engine oil
Incorrect oil level
Incorrect oil viscosity (low)
Deteriorated oil
Clutch drags
1. Clutch
Unevenly t ...
Battery
A poorly maintained battery will corrode
and discharge quickly. The electrolyte
level, battery lead connections and
breather hose routing should be
checked before each ride and at the intervals
specified in the periodic maintenance
and lubrication chart.
WARNING
Electrolyte is pois ...