Yamaha YZF-R125 Service Manual: Starting failures

Engine

1. Cylinder and cylinder head

- Loose spark plug

- Loose cylinder head or cylinder

- Damaged cylinder head gasket

- Damaged cylinder gasket

- Worn or damaged cylinder

- Incorrect valve clearance

- Improperly sealed valve

- Incorrect valve-to-valve-seat contact

- Incorrect valve timing

- Faulty valve spring

- Seized valve

2. Piston and piston ring(s)

- Improperly installed piston ring

- Damaged, worn or fatigued piston ring

- Seized piston ring

- Seized or damaged piston

3. Air filter

  • Improperly installed air filter
  • Clogged air filter element

4. Crankcase and crankshaft

  • Improperly assembled crankcase
  • Seized crankshaft

Fuel system

1. Fuel tank

  • Empty fuel tank
  • Clogged fuel filter
  • Deteriorated or contaminated fuel

2. Fuel pump

  • Faulty fuel pump

3. Throttle body

  • Deteriorated or contaminated fuel
  • Sucked-in air

Electrical system

1. Battery

  • Discharged battery
  • Faulty battery

2. Fuse(s)

  • Blown, damaged or incorrect fuse
  • Improperly installed fuse

3. Spark plug

  • Incorrect spark plug gap
  • Incorrect spark plug heat range
  • Fouled spark plug
  • Worn or damaged electrode
  • Worn or damaged insulator
  • Faulty spark plug cap

4. Ignition coil

  • Cracked or broken ignition coil body
  • Broken or shorted primary or secondary coils
  • Faulty spark plug lead

5. Ignition system

  • Faulty ECU
  • Faulty crankshaft position sensor
  • Broken generator rotor woodruff key

6. Switches and wiring

  • Faulty main switch
  • Faulty engine stop switch
  • Broken or shorted wiring
  • Faulty neutral switch
  • Faulty start switch
  • Faulty sidestand switch
  • Faulty clutch switch
  • Improperly grounded circuit
  • Loose connections

7. Starting system

  • Faulty starter motor
  • Faulty starter relay
  • Faulty starting circuit cut-off relay
  • Faulty starter clutch
    General information
    NOTE: The following guide for troubleshooting does not cover all the possible causes of trouble. It should be helpful, however, as a guide to basic troubleshooting. Refer to the relative procedu ...

    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 ...

    Other materials:

    Circuit diagram
    2. Crankshaft position sensor 4. Main fuse 5. Main switch 6. Radiator fan motor fuse 8. Sidestand switch 9. Battery 17.Engine stop switch 19.Intake air pressure sensor 20.Intake air temperature sensor 21.Throttle position sensor 22.Coolant temperature sensor 23.Lean angle sensor 24.S ...

    Ignition circuit cut-off system
    The ignition circuit cut-off system (comprising the sidestand switch, clutch switch and neutral switch) has the following functions. It prevents starting when the transmission is in gear and the sidestand is up, but the clutch lever is not pulled. It prevents starting when the t ...

    Periodic maintenance and lubrication chart
    NOTE: The annual checks must be performed every year, except if a kilometer-based maintenance, or for the UK, a mileage-based maintenance, is performed instead. From 30000 km (17500 mi), repeat the maintenance intervals starting from 6000 km (3500 mi). Items marked with an asterisk s ...

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