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