Engine
1. Cylinder(s) and cylinder head(s)
- 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(s) 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
- Clogged fuel strainer
- Clogged fuel tank drain hose
- Clogged rollover valve hose
- Deteriorated or contaminated fuel
2. Fuel pump
- Faulty fuel pump
- Faulty fuel pump relay
3. Throttle body (-ies)
- 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(s)
- Incorrect spark plug gap
- Incorrect spark plug heat range
- Fouled spark plug
- Worn or damaged electrode
- Worn or damaged insulator
4. Ignition coil(s)
- Cracked or broken ignition coil body
- Broken or shorted primary or secondary coils
5. Ignition system
- Faulty ECU
- Faulty crankshaft position sensor
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