Yamaha MBK MW125 2014, Service Manual

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TROUBLESHOOTING
9-1
EAS20090
TROUBLESHOOTING
EAS30599
GENERAL INFORMATION
TIP
The following guide for troubleshooting does not
cover all the possible causes of trouble. It should
be helpful, however, as a guide to basic trouble-
shooting. Refer to the relative procedure in this
manual for checks, adjustments, and replace-
ment of parts.
EAS31258
STARTING FAILURE/HARD STARTING
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 tank cap breather hole
• Deteriorated or contaminated fuel
• Clogged or damaged fuel hose
2. Fuel pump
• Faulty fuel pump
• Clogged fuel pump filter
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 AC magneto rotor woodruff key
6. Switches and wiring
• Faulty main switch
• Broken or shorted wiring
• Faulty front, rear or both brake light switches
• Faulty start switch
• Faulty sidestand switch
• Improperly grounded circuit
• Loose connections
7. Starting system
• Faulty starter motor
• Faulty starter relay
• Faulty starting circuit cut-off relay
• Faulty starter clutch
EAS30601
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 loose throttle body joint
• Faulty ISC (idle speed control) valve
• Improper throttle grip free play