Poor Medium-And-High-Speed Performance; Faulty Clutch; Overheating - Yamaha MBK MW125 2014 Service Manual

Electrical system
1. Battery
• Discharged battery
• Faulty battery
2. 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
3. Ignition coil
• Broken or shorted primary or secondary coil
• Faulty spark plug lead
4. Ignition system
• Faulty ECU
• Faulty crankshaft position sensor
EAS30602
POOR MEDIUM-AND-HIGH-SPEED
PERFORMANCE
Refer to "STARTING FAILURE/HARD START-
ING" on page 9-1.
Engine
1. Air filter
• Clogged air filter element
Fuel system
1. Fuel pump
• Faulty fuel pump
EAS30849

FAULTY CLUTCH

Engine operates but vehicle will not move
1. V-belt
• Bent, damaged or worn V-belt
• Slipping V-belt
2. Primary sheave cam and primary sheave
slider(s)
• Damaged or worn primary sheave cam
• Damaged or worn primary sheave slider
3. Clutch spring
• Damaged clutch spring
4. Transmission gear(s)
• Damaged transmission gear
Clutch slips
1. Clutch shoe spring(s)
• Damaged, loose or worn clutch shoe spring
2. Clutch shoe(s)
• Damaged or worn clutch shoe
3. Primary sliding sheave
• Seized primary sliding sheave
TROUBLESHOOTING
Poor starting performance
1. V-belt
• Slipping V-belt
• Oil or grease on the V-belt
2. Primary sliding sheave
• Faulty operation
3. Clutch shoe(s)
• Bent, damaged or worn clutch shoe
Poor speed performance
1. V-belt
• Slipping V-belt
• Oil or grease on the V-belt
2. Primary sheave weight(s)
• Faulty operation
• Worn primary sheave weight
3. Primary fixed sheave
• Worn primary fixed sheave
4. Primary sliding sheave
• Worn primary sliding sheave
5. Secondary fixed sheave
• Worn secondary fixed sheave
6. Secondary sliding sheave
• Worn secondary sliding sheave
EAS30607

OVERHEATING

Engine
1. Clogged coolant passages
2. Cylinder head and piston
• Heavy carbon buildup
3. Engine oil
• Incorrect oil level
• Incorrect oil viscosity
• Inferior oil quality
Cooling system
1. Coolant
• Low coolant level
2. Radiator
• Damaged or leaking radiator
• Faulty radiator cap
• Bent or damaged radiator fin
3. Water pump
• Damaged or faulty water pump
4. Thermostat
• Thermostat stays closed
5. Hose(s) and pipe(s)
• Damaged hose
• Improperly connected hose
• Damaged pipe
• Improperly connected pipe
9-2