Yamaha MBK MW125 2014 Service Manual

VALVES AND VALVE SPRINGS5-243. Eliminate:• Carbon deposits(from the valve face and valve seat)4. Check:• Valve facePitting/wear → Grind the valve face.• Valve stem endMushroom shape or diameter larger than thebody of the valve stem → Replace the valve.5. Measure:• Valve stem runoutOut of specification → Replace the valve.TIP• When installing a new valve, always replacethe valve guide.• If the valve is removed or replaced, always re-place the valve stem seal.EAS30285CHECKING THE VALVE SEATSThe following procedure applies to all of thevalves and valve seats.1. Eliminate:• Carbon deposits(from the valve face and valve seat)2. Check:• Valve seatPitting/wear → Replace the cylinder head.3. Measure:• Valve seat width “a”Out of specification → Replace the cylinderhead.▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼▼▼a. Apply blue layout fluid “b” onto the valve face.b. Install the valve into the cylinder head.c. Press the valve through the valve guide andonto the valve seat to make a clear impres-sion.d. Measure the valve seat width.TIPWhere the valve seat and valve face contactedone another, the blue layout fluid will have beenremoved.▲▲▲▲ ▲ ▲▲▲▲ ▲ ▲▲▲▲ ▲ ▲▲▲▲ ▲ ▲▲▲▲ ▲ ▲▲▲▲ ▲▲▲4. Lap:• Valve face• Valve seatTIPAfter replacing the cylinder head or replacing thevalve and valve guide, the valve seat and valveface should be lapped.▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼ ▼▼▼▼ ▼▼▼a. Apply a coarse lapping compound “a” to thevalve face.Valve stem runout0.010 mm (0.0004 in)Valve seat contact width (intake)0.90–1.20 mm (0.0354–0.0472 in)Limit1.6 mm (0.06 in)Valve seat contact width (ex-haust)0.90–1.20 mm (0.0354–0.0472 in)Limit1.6 mm (0.06 in)