Yamaha YZ250FM 2021 Owner's Service Manual

TROUBLESHOOTING9-2Incorrect idling speed or mid-to-high speed—Water or foreign material in fuel,degraded fuel Change fuel.Loose spark plug Tighten the spark plug to the spec-ified torque.Loose cylinder head or cylinderTighten bolts or nuts on cylinderhead and cylinder to the specifiedtorque.Damaged cylinder head gasket Replace the cylinder head gasket.Incorrect valve timing Adjust the valve timing.Incorrect valve clearance Adjust the valve clearance.Worn valve guide Replace the valve guide.Bent, damaged, or stuck valve Replace the valve.Poor contact between valve andvalve seatReface the valve-to-valve-seatcontact.Fatigued or broken valve spring Replace the valve spring.Worn, damaged, or stuck pistonringReplace the piston and pistonrings as a set.Seized or damaged piston Replace the piston and pistonrings as a set.Worn or damaged cylinder bore Replace the cylinder, piston, andpiston rings as a set.Symptom Possible cause ActionsSpark plug does not produce asparkIncorrect spark plug gap Adjust the spark plug gap.Worn or damaged spark plug Replace the spark plug.Defective spark plug cap Replace the spark plug cap.Defective ignition coil Replace the ignition coil.ECU failure Replace the ECU.Low fuel pressure— Measure the fuel pressure.Fuel leakage Check the fuel passage. Repair orreplace as necessary.Clogged fuel pump Clean or replace the fuel pump.Cracks or damage in fuel pump Replace the fuel pump.Fuel pump malfunctionRefer to “TROUBLESHOOTING”under “FUEL PUMP SYSTEM” onpage 8-37.Fuel not supplied Failed or clogged fuel injector Replace the fuel injector.ECU failure Replace the ECU.Symptom Possible cause Actions