Yamaha WR450F 2021, Owner'S Service Manual, Page: 373 / 402

TROUBLESHOOTING
9-2
Incorrect 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 cylinder
Tighten bolts or nuts on cylinder
head and cylinder to the specified
torque.
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 and
valve seat
Reface the valve-to-valve-seat
contact.
Fatigued or broken valve spring Replace the valve spring.
Worn, damaged, or stuck piston
ring
Replace the piston and piston
rings as a set.
Seized or damaged piston
Replace the piston and piston
rings as a set.
Worn or damaged cylinder bore
Replace the cylinder, piston, and
piston rings as a set.
Symptom Possible cause Actions
Spark plug does not produce a
spark
Incorrect 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 or
replace as necessary.
Clogged fuel pump Clean or replace the fuel pump.
Cracks or damage in fuel pump Replace the fuel pump.
Fuel pump malfunction
Refer to “TROUBLESHOOTING”
under “FUEL PUMP SYSTEM” on
page 8-53.
Fuel not supplied
Failed or clogged fuel injector Replace the fuel injector.
ECU failure Replace the ECU.
Symptom Possible cause Actions