Yamaha YZ 2020 Series Owner's Service Manual page 347

Symptom
—
Incorrect idling speed or mid-to-high speed
Symptom
Spark plug does not produce a
spark
Low fuel pressure
Fuel not supplied
Possible cause
Water or foreign material in fuel,
degraded fuel
Loose spark plug
Loose cylinder head or cylinder
Damaged cylinder head gasket
Incorrect valve timing
Incorrect valve clearance
Worn valve guide
Bent, damaged, or stuck valve
Poor contact between valve and
valve seat
Fatigued or broken valve spring
Worn, damaged, or stuck piston
ring
Seized or damaged piston
Worn or damaged cylinder bore
Possible cause
Incorrect spark plug gap
Worn or damaged spark plug
Defective spark plug cap
Defective ignition coil
ECU failure
—
Fuel leakage
Clogged fuel pump
Cracks or damage in fuel pump
Fuel pump malfunction
Failed or clogged fuel injector
ECU failure
9-2
TROUBLESHOOTING
Actions
Change fuel.
Tighten the spark plug to the spec-
ified torque.
Tighten bolts or nuts on cylinder
head and cylinder to the specified
torque.
Replace the cylinder head gasket.
Adjust the valve timing.
Adjust the valve clearance.
Replace the valve guide.
Replace the valve.
Reface the valve-to-valve-seat
contact.
Replace the valve spring.
Replace the piston and piston
rings as a set.
Replace the piston and piston
rings as a set.
Replace the cylinder, piston, and
piston rings as a set.
Actions
Adjust the spark plug gap.
Replace the spark plug.
Replace the spark plug cap.
Replace the ignition coil.
Replace the ECU.
Measure the fuel pressure.
Check the fuel passage. Repair or
replace as necessary.
Clean or replace the fuel pump.
Replace the fuel pump.
Refer to "TROUBLESHOOTING"
under "FUEL PUMP SYSTEM" on
page 8-41.
Replace the fuel injector.
Replace the ECU.