Yamaha YZ250 2022 Owner's Service Manual page 243

Symptom
Compression pressure is low
Incorrect idling speed or mid-to-high speed
Symptom
Spark plug does not produce a
spark
Fuel not supplied
—
Possible cause
—
Loose spark plug
Loose cylinder head or cylinder
Damaged cylinder head gasket
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
CDI unit failure
Fuel leakage
Clogged carburetor fuel passage
Clogged needle valve or needle
valve seat
Damaged float
Water or foreign material in fuel,
degraded fuel
Clogged carburetor air passage
Incorrectly adjusted throttle cable
Incorrectly adjusted idling speed
(throttle stop screw)
Incorrectly adjusted pilot screw
Incorrectly adjusted pilot air screw Adjust the pilot air screw.
Clogged pilot jet
Loose pilot jet
Damaged or worn needle valve
seat
Loose needle valve seat
9-2
TROUBLESHOOTING
Actions
Measure the compression pres-
sure.
Tighten the spark plugs to the
specified torque.
Tighten bolts or nuts on cylinder
head and cylinder to the specified
torque.
Replace the cylinder head gasket.
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 CDI unit.
Check the fuel passage. Repair or
replace as necessary.
Clean the carburetor.
Clean the needle valve or needle
valve seat.
Replace the float.
Change fuel.
Clean the carburetor.
Adjust the throttle grip free play.
Adjust the throttle stop screw.
Adjust the pilot screw.
Clean the pilot jet.
Tighten the pilot jet to the specified
torque.
Replace the needle valve seat.
Tighten the needle valve seat to
the specified torque.