Probable cause
Possible remedy
Lack of lubricating oil.
Fill the crankcase with proper grade and quantity of oil.
Refer to LO 5-1185.
Adjust carburetor (par. 106d).
Fuel mixture too lean.
Remove carbon (par. 135b.)
Heavy carbon deposits in combustion chambers.
Fan belt broken.
Replace fan belt (par. 130c).
Fan belt slipping.
Adjust fan belt (par. 130c).
Choke button pulled out.
Push in the choke button.
Engine overloaded.
Check load against operating capacity of the engine.
Cooling system frozen.
Thaw out cooling system.
Radiator clogged.
Clean radiator (par. 129e).
Coolant passages clogged.
Clean coolant passages (par. 129e).
Valve timing incorrect.
Adjust valve timing (par. 182e). See note in paragraph
Breaker cam retarded.
Adjust magneto (par. 116).
Exhaust pipes restricted.
Clean or replace exhaust pipes and muffler (par. 133).
64.
Engine Backfires
Probable cause
Possible remedy
Fuel mixture too lean.
Adjust carburetor (par. 106d).
Valve timing incorrect.
Adjust valve timing (par. 182e). See note in paragraph
Valve damaged.
Replace valves (par. 136c).
Valves sticking.
If the intake valves stick, the engine will backfire through
the carburetor; if the exhaust valves stick, It will fire
through the exhaust system. Remove the spark plug
and pour penetrating oil into the cylinder. Let the
engine stand idle for a few minutes. Rotate the
engine by hand and if the valve has not freed itself,
tap valve lightly with a soft copper rod. Replace the
spark plug and start the engine. If the valve is still
stuck, pour carbon solvent into the carburetor
through its air intake, and mix some carbon solvent
with the oil. Start the engine and allow it to run for a
few minutes. If the trouble has not disappeared, a
carbon and valve job is needed (par. 136).
65.
Engine Becomes Noisy
Probable cause
Possible remedy
Muffler damaged or corroded.
Replace muffler (par. 133).
Exhaust pipe broken.
Replace exhaust pipe (par. 133b).
Muffler gasket damaged.
Replace muffler gasket (par. 138b).
Exhaust manifold loose or broken.
Tighten or replace exhaust manifold (par. 132).
Manifold gasket blown.
Replace manifold gasket (par. 132).
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