BASIC TROUBLESHOOTING
14
VI. Flow blockage
A Exhaust blockage, usually accompanied by
an unusual exhaust sound.
• Just as a throttle on the carburetor
controls the engine RPMs by limit
-
ing the amount of air an engine
can breathe in, an exhaust block
-
age will limit engine performance
by constricting the other end of the
system.
• The muffler itself my be blocked.
• The exhaust valve may not be
opening fully, possibly because of
extremely loose valve lash set
-
tings.
• The exhaust valve seat may have
come loose in the cylinder head.
This may cause a loss of compres
-
sion, a flow blockage or it may ran-
domly alternate between the two.
NOTE: The cause of an exhaust valve coming
loose is usually over heating.
B Intake blockage
• An intake blockage up-stream of
the carburetor will cause a rich
fuel/air mixture and constrict the
amount of air that the engine can
draw in, limiting performance.
• The intake valve not fully opening.
A possible cause of this is loose
valve lash.
C Makes unusual smoke when running
a. Black smoke, usually heavy, usu-
ally indicates a rich air fuel mixture
• Not enough air: air flow blockage
or a partially closed choke.
• Too much fuel: carburetor float or
float valve stuck or metering /
emulsion issues with the carbure
-
tor.
b. White smoke, usually heavy
• Oil in muffler, usually the result of
improper tipping. The engine will
“fog” for a minute or so, then clear-
up on its own.
• Massive oil dilution with gasoline.
It may be caused by improper tip
-
ping. It can also be caused by
leaky carburetor float valve, if
there is a down-hill path from the
carb. to the intake port. Check oil
for gasoline smell, repair carbure
-
tor.
c. Blue smoke, usually light.
1 PCV system
• May be blocked or unplugged.
• May be over-come by massive
over-filling or oil dilution with gaso
-
line.
• Will cause oil to exit the engine via
any low-resistance paths.
2Piston rings
• Confirm with leak-down test.
• Smoke will be more pronounced
under load.
• Repair may not make economic
sense.
3 Valve guides (and intake valve
stem seal).
• Smoke will be more pronounced
on over-run.
D Makes unusual noise when running
a. Knock
• Check for loose mounting of
engine or driven implement
• Rotate crankshaft back-and-forth
to check for loose connecting rod.
b. Click
• Clicks and pops on engine shut-
down: Compression release com
-
ing into play as the engine RPMs
cross the activation threshold.
This will have no ill effects on
engine performance.
• Half-engine speed clatter: loose
valve lash.
• Half-engine speed clatter, slightly
heavier: wrist-pin.
• Rhythmic heavy-light engine
speed click: piston slap
c. Spark-knock
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