Chapter 2: Basic Troubleshooting
15
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
• Advanced ignition timing
• Low octane fuel
• Over-heating engine (check for
blocked cooling air flow)
• Carbon build-up in cylinder: glow-
ing carbon chunks pre-igniting air
fuel mix.
d. Chirp
• Compression, blowing-by the fire-
ring of a damaged head gasket will
sometimes produce a chirping
noise.
• Confirm with a compression test
and leak-down test.
e. Unusual exhaust tone
1 Splashy, blatty, wheezing or whis-
tling.
• Splashy or blatty idle usually indi-
cates a slightly rich condition.
• Whistling or wheezing may indi-
cate an exhaust blockage, usually
slightly muffled.
2 Backfire
• On over-run: unburned fuel ignit-
ing past exhaust valve. Mixture not
burning completely in combustion
chamber. It may be too rich or it
may be a spark plug or an ignition
problem.
• Occasional, under load: engine
momentarily runs lean, usually will
cycle with float bowl level or gover-
nor pull-in, sometimes sounds like
a slight stumble. Ethanol content
exceeding 10% will make the
engine run artificially lean.
c. Skip
• Usually ignition related.
• Run the engine with a spark tester
in-line between the spark plug wire
and the spark plug or use an oscil-
loscope and see if the spark goes
away at the same time the engine
dies.
E Engine over-speed
a. Continual over-speed
• Binding or damaged external gov-
ernor linkage or carburetor throttle.
• Mis-adjusted governor arm.
• Internal governor failure.
b. Momentary over-speed
• Intermittent bind (very unusual).
• Interference: This is fairly common
when debris can fall on the gover-
nor linkage during normal opera-
tions.
F Engine RPMs surge (hunting)
a. Over-governed condition- Return
spring replaced with wrong part or
hooked into wrong hole.
NOTE: This is an extremely rare condition, usu-
ally created by tampering.
b. Lean Air-fuel mixture condition-
When AFR (Air Fuel Ratio) is sig
-
nificantly below stoichiometric
ratio (14.7:1) engine RPMs sink
until they reach a point that can be
supported by the available fuel.
This causes a momentary surge in
power until the available fuel is
consumed, then the RPMs fall
again, repeating the cycle.
• Too much air: look for an air leak in
the intake tract
• Not enough fuel: look for fuel sup-
ply or carburetor problems
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