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MTD 478 Series User Manual

MTD 478 Series
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BASIC TROUBLESHOOTING
15
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 or blatty
Splashy idle usually indicates a
slight rich condition.
May indicate 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 spark-plug or ignition prob
-
lem.
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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MTD 478 Series Specifications

General IconGeneral
BrandMTD
Model478 Series
CategoryEngine
LanguageEnglish

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