MTD 70 Series Spezifikationen PDF herunterladen (Seite 20)

BASIC TROUBLESHOOTING

14

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 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 and air leak

in the intake tract

• Not enough fuel: look for fuel sup-

ply or carburetor problems

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