John-deere stx38 User Manual download pdf (Page 74)

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4/23/97

REPAIR

ENGINE

ANALYZE PISTON RING WEAR

Rin

s of the wron

size or rin

s havin

improper end

ap will not conform to the shape of the c

linder. This

results in hi

h oil consumption and excessive blowb

Rin

end

aps should be sta

ered on the piston

durin

installation. End

aps in ali

nment can also

cause oil consumption and blowb

ht scuffin

or scorin

(

)

of both rin

s and piston

occurs when unusuall

h friction and combustion

temperatures approach the meltin

point of the piston

material.

When this condition exists, it is due to one or more of

the followin

probable causes:

1. Dirty cooling shroud and cylinder head.

2. Lack of cylinder lubrication.

3. Improper combustion.

4. Wrong bearing or piston clearance.

5. Too much oil in crankcase causing fluid friction.

The en

ine operatin

at abnormall

h temperatures

cause varnish, lac

uer, or carbon deposits

(

)

form in the piston

rooves makin

the rin

s stick.

When this happens, excessive oil consumption and

blowb

will occur.

ine overheatin

and rin

stickin

is usuall

caused

one or more of the followin

1. Overloading.

2. Incorrect ignition timing.

3. Lean fuel mixture.

4. Dirty cooling fins.

5. Incorrect oil.

6. Low oil supply.

7. Stale fuel.

Vertical scratches

(

)

across the piston and rin

s are

due to an abrasive in the en

ine. Abrasives ma

airborne, ma

have been left in the en

ine durin

overhaul or ma

be loose lead and carbon deposits.

When this condition exists, check for one or more of

the followin

1. Damaged, collapsed, or improperly installed air

filter.

2. Loose connection or damaged gasket between air

cleaner and carburetor.

3. Air leak around carburetor-to-cylinder block gasket.

4. Air leakage around throttle shaft.

5. Failure to properly clean cylinder bore after

reconditioning engine.

Abrasive particles in en

ine oil cause scratches on side

rails

(

)

of oil control rin

. If inner spacer

(

)

of oil

control rin

is worn or distorted it ma

cause:

• Increased or hi

h oil consumption.

• Increased deposits in combustion chamber.

• Stickin

compression rin

ANALYZE PISTON WEAR

DETONATION—

Detonation is uncontrolled combustion

(

)

caused b

excessive temperature and pressure in the combustion

chamber. Commonl

called carbon knock, spark

knock, or timin

knock; detonation occurs when the

extremel

hot, over-compressed, fuel-air mixture

nites spontaneousl

—causin

a second burn point

durin

the normal i

nition c

cle. This second burn then

collides with the spark plu

induced burn

(

)

, causin

loud explosion and extreme shockwave.

M29943

M29944

M38101

M58335

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John-deere stx38 User Manual Page 74

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