42
Disassembly/Inspection and Service
KohlerEngines.com
18 690 06 Rev. --
Automatic Compression Release (ACR)
ACR Operation Details
B
C
E
D
A
A
Camshaft
B
Exhaust Cam Lobe
C
Decompression
Weight
D
Return Spring
E
Arm
These engines are equipped with an ACR mechanism.
ACR lowers compression at cranking speeds to make
starting easier.
Operation
ACR mechanism consists of a decompression weight
and arm mounted to camshaft, and activated by a return
spring. When engine is rotating at low cranking speeds
(1000 RPM or less), decompression weight holds arm
so it protrudes above heel of exhaust lobe. This holds
exhaust valve off its seat during
fi
rst part of compression
stroke.
After engine speed increases above approximately 1000
RPM, centrifugal force causes decompression weight
to move outward, causing arm to retract. When in this
position, arm has no effect on exhaust valve and engine
operates at FULL compression and power.
Bene
fi
ts
As a result of reduced compression at cranking speeds,
several important bene
fi
ts are obtained:
1. Manual (retractable) starting is much easier. Without
ACR, manual starting would be virtually impossible.
2. Electric start models can use a smaller starter and
battery that are more practical for application.
3. ACR eliminates need for a spark retard/advance
mechanism. A spark retard/advance mechanism
would be required on engines without ACR to
prevent kickback that would occur during starting.
ACR eliminates this kickback, making manual
starting safer.
4. Choke control setting is less critical with ACR. If
fl
ooding occurs, excess fuel is blown out opened
exhaust valve and does not hamper starting.
5. Engines with ACR start much faster in cold weather
than engines without ACR.
6. Engines with ACR can be started with spark plugs
that are worn or fouled. Engines without ACR are
more dif
fi
cult to start with those same spark plugs.
Remove Piston, Connecting Rod, and Crankshaft
NOTE: If a carbon ridge is present at top of cylinder
bore, use a ridge reamer to remove it before
attempting to remove piston.
1. Remove screws securing end cap to connecting rod.
Remove end cap. While guiding connecting rod,
slide piston and connecting rod from cylinder bore.
2. Remove crankshaft from crankcase.
Connecting Rod Inspection and Service
Check bearing area (big end) for excessive wear, score
marks, running and side clearances. Replace connecting
rod and end cap if scored or excessively worn.
Service replacement connecting rods are available in
STD size.
Pistons and Rings Inspection
Scuf
fi
ng and scoring of pistons and cylinder walls occurs
when internal engine temperatures approach welding
point of piston. Temperatures high enough to do this are
created by friction, which is usually attributed to improper
lubrication and/or overheating of engine.
Normally, very little wear takes place in piston boss or
piston pin area. If original piston and connecting rod can
be reused after new rings are installed, original pin can
also be reused but new piston pin retainers are required.
Piston pin is included as part of piston assembly; if pin
boss in piston or pin itself is worn or damaged, a new
piston assembly is required.
Ring failure is usually indicated by excessive oil
consumption and blue exhaust smoke. When rings fail,
oil is allowed to enter combustion chamber where it is
burned along with fuel. High oil consumption can also
occur when piston ring end gap is incorrect, because
ring cannot properly conform to cylinder wall under this
condition. Oil control is also lost when ring gaps are not
staggered during installation.
When cylinder temperatures get too high, lacquer and
varnish collect on pistons, causing rings to stick, which
results in rapid wear. A worn ring usually takes on a
shiny or bright appearance.
Scratches on rings and pistons are caused by abrasive
material such as carbon, dirt, or pieces of hard metal.
Detonation damage occurs when a portion of fuel charge
ignites spontaneously from heat and pressure shortly
after ignition. This creates 2
fl
ame fronts, which meet
and explode to create extreme hammering pressures on
a speci
fi
c area of piston. Detonation generally occurs
from using low octane fuels.
Preignition or ignition of fuel charge before timed spark
can cause damage similar to detonation. Preignition
damage is often more severe than detonation damage.
Preignition is caused by a hot spot in combustion
chamber such as glowing carbon deposits, blocked
cooling
fi
ns, an improperly seated valve, or wrong spark
plug.
Replacement pistons are available in STD bore size.
Replacement pistons include new piston ring sets and
new piston pins.
