Kohler K91 Service Manual page 95

AUTOMATIC COMPRESSION RELEASE
All K-Series single cylinde r engines , except the
K9 1 , are equi pped with Automatic Compression
Release (ACR) . The ACR mechanism lowers com­
pression at cranking speeds to m ake starting eas­
ier.
OPERATION
The ACR m echanism consists of two flyweights
and a spring attached to the gear on camshaft .
When the engine is rotating at low cranking
speeds ( 600 RPM or lower) the flyweights are held
by the spring in the position shown in Figure 9-1 .
Exhaust
Valve ( O pen)
Camshaft
Tab
()
Spring
Flyweights
Spring
Figure 9- 1 . Automatic Com pression Release
(ACR) - Starting Position.
I n this position . the tab on the larger flyweight pro­
trudes above the exhaust cam lobe. This lifts the
exhaust valve off its seat during the first part of
the compression stroke . The reduced compression
SECTION 9
results in an effective compression ratio of about
2: 1 d u ring cranking .
After the engine speed increases to about 600
RPM , centrifugal force moves the flyweights to the
position shown in Figure 9-2 . In this position the
tab on the l arger flyweight d rops i nto the recess in
the exhaust cam lobe. When in the recess, the
tab has no effect on the exhaust valve and the
engine operates at full com pression and full
power.
Exhaust
Valve (Closed)
Camshaft
Flyweights
Spring
Flyweights
Figure
When the engine is stopped , the spring ret u rns the
flyweights to t he position shown i n Fig u re 9-2 ,
ready for the next start .
BEN EFITS
Because of the reduced compression pressures at
cranking speeds , several i mportant benefits a re
obtained .
Automatic Com pression Release
9-2 .
(ACR) - R u n n i n g Position.
Tab
9 . 1