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SECTION 9
AUTOMATIC COMPRESSION RELEASE
All K-Series single cylinder engines, except the
K91, are equipped with Automatic Compression
Release (ACR). The ACR mechanism lowers com-
pression at cranking speeds to make starting eas-
ier.
OPERATION
The ACR mechanism 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 (Open)'-.,__
Camshaft
F=~ ~~=T7-5'r====i
Tab
"' O
t~----' L~
Spring
Flyweights
Spring
Figure 9-1. Automatic Compression Release
(ACR) - Starting Position.
In 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
results in an effective compression ratio of about
2: 1 during 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 larger flyweight drops into 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 compression and full
power.
Flyweights
Tab
Spring
Figure 9-2. Automatic Compression Release
(ACR) - Running Position.
When the engine is stopped, the spring returns the
flyweights to the position shown in Figure 9-2,
ready for the next start.
BENEFITS
Because of the reduced compression pressures at
cranking speeds, several important benefits are
obtained.
9.1
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