Automatic Decompression System - Robin EH09D Service Manual

7. AUTOMATIC DECOMPRESSION SYSTEM

EH09D and EH12-2D engines employ an automatic decompression system as a standard feature. This
enables easy starting of the engine, with lighter recoil pull.
The automatic decompression system releases the compression of the engine by lifting up the exhaust
valve at cranking speeds. The following is the explanation of how the system works.
At the end of the compression process, the release lever lifts up the tappet which in turns opens up the
exhaust valve slightly to release the compression. The release lever has a flyweight on its end and
another end of the lever is a crescent cam.
When the engine is cranked, the crescent cam projects above the cam profile and lifts up the tappet
because the gravity force on the weight is larger than the centrifugal force on the weight.
TAPPET
RELEASE LEVER
FLYWEIGHT
CRESCENT CAM
EXHAUST CAM
EXHAUST CAM
CAMSHAFT
Fig. 7-1
When the cranking speed reaches a specified r.p.m., the crescent cam is retracted under the cam profile
because the centrifugal force applied to the flyweight becomes larger than the force of gravity on the
weight and thus it is shifted to the position shown in the illustration below.
FLYWEIGHT
TAPPET
RELEASE LEVER
CRESCENT CAM
EXHAUST CAM
EXHAUST CAM
CAMSHAFT
Fig. 7-2
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