2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 156Get this manual
POWER UNIT DECOMPRESSION SYSTEM
DECOMPRESSION SYSTEM
Decompression system activates by means of the centrifugal system installed on the exhaust camsWhen starting, decompression cams lift the rocker arms and open the exhaust valves slightlyOnce the engine starts, centrifugal force closes the decompression cams, thereby returning the exhaust valves to the normal closed position The exhaust valves normally closed the decompression cams by centrifugal operation