1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 19Get this manual
MAINTENANCE
Flushing The Cooling System
Flush the internal water passages of the outboard with fresh water after each use in salt, polluted, or muddy waterThis will help prevent buildup of deposits from clogging the internal water passagesUse Quicksilver accessory or equivalent flushing attachment
WARNING
To avoid possible injury when flushing, remove the propellerRefer to Propeller Replacement1Remove propeller (refer to Propeller Replacement)Install the flushing attachment so the rubber cups fit tightly over the cooling water intake holes