Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 42Get this manual
OUTBOARD MOTOR INSTALLATION
Engine Over-Speed Protection System
The system is activated if the engine speed should ever exceed the maximum allowable limitThis will protect the engine from mechanical damageAnytime the engine over-speed system is activated, the system will automatically reduce the engine speed to within the allowable limitIf engine over-speed continues, have the outboard checked by your dealer