Yamaha F100B F100C Outboards Factory Service Manual, Page 19Get this manual
Features and benefits Cooling system
A high capacity water pump and large cylinder head water jacket have been incorporated to enable better coolingA pressure control valve (PCV) has also been incorporated to control the cooling water pressure, which in turn helps prevent water leakage from the power unitWhen the cooling water pressure becomes too high, the PCV opens and the cooling water flows through bypass passage to the upper caseAn additional water inlet, located in front of the trim tab, allows cooling water to flow into the exhaust passage in the lower unit during high-speed operationThis extra cooling water properly cools the high-temperature exhaust gas before it exits through the propeller boss, helping to prevent the rubber parts in the propeller boss from being damaged by high gas temperatures