Poppet valves?

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Have only seen them on Yams big V6s myself ... but can't give any specifics less that.

The poppet gets 'pushed out of the way' once above 2K RPMs IIRC, to allow more or easier cooling water passage. When they get old and lose their elasticiity, they don't work as well or at all.
 
Need to be more specific on this question.
Early outboards had poppet valves to serve in the induction process. These were later replaced by reed valves.
During this time rotary valves were also in engines.
Am assuming you know that an F25 would have standard intake and exhaust valves for the combustion process. The above was for that just in case scenario.

In some engine brands......
In the cooling system of later engines thermostats were encapsulated into a spring loaded valve assembly. During times of low water pressure and volume the thermostat would do its work and regulate temperature. (Idle and off idle speeds.)
When additional cooling water volume was required (with its resultant water pressure increase) these thermostat/valve assemblies would be forced off their seat and allow a massive increase of cooling water flow. (Planing speeds up to wide open throttle.)
You should be able to look in a parts book and find the answer to your question on the F25.
 

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