That engine has a thermostat set to 120 degrees.
The engine is a model:
1025201MD MERCURY 1992 25
Parts Listing:
http://www.crowleymarine.com/mercury-outboard/367.cfm
Service Manual P/N: 90-43509-3
Here is a good deal, grab it while you can:
http://www.ebay.com/itm/Mercury-Mariner-Service-Manual-18-20-7-25-HP-Motors-/120852374556
Gerry