I checked all new thermostats on every kind of engine before installing and checked it every time I changed water pumps on every kind of engine.Make sure that they start opening at the right temperature and make sure that they fully open by the right temperature.Make sure they start closing and do close at the right temperature.New does not mean it is right!I have found several new ones to be bad.Opening and closing does not mean it is right.Opening and opening fully and closing at the right temperatures has to be right.Thermostats have a correct cycle,not just to open and close as we think.This is from old Diesel Mechanic,40 Horse and Under Outboard Motor and Small Engine repair schools I have attended along with the heart break of personal experience.And yes,some small engines have been water cooled.
I set a thermostat on a ceramic or glass surface inside a pot and put the thermostat on or in it.Metal to metal contact of the thermostat with the bottom or sides of the container will cause the thermostat to heat unevenly at times.Avoid this.Also your thermometer should not not touch metal.Heat it slow and watch closely.When fully open,watch when it starts to close and when it closes completely.You will often be surprised.
Also.A "wax filled",sometime listed as "heavy duty" thermostat generally last far longer than the gas filled in everything.They are more expensive,but have been well worth it in my opinion.You take what you can get with outboards.
Also gaskets.I buy two to have a spare.Small (40 & under)outboards I Never Seize (Nickle only,no Copper!)coat the gaskets and they are reusable until they harden.If they do leak or seep it is easy to see and correct.
Some use to offer two different temperature thermostats for the same motor.I trolled and idled a lot working with my 9.8 Mercury.I believe that the slightly hotter thermostat that Mercury recommended worked better year round for me.Also,my motors were run at 2/3 throttle on the top end and never reached the upper levels of the designed heat range.Also they had a cold weather package that circulated hot water around your carb that worked great in cold water after you got some water heated in your engine.If your water never gets below 40 degrees,I don't think this would be worthwhile.It was a help the few times our water temperature got real low in South East Georgia,but really shined in northern winter trips!
I have a 2 cylinder Mariner manual 40 that has been as good a motor as I have ever had.It pulled a 30 foot Scorpion (big for it's width balloon net with lots of webbing) Shrimp net for 4 years along with many hours of "chasing the birds" offshore,inshore oystering,clamming and fishing.
I hope this helps someone.
I love you Brothers and Sisters.peewee