Raw water cooled engines used in fresh water can be run as high as 180. Raw water cooled engines run in Salt Water should be run at +/- 140. Raw water cooled engines run in BOTH Salt and Fresh can be run at 160 if they are flushed after each use in Salt.
A thermostat is manditory in either case to maintain constant engine temperature.
If raw water cooled engines are run HOT in Salt water, they will
Salt Up. Not good in the long run.
I have run my Donzi in Salt water maybe 10% of the time and 90% Fresh water for the past 14 years and no sign of salting up.....always flush after salt water running. HOWEVER.. cast iron exhaust risers will still rust out ...in my case in about 10 years. Manifolds less so.
SO: How do you tell if you need NEW Risers or Manifolds: The engine will be reluctant to Start..when it does it will run ragged. You may be able to run fast, but when coming off plane to idle the engine will run
Sucky.
Start pullung the spark plugs one at a time on ONE side of the engine. If you find a plug....probably more than one..with a big drop of water on the plug electrode...ie: WET plug. You probably need to replace the riser on that side. The riser goes first. Manifolds last longer.
My experience has been (350 V8 SBC) the left side always seems to go first. I can't explain why.
What is happening: Water is getting into the cylinders from the Rusted Through riser or manifold. Time to replace. There is no Shortcut. NB