Re: Perkins 4108 Overheating
You don't mention the out flow from the exhaust, it should be a be solid stream of 8-10 gallons per minute. That's hard to measure but for reference a kitchen faucet is half that.
If it's less than that you have blockage or a bad impeller, or a loose belt to the pump.
I'd suggest that (bad flow) as the most likely problem as that explains the symptoms as presented (adequate at idle, inadequate even a tad above that).
I'm confused on the lack of thermostat - That's what is supposed to control the flow of raw water through the hot bits.
I'd not put raw water through any part of the engine not specifically designed for it and protected with zinc's. The best way to turn a hunk of steel into a hunk of rust is to introduce it to hot salty water.
Lessons learned are opportunities earned.