Try replacing the copper water lines on the front of each cylinder with clear flexible tubing. When you run the motor on a boat, it will be easy to see if your’e getting water flow to the cylinder. This is a pressure vacume cooling system, so it’s essential that all lines, fittings etc. are air tight.
I’m sure you realize that the PO 15 needs to be throttled up immediately after starting, to allow the system to fill with water. This ”primes” the cooling system, so that after the engine is slowed down to a trolling speed, the vacume part of the system will keep the flow of cooling water constant. Any vacume leaks through out the system will cause a loss of prime, and result in overheating.
Here is a pic of my V45, with the clear hoses. I wrapped the threaded fittings with teflon tape, then used a hose clamp to secure the tubing. This protects the tubing from being ”chewed up” by the fittings threads.
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