The only thing that would be different with the RH rotation engine, would be the engine circ pump. This requires a circ pump that is bi-directional. However, this would likely not cause any difference to exhaust man/riser temperatures without some tell tale signs of engine temp being too high.
The composite material (rubber), on our Sea Water pump impellers, takes a "set" when left in the pump housing...... and particularly while not being used for any length of time.
These can actually last longer on a boat that is used more regularly, than on one that is laid up for the winter (impeller left in pump).
Maybe these should be replaced.
(I always remove my impellers for the lay-up duration)
I still maintain that you have a restriction, of some sort, somewhere in your raw water system.
Are the tube passages within the H/E clean and free from any debris?
Did you by chance loose any vanes from a previous impeller failure...... vanes that may not have been retrieved?
Don't discount interior hose delamination.
Even while new...... you may want to remove the risers and give the transfer ports a good inspection. With this being twins...... we can even swap risers from one engine to the other...... although a considerable amount of work.
What about portions of the exhaust system beyond the manifolds and risers?
A long shot: I recently read a thread whereby a person inadvertently received a bad manifold casting from a major manifold supplier. This caused damage to an exhaust valve. There was no mention of high heat during the failure. I don't know if the owner had even considered manifold temperature prior to the failure.
Point being..... try your best to not over-look anything, no matter how silly it may seem. Sometimes these are very small items that cause huge issues.
Remember........ the camel didn't realize that one small straw could break his back.
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