Your best friend will be using the P of E..... process of elimination.
Test/change one item ONLY at a time.
When used systematically and methodically, the P of E won't let you down! There is no disputing this!
**Understand that nothing up-stream from the T-stat housing will be causing an un-equal seawater flow to the exhaust system.
In other words, any seawater being introduced into the engine alone has not yet been split or divided in order to equally supply the exhaust system. That split or division occurs at and within the T-stat housing.
With exception to some jet pump systems, this will apply to any Raw Water Cooled Merc, OMC, Volvo Penta, Indmar, PCM, etc.
In other words, you do not need to be a Merc Tech in order to make a suggestion re; this particular issue!
In post #1, you mentioned; "Obviously there is a restriction somewhere".
In your most recent post #31, you have made that assessment again.
I would say, yes, that is very likely what is going on!
I have occasionally read whereby someone has added restriction to the better flowing side in hopes of increasing flow to the problematic side. I would not suggest doing that.
Find the restriction, and you will have found the problem.
Chamfer any sharp edges that may be restricting flow.
Look at the interior of the hoses for any signs of delamination.
Look at the ID of any 90* fittings that may be causing a restriction.
During the process, you may find an object that is causing the restriction (i.e., impeller pieces, for example).
Occasionally Starboard parts can be swapped to the Port side (or visa-versa) as part of the P of E.
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