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Your symtoms could be caused by this:
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My '86 Minx. I replaced both risers/elbows (Merc Cast Iron) at about 13 years old because..as it turns out, one was pissing water into the cylinders (rust-pinhole) on the PORT side. (@190 hours)
Everything is fine until last season (24 years old). More pissing. I replaced BOTH the Riser And the Manifold (PORT Side) because I figured the manifold was probably about due. Lets do both. What a Real PITA that was. Broken bolts......... So now it's all done.
NOW.. After the fact..I take a GOOD look at the old riser which seemed to be fine on first inspection off the old manifold...prior to replacing ALL. The F-in riser had a pinhole. The original old manifold was NOT the problem.
This boat is raw water cooled...and flushed after every use.
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Pull some spark plugs and look for Water. Replacing an elbow is pretty easy.
Avatar: RELIANCE (1903) The Largest single masted sailing yacht ever built (Until 2004)...with over 16,000 square feet of sail. Successful Americas Cup Defender.
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1986 Minx. SBC 350/260hp, Alpha I. Red, Honey.