This post is only for gearheads that know something about engines. If that is not you, read something else. Perhaps Huckelberry Finn.
I finally fixed my boat with the help of our marina mechanic. We, and the rep from Merc Cruiser who built the engines were baffled for quite a while and it was the simplest and stupidest issue.
It started when my exhaust manifold rotted through so I replaced all 4 of them.
This is one part of one of them.
Then I went out and all was well for an hour until one engine cut out. No problem as I have two engines.
The other engine cut out so I had to get towed.
After much testing It was the coils which is ridiculous because those two coils sat in that engine a couple of feet from each other for 20 years and they both croaked minutes apart and the weird part is that the both coils worked. They just didn't work hard enough which I have never seen and I am a pretty good mechanic as is the boat mechanic.
I replaced the coils and spark plugs, wires and went out. After a little while the starboard engine slowed down and sounded like a machine gun and trust me, I know what a machine gun sounds like.
Back at the dock I took a compression test which was good. I figured a lifter was hanging up so I removed the exhaust manifold (again) so I could remove the intake manifold and the lifter, which looked perfect so I put it back together.
Same noise so I found out a valve spring was broken near the bottom so I couldn't see it.
I replaced the spring and went out. Now that big noise is gone but over 3,000 RPM the engine slows down and makes popping noises.
We swapped from one engine to the other the coil, distributor, carburetor, "computer" and fuel pump. Took it out, still no power in starboard engine.
Swapped spark plugs from one engine to the other. AAAhhhha
The other engine, which was fine no longer started.
"Spark Plugs" should not cause that especially spark plugs with less than an hour on them.
Put in new plugs and took it out. Wide open throttle for 20 minutes and it ran perfect.
I am freekin thrilled. The mechanic took it out because I was busy but now I want to take it out to see for myself.
Now all I want to do, besides go out to dinner on it, is to remove one carburetor because the lower butterfly is severly sticking . Easy fix (I hope)