Re: Mid 80’s E/rude V-6 150hp. Loses power after planing

[Don]

Well
Could be just about anything, so starting with new plugs is a good idea. Do your carbs have metal or plastic bowls? Do you have an OMC jet screwdriver? If you have metal bowls and a jet screwdriver, I would pull the high speed jets and inspect. The plastic bowls make their own debris when threading jets in and out, so you have to be extra careful during removal/installation.
You could isolate problem cylinders by removing the large plastic air box cover and injecting each throat with fuel/oil mix (from an oil squirt can) while the engine is misbehaving. The RPM should drop some when injecting a good cylinder. If there is a fuel delivery problem on a particular cylinder, the RPM will spike up for a second indicating a lean condition for that cylinder. If there is no RPM change when injecting a particular cylinder, that would indicate some other problem like ignition.
You could try pushing the key in (to prime) for a second while the engine is misbehaving, the RPM might spike back up for a second if there is a lean condition in a cylinder(s). Unfortunately, this method can cause confusion because you are richening up good cylinders while supplying needed fuel to the lean cylinder(s).
Sounds like you are going to check your fuel system, which is the place to start. Do you have a spin on water separator filter, that might have water in it. Your fuel system may have an air leak/restriction causing a lean condition at high speeds as well. You could try pumping your squeeze primer bulb while the engine is misbehaving as well, an RPM increase would indicate an air leak/restriction/filter, or fuel pump problems.
Sorry I can’t give you any specific advice, except to say inspect the simple stuff to avoid getting buried in confusion.
Let us know if we can be more help….D