Posts: 3508
Location: Elk River, Minnesota | I'm not too familiar with the johnson 90hp, but I do have a couple of ideas you can try...
Take the boat out, get it good a warm, then run the motor wide open until it overheats (that is what I am thinking is happening.) Take the cover off and feel place a hand on the cylinder heads, checking each particular cylinder. If you can, find the hot spot by touch. You should be able to feel it pretty easilly. Once you have found that, you have now started to narrrow down your problem.
What I am thinking right now is that somewhere in one of the two water jackets or passages leading up to the heads in the motor, there is a clogged passage, and cooling water is not getting to part of the heads. You probably still have water coming out your pee hole, but something is not flowing like it should. That would predicate the hot warning and the low rpm. Since the thermostats might be right on the back of the water jacket as well (at least they are on a yamaha....that is where I am unsure on the johnson) you can throw those into a pot of water, heat them up and see if they open like they should.
My suggestion: Get new head and water jacket gaskets, and have them replaced. At that time, have the water jackets cleaned and checked. Reinstall, and head to the lake. you will see two things: better compression (more power) and should have better water flow. This is also good preventative maintenance as well.
I'd definitely be wanting the old parts back from the dealer/mechanic you worked with. it would be very interesting to find if the thermostats were NOT replaced and that was your issue all along.
Something is clogged....everything else has been done, so that is the only thing really left to check.
Steve
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