Been reading about TON of folks troubleshooting same problem with same or similar outboards... motor fires right up, idles perfectly, revs up in neutral, but RPMs limited under a load. I have a sneaky suspicion that the motor is in 'limp' mode or 'failsafe' mode. After seeing the debris in the cooling channels in the head and thermostat pocket, I'm thinking I had reduced water flow in the head causing temps to run a little high... Not so high to trigger the alarm and shut the motor off completely, but enough to keep it in failsafe mode to prevent complete overheating. I REALLY think its going to run perfectly once we put the carbs back together and reinstall... not from the carb refreshing, but from cleaning the snot outta the water jacket.
Regardless, have a compression tester coming in tomorrow and after putting it back together, I will check fuel pump, check spark/ compression on each cylinder, then run it in a bucket using my buddies portable gas tank, fuel line, primer bulb, and fuel connection to motor. Then will run on the lake using the accessory tank and line to see if I get full RPMs. If so, I will then switch back to my internal tank and lines and run it with those just to be sure there isn't a collapsed or restricted fuel line in the boat itself.
If it won't run wide open, then I may have to take it to a honda mechanic on the coast to check the computer brain.
I called the prior owner just to get some more information from him which might help troubleshoot. We had seen where 1 carb had been pulled at some point (had a different gasket than the other 2, so knew at one point they had been worked on).
Super nice/ helpful. Apparantely 3 or 4 years before it sat, he went way back into the marshes and sucked up a bunch of mud/ grass into the lower unit. On the way back from that trip, motor went into failsafe mode and would not plane. He took it to a mechanic who naturally thought it was the carbs. The mechanic told the prior owner that he cleaned the carbs and then ran fine. I suspect the mechanic just pulled the top carb, found it clean, and didn't disassemble the other 2 carbs. Probably just flushed the cooling system and then motor ran fine.
Regardless, prior owner said motor ran just fine for a year or two, then he let it sit for about 6 to 9 months. When he went to take it back out, motor fired up, idled, but zero response to throttle in neutral (the rusted/ seized throttle cable I fixed a few weeks ago). He never got that addressed, and then the boat sat for another 1.5 years until he sold it to me.
Best part... he gave me his best fishing honey holes on the east side of the MS coast! Super nice of him to do that!