I would run it again at WOT and note the throttle cable position then when the RPM drops take off the cowling and check the throttle cable position. It may be slipping as mentioned above. I have had to adjust my cable once.
The Nissan/Tohatsu is pretty simple. If it is having fuel problems due to jets clogging, or other fuel flow problems it would start running really lean for the amount of airflow. It would likely either die or run roughly (cough) when this happened. If it was clogged up I doubt it would run at WOT for any length of time. The overheat system may cause this as mentioned, I am not sure the logic the engine uses to do this.
If it is not the throttle cable I would clean out the carbs as mentioned as the next step. The sediment in the bowl could have worked through the screen.
I had a I/O boat that had fuel sit in the carbs for a few months and gum up the carbs. It would start coughing at higher throttle settings (too lean since carbs were gummed up)- cleaned the carbs- worked great.
My boat is in the water, so I tilt up to store. When I flush the motor I disconnect the fuel line at the motor and let it run until it dies to get as much of the fuel out of the carb as possible. It runs for a few minutes without the fuel line at idle. I have also installed a clear inline fuel filter in the fuel line. It is quite amazing how much sediment gets trapped in that filter. I would rather trap it there then the engine filter.
98 carburated Nissan 50 sudden loss of RPM / Power
I had the same problem with my 30HP Nissan, it runs nicely and all of a sudden drops down for a couple of minutes than comes back etc. It happened on the cruise and in tough spot, last mile before the harbour. I thought I have gas problem and I switched tanks, it worked and I docked. next morning I could not start the engine at all. Luckily the other Mac captain is Nissan car mechanic and we started looking into it with many theories, flooded etc. We were in hurry to fuel and hit the slack on the pass. Finally started the engine, running nice. As soon as I put the cover back RPM drops down, I just left the cover off. While I was fueling my friend was floating behind and looking into my engine still puzzled. Finally, Eureka, he asked me to park it on the dock and he came over again. It was a spark plug cable. It was arching and as boat was moving cable was getting closer to the cover and arching, killing one cylinder. As a temporary measure we run the wire different route and had no more problems after that. So check your wires.
Zoran
Zoran
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Zoran
Interesting you located the problem as a secondary ignition failure. The problem may not be wholly cured, as the ignition wire jumping to the cover must have a bad spot in the wire, or a high resistance before the plug fires. Dominant Gap Theory says the kilo-volts flow like water to the least resistance (The Cover). I would check the Plugs, maybe change them, and OHM the Secondary Plug Wires to be sure they have not failed, having a large separation, or failed carbon impregnated fiber, before the plug clip/boot. A large gap creating a higher resistance forces the spark to jump the gap and creates a very high KV voltage in doing so. A scope would pick this up, or a meter to read the individual excess KV built up to jump the gap, making it easier to jump to the cover as a ground.
Interesting you located the problem as a secondary ignition failure. The problem may not be wholly cured, as the ignition wire jumping to the cover must have a bad spot in the wire, or a high resistance before the plug fires. Dominant Gap Theory says the kilo-volts flow like water to the least resistance (The Cover). I would check the Plugs, maybe change them, and OHM the Secondary Plug Wires to be sure they have not failed, having a large separation, or failed carbon impregnated fiber, before the plug clip/boot. A large gap creating a higher resistance forces the spark to jump the gap and creates a very high KV voltage in doing so. A scope would pick this up, or a meter to read the individual excess KV built up to jump the gap, making it easier to jump to the cover as a ground.