warning horn??

I've a friend who has a Evinrude Special 88:

Model# E88MSLCCC

Serial# R1807132

He ran it hooked up to the hose the other day and had what he assumed to be a warning horn going off from the throttle/ignition area. He shut it down and started again, this time no noise. I'm not astute enough to know if this model/year has any type warning horn, nor the sound it would make. He described it as a steady pitched humming noise. He's mixing oil in gas tank, so could it be overheat warning or low water pressure? Thanks for your time!

That would be a good place to start by checking H20 flow and engine temps.

That is most likely the overheat warning horn. My guess is that one of the two thermostats stuck in the closed position.

Have him run the motor and lay one hand on the top of each cylinder head while it warms up. Both sides should warm up until it's not comfortable to leave your hands on the heads but you can touch them for a couple of seconds.

If you have an infrared temp gun, you should see about 125 degrees at the top of the cyl heads when the motor is at operating temperature.

Sometimes revving the engine a couple of times will "burp" the air from the top of the head and resolve the issue.

If this happens on a regular basis, the thermostats and pressure relief valves should be serviced along with the water pump.