Saying that you've done all the test pertaining to the horn isn't telling us exactly wht you've done, so..................
(Odd Soundings Of Warning Horn)
(J. Reeves)
Should the warning horn beep constantly when the key is turned to the ON position rather than sounding just a short beep as it should, do the following test.
Remove the TAN wire from the horn. If the constant beeping continues, the horn is faulty, in which case replace it. However, on the other hand, if the beeping stops, something has gone wrong with one of the sensor components at the engine, in which case start disconnecting the TAN wires that are connected to the various components (Heat Sensors, VRO, Vacuum Switch if so equipped, Oil Tank). Disconnecting a unit and having the beeping stop...... you've found the faulty component.
NOTE: The small black plastic horn that has a built in black ground wire...... The TAN wire must be attached to the terminal closest to that black wire. Otherwise weird things will happen.
Now, the fuel primer bulb going flat (engine usually stops) indicates a fuel restriction between the primer bulb and the fuel supply, usually the anti siphon valve as follows:
(Fuel Anti Siphon Valve)
(J. Reeves)
Many of the later OMC V/6 engines incorporate a fuel restriction warning via a vacuum device attached to the powerhead. If the engine overheats, or if you have a fuel restriction, the warning is the same.... a steady constant beep.
NOTE... Only the V/6 & V/8 engines have the above "Fuel Restriction Warning". The warning horn will not sound on the other models.
The fact that a engine is not overheating, but the warning horn sounds off with a constant steady beep, and that the rpms drop drastically would indicate that the engine is starving for fuel due to a fuel restriction. Check the built in fuel tank where the rubber fuel line attaches to the tank fitting. That fitting is in all probability a "Anti Siphon" valve which is notorious for sticking in a semi closed position. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. If this valve exists, remove it, knock out those inner components which will convert it to a straight through fitting, then re-install it. Hopefully that cures the problem.
The above procedure will cure a restriction problem with the anti siphon valve as stated. BUT, it may also allow fuel to drain backwards to the fuel tank when the engine is not running (siphoning backwards) due to the fact that the carburetors/fuel pump etc are higher than the fuel tank. This condition is not an absolute as the valves in the fuel primer bulb usually prevent this backwards siphoning problem. However.... if this does take place, the cure would be to install a new anti siphon valve.
