The vacuum switch has nothing to do with running out of gas, that is... actually running out of gas as in no more fuel in the gas tank. It rather pertains to plenty of fuel in the tank and something preventing it from being drawn out. Years back I did the outboard work for the Marine Police in the Tampa Bay area who encountered that problem frequently. As fate would have it, every time I looked at it, all was well. The third time they brought it in, I really tore into it as these aren't stupid men nor do they simply bring that large boat in because they have nothing better to do. I removed the console, took up a good size portion of the floor, then started removing access panels from the tank itself. I found a heavy duty large plastic bag inside the tank that someone at the factory left in there which floated around and every so often closed off the fuel intake screen.
Normally it isn't as bad as what's mentioned above but rather a common problem... a clogged/sticking Anti Siphon valve as mentioned below.
(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.
NOTE: There has been cases when the output valve in the fuel primer bulb would come apart, and the inner portion of the valve would actually reverse itself and be drawn back into the primer bulb's output valve body. This in effect would create a shut off valve and result in a fuel restriction. If this is the case, you should be able to feel something laying in the bottom of the primer bulb when held horizontally.
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Note that actually running out of gas will not cause damage to a fuel pump nor will it blow cylinder heads. It most certainly will cause the engine to stop running.
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(Running Engine Out Of Gas)
(J. Reeves)
If the engine has but one carburetor, it's unlikely that running the engine out of gas would do any harm. When the carburetor runs out of the fuel/oil mixture, the engine stops running.
However, a engine that has more than one carburetor should simply be shut off. The reasoning is that the top carburetor will run out of fuel first due to gravity and the engine will continue to run on because of the still existing fuel in the other lower carburetors.
This results in having at least one aluminum piston running up and down a steel cylinder wall with improper or no lubrication. Not a good idea!
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The importance of the vacuum switch is that if running a VRO setup and one encounters a fuel restriction, the fuel (gas) flow obviously drops off... the oil side of the VRO is working normally and as such, the fuel/oil mixture starts to change until eventually the carburetors load up with oil. The steady beeping gets your attention before that happens BUT as mentioned in my previous reply, one has to determine if the engine is overheating or a fuel restriction is encountered.
BTW... if running a VRO setup, the VRO does have a "No Oil" warning as does the oil tank have a low oil warning (see below)
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(VRO Horn Warnings)
(J. Reeves)
NOTE: I retired around 1991/92. Possibly some of the later V4 engines and others may also incorporate a fuel vacuum switch that would enable a fuel restriction warning to sound as mentioned below, an unknown factor to myself.
1 - A steady constant beep = Overheating - The V/6 engines, possibly some others, have a fuel restriction warning which is also a steady constant beep.
2 - A beep every 20 or 40 seconds = oil level has dropped to 1/4 tank. (Late model engine = Every 40 seconds)
3 - A beep every other second = VRO failure, air leak in oil line, oil restriction, (anything that would result in a lack of oil being supplied to the engine).
NOTE - If the warning horn is the black plastic (overpriced) three wire type horn, the warning horn should beep once when the ignition key is turned to the ON position. If it does not, it is either faulty or someone has disconnected it (a stupid move!). At any rate, if it does not beep which indicates that the horn is non functional, find out why and do not run the engine until the problem is corrected.
