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What's this little fitting for on my new fuel pump? 1976 120hp

F

fatbear

Contributing Member
The fuel pump on my old (1976) Mercruiser 120 gave out and I got a new one. The new one looks identical except for two things: it's not black and it has this little mystery fitting on the bottom. I have some black paint, so that's not a problem. But what do I connect this third fitting to?

The pump goes on the engine with the filter on top. (Just so you know what I mean by "on the bottom".) The fitting is a hose barb that protrudes downward at a 45 degree angle, so not exactly all the way at the bottom. It is below the lower diaphragm, so I'm guessing that it is to drain away fuel in case both diaphragms blow out. Could that be true? If so, what should I plumb it into? I don't want fuel just dribbling into the bilge. Or should I just remove the fitting and plug the hole?

Thanks,
--FatBear
 
Re: What's this little fitting for on my new fuel pump?

should have yellow tygon tubing which runs back up to the carb so if diaphragm leaks it goes back into carb and not leak into bilge per USCG regs

a marine carb should have a port for that hose to install to
 
Re: What's this little fitting for on my new fuel pump?

It's there to send leaking fuel to the carb airhorn via a plastic hose that connects to the side at the top of the airhorn. Go to a dealer and look at a carb in the box. The plastic hose has to be fuel rated.
 
Re: What's this little fitting for on my new fuel pump?

The old fuel pump had inlet and outlet only. No vent. No tube. The carb has no fitting for a tube. It's an old boat and I'm getting used to this kind of thing. Can I vent it out the transom?

Thanks,

--FatBear
 
Re: What's this little fitting for on my new fuel pump?

Then it was not a marine carb nor a marine pump. You sure someone didn't just plug the holes for the tube, in the pump and plug the hole in the carb and did away with the plastic tube.

I have the same pump, 3.0 and the old motor i had was a 2.5 same pump, filter on top with a bell.

What carb a rochester 2gc maybe ? 2 screws to adjust fuel / air ?

Or a maecarb one screw ?

Look close see if it's plugged. Will be at the top end.
 
Re: What's this little fitting for on my new fuel pump?

fatbear said:
The old fuel pump had inlet and outlet only. No vent. No tube. The carb has no fitting for a tube. It's an old boat and I'm getting used to this kind of thing. Can I vent it out the transom?

Thanks,

--FatBear
Click to expand...

all fuel pumps of this type have a vent for the diapram. since it is in a boat this pump vent must go into the carb . if the pump leaks internally the engine will suck the gas / vapor into the engine to be consumed. if this does not vent into the carb gas vapors could unexpectedly build up and you may hear a loud noise for a sec.
 
Re: What's this little fitting for on my new fuel pump?

I have looked carefully at the fuel pump on the boat. It has no vent that I can see. It is possible that there is just a small hole or something to vent it, but there is no fitting. But I do see the serious need for this vent (I don't want the last thing I hear to be boom!) So I would like to connect the new fuel pump correctly to the carb.

I just took a serious look at the carburetor. (It is a Rochester 2GC.) There is a threaded hole in the side of the throttle body opposite the two idle mixture screws. It is centered between the two barrels and it is plugged with a screw in plug. Is this where I could attach a fitting and run a line from the fuel pump vent to the carb? Someone said the tube should go to the air horn, but this is the only place I can find on the carb to attach any kind of fitting.

This is a "remanufactured" carb that I installed a couple of years ago. So thinking maybe I got the wrong carb, I went and looked at the old one which I still have. It does not have any possible place to attach a vent line, not even that plugged hole like is on the newer carb.

I also discovered that the gasket that goes between the top part (air horn?) and the center part (where the venturis are) is being sucked into the barrels in the vicinity of the choke valve. No wonder the damned think suddenly shuddered and quit! So I guess I'll either have to get yet another carburetor or rebuild this one myself. The decision will rest on the answer to the earlier question about attaching the fuel pump vent line.

Thanks,

--FatBear
 
Re: What's this little fitting for on my new fuel pump?

Oh, and about the sight glass. There is a glass bowl on the inlet side of the fuel pump. The fuel must go through it to get to the pump. Inside of the glass bowl is a white ceramic element which I took to be a fuel filter. Is this the sight glass? If so, how in the world does it rescue me from a leaky diaphragm?

Thanks,

--FatBear
 
Re: What's this little fitting for on my new fuel pump?

If the pump was original, it is possible that it had no barb for the 'sight tube'. that said, all replacements are required to provide this feature. The other possibility is some uneducated previous owner installed an auto/truck pump at some point in time.

either way, you'll need to add a fitting to the carb or flame arrester to complete the install. This fitting must be installed on the atmosphere side of the throttle plates. The purists bore a hole in the carb, tap it, and then install a hose barb fitting. Most opt to add the fitting to the flame arrester and then be done with it. The best implementation of this approach I've seen is to use a 90 deg hose barb fitting into the top of the flame arrester, with a reinforcing block on the 'inside'.
 
Re: What's this little fitting for on my new fuel pump?

OK, I took my two carbs up to Island Marine Salvage and compared with some old ones they have on the shelf. The hole between the barrels in the throttle body is not the place for the fuel pump vent to enter. There is a hole drilled between the two barrels up high in the air horn. It is apparently tapped to 1/4" NPT pipe thread and has a hose barb screwed into it. I can make that modification to either or both of my carbs.

Thanks for all the help!

--FatBear
 
Re: What's this little fitting for on my new fuel pump?

The hole in the throttle body is a vacuum port....only use for it on a marine engine is to measure intake manifold vacuum.

Glad you know what is remaining to be done.
 
Re: What's this little fitting for on my new fuel pump?

I have a 1967 120 2.5L with a two barrel. My old pump was toast. It had a glass bowl and no vent connection. So I purchased a new marine pump. What I did was put the valve cover hose to the vacuum port with a reduced fitting. I purchased fuel hose from the Harley shop and ran the pump vent to the pcv connection at the top of the carb. This is a bit unorthodox but so far it seems to be working like a charm. Just my 2cw.
TxGalz Dah
 
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