MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuel Pump vent lines

Due to some suspension upgrades on my 79 MGB I am having to move the fuel pump from its original location in the boot/trunk to the location originally used on the chrome bumper cars using a bracket attached to the riser outboard of the right hand battery box. I have an electronic SU pump that has two small vent outlets. The one closest to the fuel hose barb end of the pump has a typical barb type fitting on the end to accept the vent hose which was present as standard on my car outside the boot. There is a second vent pipe at the other end of the pump that projected into the boot that does not have the barbed (thickened) end but is the same thickness throughout. Is this also a vent pipe and if so I would assume it needs to be connected to the same tubing (using a T connector) that vented the other end. Is this correct? Also just to confirm the wiring....the spade fitting at the boot end of the pump is the ground fitting?
Frank

Your proposed venting is fine. The ground (-) connection is the spade lug at the end of the pump where the lines fuel lines are connected, It is the black wire. The positive connection is on the plastic end cap and should be a white wire.
John H

After searching the archives I have read that the fitting on the end away from the hoses is a check valve and needs to be positioned facing up (Per David DuBois). Is this the case?
Frank

This is one I haven't heard. Moss lists the same pump from mid sixty's on. Anyone else care to comment?
John H

Hmmm...I never paid attention the the orientation of the vent check valve (inside the plastic elbow that is on the housing end cap (points cover). Inside
that vent elbow is small a roller "BB".

On chrome cars, the pump is laying horizontally;
and the fuel banjos are exactly vertical to each
other. The IN banjo is positioned at the bottom;
and the OUT banjo is positioned at the top.
The orientation of the vent check valve is
positioned correctly by default.

The pump vents are there to provide a route for
internal air (displaced by each "click" of the pump
diaphragm) to freely inhale and exhale through the
pump body and end cap.

The factory ran the two vent hoses separately into
the forward section of the boot (trunk) and are not
connected to each other.

Each vent hose terminates with a plastic "T" fitting
to prevent them from falling out through the
rubber bulkhead grommets.

Without these vent hoses, passing road dirt and
water may get sucked into the pump and goof up
the points, or the internal pump mechanism.
Daniel Wong

The vent on the end cover does have a small steel ball in it that acts as a check vlave (at least until it becomes rusted and sticks in one position or the other). Since there is no spring associated with the ball, the only way is will be effective as a check valve is if the vent is pointing up. That said, I am not sure that it all that important. Over time the ball becomes rusted or corroded and does not provide very efficient check action. I also get pumps where the vent is missing completely or the end cover does not even have the hole for the vent to fit into (the end covers I get as replacements don't have the provisions for the vent). As long as the lower vent is open, there is no problem as there is sufficient room in the core of the coil where the shaft for the diaphragm to pass through to vent both ends of the pump sufficiently. If however, the single vent gets clogged up and the end cover is sealed tot he coil housing as it should be, the pump will become erratic in operation. Cheers - Dave
David DuBois

David,
Some in past threads have suggested it may not be wise to connect(T) the two vent lines due to the risk of gas fumes from one side reaching the electrical side and igniting. What are your or others thoughts on this?
Frank

Frank - The top (where the points reside) and bottom of the coil housing are connected by a 1/4" hole through the coil core where the shaft of the diaphragm goes through to the points toggle, so the two areas are already connected. The main thing is to have the tubbing sufficiently long to prevent an exchange of air under the end cover. With sufficient length of tubbing, the column of air in the tubbing just moves back and forth without drawing fresh air in (Thanks to Daniel Wong for this explanation). Without any fresh air being drawn into the area under the end cover, any gas fumes that might be collected there will too rich a mixture to burn. Dave
David DuBois

David and Daniel...thanks for your responses and explanations. One additional question....because of interference with the rear sway bar bracket I have to mount the pump with the check valve facing down which effectively ceases its function...can I just remove the BB and connect a long hose as you suggest or alternatively can the points end be rotated 180 degrees after unscrewing it (admittedly I have never taken one of these apart to even know) to return the check valve to an upright position? Also mine is the electronic version of the SU if that matters.
Frank

Frank - With the vent facing down, it remains open all the time (the BB drops off of the seat in that position). You can leave the BB in place or remove it. Either way, it will serve to vent the upper area under the end cover. As long as a vent tube of sufficient length is placed on it, it will not draw any fresh air into the area. Also, with an all electronic pump, there are no points to arc under the end cover so there is no chance of igniting any fumes that might collect under the end cover. Dave
David DuBois

This thread was discussed between 11/01/2006 and 12/01/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.