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MG MGB Technical - SU Fuel pump vent pipe

Hi all,Could someone tell me what the vent pipe from the pump to the boot is for, I'm in the process of installing a new SU pump, should it be connected or not? Thank you in advance
B Shnabel

It should be connected. As the pump works, it pumps air in and out of the body of the pump as it's pumping fuel in and out of the head of the pump. The vent pipe allows it to breathe nice clean air from the boot, not road spray and trash from under the car.

Some if not all have two vent pipes -- one to the body of the pump and one to the points cover. Both terminate in the boot....

HTH!
Rob
Rob Edwards

Rob- Thanks for your help, I only have one clear plastic pipe going to the boot, also do I need to prime the pump after installation ?
Cheers
B Shnabel

It is not normally necessary to prime the fuel pump to get it started, particularly not a new one with good check valves in it. If there are two vent pipes, they can be 'T'ed together or one of them (ususally the one on the cap) can be closed off. They both vent the same space (the area above the diaphragm). Good luck - Dave
David DuBois

Question: Before you all start laughing or making jokes I want a serious answer. Has anyone thought of adding a mini-airfilter to the vent pipe to make sure no small debris might be sucked into the pump?

The reason I'm asking is because when I took my SU pump off my car when it failed I found a whole crap load of dirt and debris inside. The tube was still up in the boot, but it was filled with dirt and debris too. So any answers or suggestions?
CJD Dark

David,
I only have one line to the boot, don't have the new pump yet but i think there is only one vent on the cap, what do you suggest ?
Cheers, Barry
B Shnabel

Keep your boots clean!

I've replaced both low-mounted chrome-bumper and high-mounted rubber bumper pumps and not needed to prime in either case. There is good suction even in air, run the pump in air and it chatters away like billy-ho (do this momentarily before fitting just to make sure it is OK) and putting a finger over the suction pipe slows this right down.
Paul Hunt

Barry - Just 'T' the two vents together. Cheers - Dave
David DuBois

Thank you all for your help.
cheers, Barry
B Shnabel

They are clean mind you, but I still had alot of junk inside. Dunno how or why though. Maybe its from the absolute age of the pump or from alot of driving in texas's western ares. Who knows?
CJD Dark

I have abandonded the SU pump since but when I had one the vent tube used to run into a tight hole in the chasis. It might not be orrigional but it worked as the air is debris free in there.
.
Peter

"Just 'T' the two vents together"

Two separate pipes and Tees in the boot on my 73.

Later pumps on the RB with its electrical end in the boot/trunk only had the one vent pipe off the pump body.
Paul Hunt

Paul-How many vents on the pump itself ? according to the Bently Manual there is only one vent valve on the end cover (AZX1307 pump type)Thanks, Barry
B Shnabel

Barry - there is a check valve in the vint on the end cap (that only works it the vent is pointing up). and there is a vent tube on the coil housing by the flange where it attaches to the aluminum pump body. Cheers - Dave
David DuBois

Barry - I guess that I should clarify that the end cap vent check valve just stops air from being drawn in through that vent only if the vent points up. If it points down or horizontaly, the little ball that blocks the vent won't drop back to close off the vent. The reason for the check valve is to keep fresh air from entering the area of the pump where the points ar located in an effort to keep any gas fumes that may accumulate in that area from igniting when the points arc. This is not a real great problem and if the fumes do ignite, they are not going to set the car on fire, it just makes a mess out of the area under the end cap. Having a long vent line going into the trunk should also preclude any exchange of air inside the pump due to the length of the tubing. Daniel Wong told me of this situation and I didn't know whether or not to believe him until I ran across a pump that had suffered that exact problem recently. Cheers - Dave
David DuBois

Barry - two (main body and electrical end cap) on the chrome bumper where the whole of the pump was exposed to the elements, just one on the pump body of the rubber bumper types where the electrical end was in the boot and could 'breathe' relatively clean and dry air - one assumes. The AZX was only introduced in 77 hence would only have had one vent.

Interesting point about the check valve - which I wasn't aware of even after having repaired one. Seems like it would pump gas vapours out but not allow any air back in, which would tend to cause a partial vacuum, which would tend to retard the return of the diaphragm under spring pressure but aid the pull of the solenoid.
Paul Hunt

I think the points cavity is open to the back side of the diaphragm via the hole through which the diaphragm rod runs. With the check valve on the points cover, that would mean the pump would inhale through the body and exhale through the points cover. Maybe. Or, if the path of least resistance is back through the pump body, the check valve might stay closed all the time and the air would just "wiggle" back and forth as the diaphragm moved...
Rob Edwards

Paul- since I'm geting a AZX1307 pump with one vent I would have only one vent pipe to the boot. Thanks for the help. Cheers, barry
B Shnabel

Barry,
The last AZX1307 I bought (about 5 years ago) had two vents....
Rob Edwards

Rob - Wonderful choice of word ("wiggle") regarding the air circulation in pumps. That is exactly what it does if the tubing is set up correctly. Barry - the AZX 1307 may or may not have the vent on the end cap as tehy come both ways. If the vent is there and you don't want to fuss with it, just cap it off, but you then must make sure that the vent on the coil housing is kept open. If it get clogged and the other vent is not operational, the pump will get very unstable in it's operation.
Cheers - Dave
David DuBois

Thank you all very much for your help, when I receive the new pump I'll know how many vents to deal with. cheers, Barry
B Shnabel

This thread was discussed between 25/02/2004 and 29/02/2004

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