SU fuel pump questions

[Bayless]

Ok, please excuse but I have posted these same questions in the Jag forum. This group just seems to be so much more active, I hoped for better response.

I am working on a XK140 that has fuel pump problems. I have done about all the good I can for it so it's about time for a professional. First question: Should I have it rebuilt or try to purchase new (Jag pumps run about $450+)? Second: If rebuild, who do you recommend? Finally, do you prefer the electronic conversion or leave it with the breakers? Thanks for any advice.

 

[DrEntropy]

My input may be worthless, but my opinion is that it should be rebuilt. Not only that but YOU can do this. If it's the "double sided" one, the points are the same as every other SU, just two of 'em.

To better clarify just what's happenin' there, please give us the particular things you've done so far?

...not trying to be annoying, rather trying to get a 'feel' for just what you're facing.

 

[scoutll]

You can also look at John Twist from University Motors video on SU pumps. It's on YouTube. It gives some good info.
https://www.youtube.com/watch?v=_VGQPg15-8E&feature=channel_page

 

[dklawson]

As stated above, you can do this yourself. If however, you do NOT want to rebuild the pump at home, contact Dave DuBois. He is a member of this board and he is the guy whose articles you're going to find if you start Googling for information on electric SU fuel pumps. (He's also a very nice guy).
https://homepages.donobi.net/sufuelpumps/

 

[glemon]

All the parts that wear out are replaceble, I rebuilt one for the first time a year or two ago, two keys, getting the "throw" or distance the pump right, second really helps to set up a test rig to allow the thing to run off the car so you know its good before you have to rely on it plus remove it if there is a problem. You will need a 12 volt power source, some rubber tube a container and something of similar viscosity but less volatility than gasoline to test it with, I used Marvel Mystery Oil.

I think there are some good tutorials on the web in addition to John Twists (not putting his down at all, just saying there are other descriptions of how to do it out there as well)

 

[Bayless]

Thanks for the input guys. This is the single pump. When I first got the project, the pump did not work at all. I fiddled around with it a couple of time and finally got it to pump. It still seemed to tick too slowly but the owner has put several miles on it between trips to the trim shop and the glass shop then finally the Concours last fall, all with no problems.

It appears now that it has all but quit ticking entirely. Result, of course, is that the float bowls go empty and she won't run. I haven't torn it down again yet but was beginning to wonder if there was just something "over my head" here. I know that getting the "throw" right is important and thought I had done so, at least according to the instructions I have. Maybe not. Also the diaphram appears all right, no leaks and not too stiff, and the valves were good when last checked.

I'll tear down once more and report back. Thanks again.

 

[DrEntropy]

Do a check of the points. Dave (DuBois) and Doug (Lawson) have both hit all you need to make this ting go under its own power. Clean the pump points with some abrasive. An "emery board" from a fingernail kit will do.

 

[Bayless]

Thanks Doc. Will do. I'll try to get it off over the weekend. Found out from the owner that it is either new or rebuilt but that was several years ago although it has not been run at all until recently.

 

[Bayless]

Ok, here's the latest. I got the pump off and redid everything. All looks good inside. Bench tested and it ran better than ever. Reinstalled, started the motor and ran fine for about 10 minutes. Then the pump quit again. By the time I removed it again and got back to the bench, it was working again.

The diaphram and contacts seem fine. I can't see anything wrong. Could it be that the capacitor heats up and fails? Then after resting several it cools down and works again? Or could something similar happen in the magnet coil? Or any other suggestions would be most welcome.

 

[JPSmit]

could it be fuel line related? some kind of blockage/ debris? or electrical getting to the pump?

 

[jlaird]

I have money rideing on a bad ground.

 

[dklawson]

10 minutes of running on the car and then the pump stops? Is there any chance the vent for the fuel tank is blocked? The amount of time sounds about right for running until there's too much vacuum in the tank for the pump to draw against.

 

[JPSmit]

you could run it with the gas cap off to check - right?

 

[dklawson]

Absolutely. That would be a great test.

 

[DrEntropy]

If that yields the same result it could be (from the comment about it having possibly been "rebuilt") the adjustment of the "throw-over" on the diaphragm.

In about every Bentley repair manual for any English car is a section on the SU pump and how to go about adjusting the diaphragm throw.

You did "dress" the points, right?

A cap(acitor) usually either works or doesn't. Rare to have one "valve" IMO.

Other concern is the possibility of 'junk' built up on the diaphragm rod itself or the bore it goes through. Heat could be making it semi-solid and it'd interfere with the stroke the same as if pressure had been attained in the fuel line to the carb.

Just a couple more "possibles"...

 

[Bayless]

Thanks for all the suggestions guys. Owner said the pump was actually new rather than rebuilt, but laid in the box for a few years. No, inside is clean no rust or other crud. It actually does look like a new pump. I did dress the points although they really didn't need it. And I'm pretty sure I got the throwover right this time. But DUH! I didn't think of the tank vent. This thing has a cap that's part of the tank door and just closes automatically when you close the door. The last thing I did before this problem started was add the missing gasket to that gas cap. And I'm pretty sure there is no vent in the cap. Geez! Dumb me. I'd bet a case of Guinness that's the problem.
I'll report back tomorrow.

 

[jlaird]

LOL, I wonder how many times the no vent bug has bit over the past few years and it still takes a half dozen posts to get it.

 

[Bayless]

Yep, I think it was just a couple of months ago, probably in this very forum, I and others were suggesting the same thing to another poor bloke.