SU Carburettor Fuel Leaks

The shoulder of the needles should be at the same level as the bottom of the piston. If the shoulder is further in then that will give a richer fuel fixture.

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Peter

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Yes, ive got one adjusted correctly, but the front one is inserted a bit too far, and I can’t get the needle out to check its letter code and properly reset it.

Are you able to get the clamping screw out? If so you might find that needle will come out after applying a little heat to the piston.

Peter

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Yes. Screw is out. I’ve got it soaking in carb cleaner right now. Needle may be bent from trying to pull it out.

You could also try soaking in boiling water for 5 or 10 minutes and hope the difference in expansion coefficients breaks the grip. If you do need to resort to ‘brute-force-and-ignorance’, attach several layers of protection, eg masking tape, to the jaws of the pliers and twist the needle back and forth, gripping at the base. This will reduce the risk of bending.

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You could even chuck it up in a 3-jaw drill press, clamp the piston to the table, and see if you can draw the needle out with the lever.
If you end up trying the propane torch, just go round and round the base near the needle.
The Haynes SU Carb Manual gives the diameters of the DY needle, and the applications list, so there may be some other car that used it.

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Good idea, and a light 0000 steel wool rub on the needle root, plus a very teeny tiny bit of anti-seize would be a good idea.

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I’m leaving it to soak in carb cleaner overnight. Good idea with the drill press. I’ll try that tomorrow morning.

Could you post technical information about your fuel pump(s) and the carbs? It would be good to know the fuel pump manufacturer, part number, and any external specification. Somewhat similar question for carbs, including throttle diameter.

Jaguar Service Manual 1946-48 says on page 19 that 3 1/2 litre RHD cars use DY needles and LHD cars use FL (same needle spec on page 14). Page 118 specifies two Skinner’s Union Type “L” pumps for the 3 1/2 litre. Note that Type “L” pumps are not the same as Type “LCS”.

The SU Workshop Manual page D.5 specifies that Type “L” pumps have maximum suction lift at carb height on engine of 48 inches but also maximum output lift of 24 inches. This means the output pressure of a Type “L” pump is 1.3 psi. Many people working with automotive fuel systems are not accustomed to working with a pump with such low pressure. As an aside, if there is a pressure reduction valve between the fuel pumps and your carbs, be suspicious that it is not working as you hope. I have not found a commercial PRV that can work for this circumstance even though the manufacturer markets this as within the PRV operating range.

As for new float chamber needle valves, I have found some brand new ones do not work and instead give the results mentioned in my earlier post. Unfortunately, solely because a part is new does not mean it was engineered and produced to the original functioning circumstances. This problem is the bane of old car repairs which would benefit from new parts but where the manufacturing run numbers do not motivate careful engineering or production. Sometimes new is good, sometimes not (and this is not limited to old Jaguars or Lucas electrics).

Photos of your carbs, fuels pumps in place would help guide thoughts.

Here is a type L in my 2-1/2 Liter. A 3-1/2 Liter would have two of them side by side here working in parallel.


MGTC uses this same pump so they are readily available.

I did not find any other cars using the DY needle, but the DZ needle is almost the same, a few thousandths larger diameter all along, and used by many BMC 1275cc engined cars, so if nothing else turns up, an expert machinist could turn down a DZ to make it a DY.

I imagine the LHD uses a different needle because it has a different air cleaner.

Got the needle out of the front piston. It was still very tight after soaking in carb cleaner overnight, so I put 3 layers of electrical tape on it and carefully pulled it with vice grips. The jaws still left slight indentations in the needle. Anyways, it’s marked ‘DY,’ the same as the rear. IAW the service manual. this is correct for my RHD car with a 3.5 liter engine. As I don’t have another needle and haven’t found a source for a new one, I’m going to carefully polish this one, straighten it, and reuse it. I will reinstall it out a bit farther where the extreme top of the shoulder is even with the bottom of the piston. That’s about 1/16" farther out than it was.

I pulled the carbs. The bodies of both are marked 4031. Other than numbers on the bowls, I couldn’t find any other numbers on them.

The fuel pumps were wired to the inside of the left inner fender. There was no inline pressure regulator. I completely removed them to look for any numbers or other identifying marks. The body of one pump was marked B 10-563 and the other was B 10-999. No other obvious numbers on either one. There is an inline filter between the tank and the first fuel pump, and a second inline filter between the second fuel pump and the carbs. As long as I’ve got them completely removed, I’m going to put them back where they belong on the top of the firewall.

Now that I’ve got 2 complete rebuild kits from Moss, I’m going to tear the carbs apart again, looking for anything out of place. I’m open to suggestions on how to lean them out. I’ve pasted a bunch of pictures here. Hopefully they will all post to the thread.









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We you not able to get one from Burlen or Midel?

I don’t know the codes for the high pressure or low pressure pumps but the only difference between them is the strength of the diaphram return spring. If you are buying a needle the supplier should be able to send you the low pressure springs.

Peter

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Had never heard of Burlen or Midel. I am looking at Burlen’s site right now. Are SU carbs supposed to have a spring on the top of each piston? Mine don’t. Burlen site says different color springs for different tension. What would be correct for mine?

No, no spring back then. Your carbs are H4.

Peter

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Okay Peter, thanks. I clicked post-war Jag 3.5 litre on Burlen’s website and went to SU carb. An IPB came up which showed a spring. That’s why I asked.

I’m sure that 1/16" was making that carb too rich.
I believe with the brass piston there was no spring, but when they went to the aluminum pistons they added the spring.
I looked on the Burlen and Midel websites but did not find metering needles. Maybe you have to ask.
BTW on leaking, be aware there is a fiber seal or gasket at the bottom of those brass hex caps, and you have to get the right one so it doesn’t catch on the adjustment screw and turn it as you put the cap on. Guess how I know. :slightly_smiling_face:

So it looks like you have the original AUA-25 Type L fuel pumps after all. Maybe they are in marginal condition, stiff diaphragms, and were not pulling up from the tank so somebody moved them. I can’t identify the B numbers. Mine has only a date code G12.
They can be rebuilt with kits from Moss, Burlen et al.


There are 2 or possibly 3 different diaphragm stem lengths, Moss has 3 different part numbers anyway, so be sure you get the right one.

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George
My knowledge is very limited here, however, I would support what Rob just posted, the needle position, the correct small gasket / joint for the cap are very important, also, ensure the correct gasket / joint fitted to the float bowl bolt has the cut outs to allow the float bowl to “breathe”.
It is likely the pumps were moved to the upright panel due to vaporization of the fuel in high temperatures or high altitude service, quite often alternative pumps (Facet) were fitted at the rear of the car level with the fuel tank outlet to overcome this problem.
Regards
David

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Hanging on the shirt tails of this thread, does anyone have, or know where to get, a set of the fuel pipes connected to the twin pump assembly as shown in one of George’s photos above? A previous owner fitted a modern in-line pump under the running board area and retained the SU pumps but discarded the connecting fuel pipe assemblies. I could make up an acceptable set but they would not be as good as the original.

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George, thanks kindly for posting the photos and further information.

For the long-stem carb suction chambers I’ve worked on, no spring has been present or called out in the parts manuals. I have the impression springs were added later for short-stem carbs where engine compartment clearance issues encouraged shorter carb stacks.

For the fuel pumps in the picture, can anyone tell distinguishing marks or numbering to tell if they are Type “L” or Type “HP” ? Type “HP” have twice the output pressure of Type “L”.

The parts manual says '46-'48 cars used C.1455 Petrol Pump (S.U. Electric) Complete, quantity of 2. The '46-'48 cars also used in the fuel pumps Ass.1438 Rocker Assembly (complete with Contact Points), quantity 2. Early Mark V cars had Type “L” fuel pumps but carried a Jaguar part number C.2822. After several dozen cars, Mark V used the single C.2823 Type “LCS” pump. I don’t know if C.1455 is a Type “L” pump.

As the pumps are out of the assembly, the opportunity will exist at some moment to measure the pump output pressure. That can be done with a suitable pressure gauge. It also can be done easily with a length of transparent tubing perhaps six feet long attached on the output side to run vertically and see if the pump supports a gas column of height 24 inches (Type “L” specification) or 48 inches (Type “HP” and Type “LCS”). We know from your photos that your pumps are not Type “LCS”.

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A good question, Roger.
This is from an SU bulletin.


There may be an AUA metal tag indicating the spec, e.g. 6V or 12V and optional pipe connections.
HP pumps had AUA numbers in the 40s-70s and a thing called an air bottle on top, sort of like the water hammer chambers in residential water piping.

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