SU Carburettor Fuel Leaks

Just rebuilt a set of SUs for a 3.5 litre engine. I am familiar with SUs on MGTs & Big Healeys where the jet is sealed with two cork washers. On the Jag the top of the jet is sealed with a cork washer #37 but nothing below. I am finding petrol is leaking out of the adjusting screw jet #34. I know that the cap nut #45 washer has fiber washer to stop this. Does this mean when adjusting the carby mixture you have to put up with this petrol leak? Also no matter how tight I tighten the nut #44 the bearing jet #41 turns when you tighten the cap nut #45, is this normal? Looking at the diagram it appears that the nut #44 doesn’t lock the bearing jet #41 & the bearing jet is only secured by the spring #39?
I’ve used new washers & seals in the rebuild.

Regards Peter

I’ve just done mine and similar experience re dripping fuel when cover off and rotating of jet. I had to recentre the jet and the rotation stopped after this repositioning and tightening but fuel still leaking around the adjustment screw

I found it helpful to look at the MGTC carb diagram on Moss.

It explodes out all the washers more clearly, and you can see that most of them are the same positions and functions as in our SS and Jag carbs. Just the adjuster screw and cap are different. And with all new washers in there I did not get much leakage around the adjuster screws, and none after I put the caps on with their fiber washers.

Just an idea from a long since departed carburettor specialist. Soak the cork washers in brake fluid for a few hours or overnight to plump them up. That helps them to seal better. You shouldn’t have any leakage if you do this.


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Thank you Gentlemen for your comments. Thinking about it I decided that unless the bearing jet was stopped from turning then it would always leak fuel. I measured the thickness of the new copper washers & found that they were thinner than the originals. It was a tossup whether I put two on the top bearing or two on the bottom bearing, I went for two on the top bearing & now I can tighten the sealing ring & lock the bearing jet from turning. Look likes replacement service kits aren’t made to the same specs as the originals.

Regards Peter

Both of my Mark IV SUs seem to be running extremely rich, spewing black soot from the tailpipe. New points, plugs, and condenser. Old plugs black with soot. New ones sooted up almost immediately. Not oil, soot. Th carbs also leak when the caps are removed for adjustment. But I don’t think that’s my problem. However, there is not any appreciable change when turning the adjustments all the way in or almost completely out on either carb. I tore them down and soaked in carb cleaner overnight, then blew them all dry before reassembly. Each needle shoulder was flush with the bottom of its piston, as described in the maintenance manual. No damage to either of the needles…both clean and straight. Once reassembled it didn’t make any difference. Thoughts? Who has rebuild kits for these carbs?

It might be worth checking that the starting carb is not remaining on even after the car has warmed up.


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Thanks for the thought Peter, but it doesn’t seem to be as it idles down after running for a minute or so.

It’s possible that your float levels are wrong. If you remove both dash pots and pistons/needles so that you can see the tops of the jet tubes then turn on the ignition so that the fuel pumps are operating the fuel level in the jet tubes should be just a fraction below the tops and certainly not with petrol flowing out of them. If the latter is the case then check that your floats don’t have fuel in them by shaking them and if they are empty then set up the lever arms that act on the floats. They are set by inverting the lids and checking that drill (is it 3/8" I can’t remember) positioned under the arch of the lever and resting on the now upper edges of the lid does not lift the levers off the needle jet and the arch is just touching the drill.




A cold engine will spew soot from the tailpipe. The rich mixture and cold pipe means water condensation inside the exhaust line will get blown down and out the pipes carrying the soot from the exhaust line walls. Once the engine warms up to where the exhaust line is hot enough to prevent internal condensation then the soot spewing out the back stops.

Engines run for brief periods (e.g. less than ten miles from cold start to shutdown to cold) will experience internal soot buildup also. This is common in old collector cars which are driven only short distances and also on modern collector cars driven only short distances (e.g. common on Ferraris). I’ve experienced this in my Mark V. De-coking used to be done and still is done in these circumstances.

Starting up these pushrod engines does run them rich for the first couple of minutes, then the mixture should be leaned out to run well without sooting up the plugs. I noticed sooted plugs on my Mark V when pulling the engine to replace a broken transmission tooth. With only 5,000 miles since rebuild I came to realize I had changed my driving style from daily driver to weekend ten miles a day. Borescope showed buildup on piston crowns and head off showed the buildup clearly. You might consider plug heat range change as one option.


And to add to Peter’s solid advice, here is what the S.U. Workshop Manual says for fuel level associated with float setting. From page B.4:

“Regarding (a), the position of the fuel level in an S.U. carburetter need not be exact and to a critical dimension, but the standard level is 3/8 in. (10 mm.) below the level of the rectangular facing known as the jet bridge.”

I have not found a float level adjustment in the Service Manual for All Models 1946-1948. That manual does describe adjusting richness of the starting carb. In the Mark V Service Manual, consistent with Peter’s advice, page C.14 says:

“Remove the float chamber cover and holding this inverted, press the forked float lever downwards with the finger above the hinged end, thus ensuring that the needle valve is in the closed position. If the petrol level is correct a 7/16” (11.1 mm) round bar will just slide between the lever and the spigot on the lid. If the level does not touch the bar at the same time as it holds the needle on the seating adjustment must be made by bending the lever."

I have found that using the 7/16" round bar method may give a fuel level higher in the jet bridge than what S.U. recommend.


Bingo…I think. That’s one thing I noticed before tearing them down that with the dashpots and needles removed and the ignition on, fuel dribbled up through both jet tubes. Not a lot, but it doesn’t take a lot for an engine to run rich. Floats were both okay when I took them out during disassembly. Probably a day or two before I get to checking it out. Thanks guys.

Excellent advice as usual from Peter and Roger.
Moss Motors sells all the gaskets and other parts, listed under MGTC.

I can only add that I have had a sunk float on two occasions, and the symptoms were overrich running and black smoke. The first occasion I was 20 miles from home, and I removed the float lifting it out with a paper clip, smeared RTV sealant on it, and it lasted long enough to get me home before it was sunk again. The second time I was 100 miles away from home but I had a spare with me, so I changed it at a truck stop with a bunch of truckers looking over my shoulder. So I always carry a spare float.

Then there was the time I was at a British car show in the Chicago suburbs, and the announcement came over the PA system, “Anybody have a float for an SU carb, please bring it to the Triumph area.” So I drove over in the XK120, holding the float out the window, to where I found a crowd around a sad looking guy with a TR3. I got a big round of applause and cheers from the Triumph people, “Jaguar saves Triumph”. He mailed it back the next day from Wisconsin with a note of thanks.

That was standard operating procedure when I did the early SUs: I soaked them in motor oil, but I don’t think it really matters which one.

Okay. Confirmed floats not sunk. I checked the float levels…one was a tad under 1/2-inch and the other was a tad over. Carb rebuild kits came in from Moss today, so I replaced both float needles and seats. I set them both to 7/16. Put it all back together and now there’s no more gas seeping up from the needle seats when the pistons and needles are removed, and ignition is on. However, it’s still running very rich. I turned the adjustment on both carbs all the way closed and made no difference. And warm-up carb is definitely disengaged. It runs the engine at about 1500 rpm on start up, then after a couple of minutes, settles down to about 700 rpm. And even with new gaskets on the bottom end, both carbs still leaking. :frowning:

It might worth checking your needle profiles. I think the needle code is stamped on the part of the needle that is clamped in the piston. It’s quite faint and I couldn’t see it without a lens. I would expect your needles to be stamped FL or possibly DY. Both of these are standard needles. (Could also be FW.) Rich needles would be AQ and weak needles GA.


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The needle operating on the seat for the float action to determine the fuel level in the float chamber and associated level in the carb bridge may not function the same when the engine is running and when it is not running but just has fuel pump clicking over.

There are a variety of needles in use and some vibrate to a leaking condition when the engine is running. This happens when the engine vibration on the needle, in combination with fuel feed pressure, is enough to unseat the needle when the float chamber should be full. On the needle side, I’ve seen this for some needles even when the fuel pressure feed is correct.

More often I’ve seen this condition when the fuel pressure feed is too high from the pump. These carbs work at relatively low pressures and the correct fuel pressure is essential.

In terms of leaking on the bottom end, it is quite normal to have some leakage there if the nut caps are off, but one must use the nut caps judiciously to eliminate all leakage once the jets are set. Jaguar provided a protective fuel spill tray under Mark V carbs yet I’ve seen several burned bonnets with scorching over the carb region.

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I’ll pull the pistons today and check the throttle needles.

I understand what you’re saying about float bowl needle vibration from engine operation. I’m not sure what can be done about that other than brand new needles and seats, which I just installed.

As far as overpressure from the fuel pump elevation above the carbs, perhaps that’s why my fuel pumps have been relocated. They now sit vertically, wired to the inside of the left inner fender. I had no idea why a previous owner might have put them there, other than for the exact same reason you’ve stated about their normal position being above the carbs contributing to an overpressure condition. I was going to put them back to where they are supposed to be mounted on the top of the firewall, but for the time being, I’m going to leave them where they are until I get this rich fuel problem corrected.

Thanks to you both for help. More to come.
Regards, George

Peter, one needle marked “DY.” Cannot get the other out. Soaked in WD40. Didn’t help. It seems to be very slightly recessed into the piston further than the other…maybe 1/16 of an inch.

I’ll most likely destroy the needle removing it. Where can I find a replacement?