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.


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