mysterious leaking of SU HD8

The rear SU HD 8 on my Series 1.5 E-Type is leaking through the jet when the fuel pump is on. A stream of gasoline about three inches high comes from the jet when the needle/piston/chamber is removed. The other 2 carbs do not leak and the fuel level in them is correct. Most of the typical culprits have been checked: float level correct, float needle/seat replaced, float is correct weight and has no leak when placed in hot water, jet/diaphragm replaced, float bowl overflow pipe is clear (no leaking through the overflow), float needle lever pivot adjusted, many expletives directed at the problem area.

Recently I let the pump continue and the stream of gasoline stopped after about 10 seconds during which about a half cup of gasoline came out through the jet of the rear carb and none through the other two. The gasoline level in the jet is now about 1/16 inch below the bridge in all three carburetors. I have no idea why it stopped leaking. I assume the float bowl is full and the needle is closed. What I don't know is what will happen when the float chamber becomes empty after sitting several weeks. For example, it may put another half cup of gasoline through the rear carb before closing the needle; this is a bit too rich for a typical cold start!

Does anyone have suggestions for what might be the problem?

Thanks for your ideas.

billca

Your float valve is stuck, try opening the float bowl lid and cleaning it and see if that fixes this issue.

I'd like to think you are correct; however, I've cleaned the bowl, replaced the float valve, and reset the float a couple of times (It's now at 1/2" instead of the recommended 7/16".) The float/needle mechanism works smoothly when removed from the carb. I've also tapped on the float bowl lid with a hammer with no results. What is unknown is why the float valve is still sticking and why all the gasoline is going through the jet and none through the overflow pipe.
Thanks for your response,
billca

Because your overflow pipe is blocked !

Because your overflow pipe is blocked !

It needs to vent so that the float will rise and close the needle and seat. Once you remove it completely and run mechanics wire up the inside of it from the end to unplug the rust and crap , your worries will be over!

Thanks for the advice about the overflow tube. I have checked by blowing through it which confirms that it is not totally blocked. However, it may still be partially blocked so that it doesn't vent fast enough. I'll run a wire through it. Wish me luck.
Billca

For the overflow, make sure you have the two washers, one on either side where the float bowl nut passes through it, in the correct locations. The "segmented" one goes on the bottom and the full one on top. If you have a full diameter washer on the bottom then gas cannot flow to the overflow pipe so it won't matter of it is free or plugged. As for why you're getting the flow, I agree with the others that the float valve is hanging up.

Just had the same exact problem with an XK 120 that I revived . Had plenty of hung up float valves but never one that caused fuel to shoot out the jets.

Conclusion: I ran a stranded wire entirely through the overflow pipe both from the top and from the bottom. In neither case did I observe any gunk come out on the wire. However, this did fix the problem. The float bowl fills, the float valve stops the flow when the bowl is full, and no fuel comes out the jet. (That was a good suggestion about the segmented washer on the overflow fitting. I did have the segmented washer in the proper place.)

Lesson learned: My original test of the overflow pipe was to blow through it with my mouth from the banjo fitting end at the top. No blockage was observed, but there must have been a partial blockage. On reflection, I recall that when the bowl was filled from being completely empty, a 3 inch high stream of fuel came from the jet. When I tested it again after a few minutes, the stream of fuel was only about an inch high above the bridge. I didn't recognize that this might have indicated a partial blockage that I missed when blowing through the pipe.

Now my only problem is that all of the 'new' fiber gaskets on all three fuel inlet banjo fittings slowly leak. Fuel seeps through the gasket material as if the material is old and dry and porous. At least I can see the problem and have ordered some more gaskets.

Thanks for all of your suggestions.

Billca