ok, so,
i'm here to put the final nail on this problem's coffin. Hope this helps someone eventually.
So in last resort, I had to resign to call the Man, and he came pick-up the car to work on it in his shop because clearly, I could not figure it out. even on the leanest of the leanest, the carb was spilling fuel like the Bellagio fontains if connected to a gravity fuel tank.
So, several thing: I was actually right about the fuel pump: it was scrap.
and turned out I did nothing wrong while puting back together the carb. I was very afraid of having messed up this one. even though I did it twice, by the book and all, step by step to make sure I did not screw it up.
once he replaced the fuel pump, his diagnosis was the same as mine: could not set it lean enough.
so he dismantled the carburator again, looking for issues. and turned out that the faulty part is the lil tube that plunge into the carburator tank to pick-up fuel. It was sligly too large an a bit oval. and of course, because of that, you could lean it as you want, fuel was still getting up the jet.
not so fun fact: this part was new, genuine SU part, from the HIF carburator repair kit. my fault was not to suspect a brand new part from being faulty. such a messed up story...
So, if you do refurb your carb, check the parts that you get from the vendor. it will save you some money and trouble. Lesson learned, the hard way.
Edited by Benoit_Dupont, 04 October 2019 - 05:27 PM.