Not meaning to distract from this thread being called "looking back in time -Archive Bluebird PHOTOS"
Now here's a curve ball for you. The system wasn't gravity at all fed. Fuel was siphoned from the main tank into a swirl pot with its own boost pump and from there into a second, auxilliary tank in the bottom of the hull - again with a boost pump - then into the engine. Because all the breather plumbing was thin-wall steel pipe it's long gone so we can't be sure how it was set up.
Now I have been reading back on this thread, This part started me thinking (stupidly thinking, probbably utter rubbish) but it was something I have heard of elsewhere.
You say that the "BREATHERS" are "thin wall" Is at all possible that the "BREATHER" somewhere along its length/or AT the TANK could have been squashed, crushed,dented or blocked? during refit or under more demands on them with the fitment of the Orph.
I know that the tank is a hard tank, not like a modern collapseable racing fuel cell, but I think it could be possible that "IF" the "BREATHER" system was strangled or had collapsed due to the higher demands of the orph, you can create a vacuum in the tank, as the space where the fuel used to be (at the start of the run) is now a void, and "SHOULD" now have been replaced by air from outside the tank via the tank breather/breathers.
If it has a resitrction in the "BREATHER" or blocked, collapsed section, does this reach a point where the pump can't siphone any more fuel even if it has fuel in the tank, because the pull of the pump equates to the same pull of the vacume above the remaining fuel? ,
is the pump turning but not able to draw any fuel?
Sorry for the bad wording/explanation of this question But the wife understood what I meant
Feel free to shot me down in flames I probbably talking C*&$
I'm not intending to blame anyone just tring another angle on the debate.