Do you have a fuel regulator to get the line pressure down to 2-3 psi ?
My Weber runs on, but that is caused by the fact that I never installed a "Dash Pot" on the throttle plates.
(With loss of engine vacuum, it closes the throttle plates tight to stop run on)
I just turn it off in gear.
I am sure there may be other issues, but these cured my Weber that was New out of The Box install.
CAPT