RonBoth fuel pumps ( Electric SU & the AC Mechanical ) have separate feed from tank, one from either side of the fuel tank, My electric SU pump in the boot is used to prime the Solex by cutting into fuel rail and feeding front Solex first whilst the mechanical pump feeds the rear Solex first.
On continental runs when touring with mainly Cobras I often run both pumps to ensure adequate fuel supply. ( otherwise I have problems keeping up with
the V8s)
The rubber ? diaphragm in the AC mechanical fuel pump can be effected by modern fuels and should be checked and replaced regularly.
On the ACOC Luxembourg International, I rebuilt the The AC mechanical pump , but I found it could not supply enough fuel due to fuel vaporisation ,
mind you we were driving in temperatures of 39 /41 degrees C.
One or two of the racing Ace Bristol's have a small fuel reservoir in line under the bonnet so they always have a continuous fuel supply especially when
low on fuel and cornering hard.
Hopefully the above is of interest and you resolve your problem.
Keith