I'm having the same problem with my 72 240Z I put clear fuel hoses filter all the way to Su carbs so I can see the bubbles ..runs fine for about 10 minutes gas are in the fuel hoses then bubbles start to appear from the tank Hardline hose to the filter. So I install electric fuel pump in front of the mechanical fuel, pumped all the gas out of the tank into a container. Gas ran freely there were microscopic bubbles but not large ones. Remove the drain plug on the gas tank no gas came out it was completely empty.. so is pick up tube good seems like it is? metal line might be bad ..next I'm going to run a rubber braided fuel line to the mechanical fuel pump from tank. Any suggestions would help.