Another idea: Add an electric pump in the back near the tank. That will eliminate any potential for vapor lock. Since you will be pushing the fuel, any air or gas vapor gets pushed out the carb.

By the way, the fuel drains back from my carb after I shut it down, sucking air back through the line and into the clear filter. I've got an electric pump with no mechanical pump and the electric pump allows some fuel to run back through the line. If your fuel pump is shot, it may be allowing the fuel to drain back and suck in some air.