1974 grew with omc 307 inboard/outboard?

I'd bet your fuel pump is not original, so it's difficult to tell what is happening. A lot of the double diaphragm pumps have a "sight bowl" there fuel collects if the pump malfunctions. You really should have a marine mechanic have a look at this, because it's both an issue of safety, and you may be missing other parts in the fuel system.