If you have a cap on both tanks you need to have an unvented cap on the top tank and a vented cap on the bottom tank.
The engine gets its fuel from the top tank, the vacuum that is created in the top tank pulls the fuel from the bottom tank up to the top tank.
It really is a stupid setup and a lot of them have had an electric pump added to the bottom tank to pump it into the top tank.
white 2-155 fuel tanks ,
