Not worked on one of these before, however the thin pipe is nearly always the fuel inlet which will have a fuel filter on the end in the tank, this will go to the fuel inlet elbow on the car, if this machine has a primer there will be a pipe from the other side of the carb too the suction side of the primer, there will then be a pipe from the pressure side of the primer and back into the tank, so the primer pulls fuel from the carb into the bulb and back to the tank.