Fuel Return Line to tank

In the standard Rotax 912 installation is a return fuel line, that branches off the line between the fuel pump and the carbs. Via this return line surplus fuel flows from the fuel pump back to the tank. Most cars with carbs have a similar installation.
This return line serves to avoid the formation of vapour locks.

As per the ROTAX installation manual, a Mandatory “restricted” fuel return line is to be incorporated within the aircraft's fuel system. The purpose of the fuel return line is to help bleed off any vapors that may form within the fuel system that could cause vapor lock, resulting in a possible loss of engine power.

Please reference the latest ROTAX installation manual and check with the manufacturer of the aircraft to verify that a fuel return line has been incorporated within the design of the aircraft's fuel system.

one of design is shown in picture.

 Have a read through this safety article (Which xìng wang quoted from above)