A fuel return line is required for the 912ULS, but many people simply route it back to the gascolator (instead of to a header tank or back to the wings, which would require more plumbing). I'm not aware of any Zeniths suffering from vapor lock problems with this setup, but vapor lock is the reason some people are concerned about having the return line go just to the gascolator instead of all the way back to a fuel tank where it would mix with a higher volume of cold fuel. I think that's probably a bigger concern on tightly cowled airplanes where there isn't much airflow over the fuel lines, but maybe other people have had a different experience. Use fire sleeve to insulate the fuel lines and route them away from the hot exhaust system. 

The best thing, by far, is to defer to official Rotax installation manual. And then defer to the official Rotax operating manual.

Hi Rich,

I am flying a STOL 750 with a Rotax 912 ULS. Very happy with it. I attached a drawing of my fuel system. Rotax asks for a return line since some time. When I started my build the return line was not necessary. So I had to route it back to the gascolator because the wings were already closed.

So far that works fine (no vapour lock occured so far). But if I could redo it, I would do it differently and route a line back to the left wing tank. It could serve for the return line but also for a venting line in case you want to install a header tank what I recommend.

I want to refit a header tank cause in my case the wing tanks drain unevenly (most builds do). The header tank helps to prevent lack of support from either port while flying turnes in descends and low fuel situations. With a header tank I can reroute the return to the header and use the line back to the wing tank as venting line.

One day I will cut a hole in the finished wing to access the spare port on the left wing tank..... Should have done it right away.