The recirculating line usually comes off the fuel assembly block that is attached to the carb cross over tube. Look in the parts catalog. If you don't have that which was supplied by Rotax then a separate line that comes off up on top where your fuel lines split to go to each carb. This line on many aircraft goes back to the gascolator. The gascolator has a port for the fuel in on one end and a fuel out on the other. The recirc line goes in on top. Others that don't have this type of setup hae routed the recirc line back to their header tank or fuel tank. This does two things. Helps prevent vapor lock back circulating cooler fuel and helps from an over pressure situation. Many older planes or kit built planes didn't have them, but they really should. It is an easy line to install.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
As Roger says the line splits off the fuel line going to the carbs. On my aircraft it is a steel barbed T-piece that is in the fuel line after the fuel pump and before the T-piece that splits for the carbs. The branch of the T- part that returns to the fuel tank has a restrictor in it which is a brass plug, braised in (I think) which has a hole in it about 1-2mm diameter. This allows fuel to keep flowing and so helps prevent vapour lock, and also helps stop the pressure building up too much. I am sure Rotax will sell the return T-piece.
I had a friend who changed his and didn't realise the branch of the T-piece had a restrictor in it. He flew hundreds of hours without any problem, but I wouldn't recommend it.
If you buy the part from rotax for the return line that attaches to the fuel assembly block, the restrictor comes with it. If you make up your own tee, you can make a restrictor to press into the tee. Easy to make from a brass plug. Hard part is drilling a hole that small. Rotax specs are 0,35mm (.014in). I have made one and got a local watchmaker to drill the hole for me.
Ok, I'm not real esperienced in Rotax, but, if you run a return line to the header tank, (Which is Full of Fuel All the Time) where does the fuel from the return line really go? Does it actually back- pressure the system to eventually push its way into the wing tanks above it?
Why not install a pressure regulator and eliminate the extra fuel line?
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