Bruce,

There is quite a bit of historic information on this list regarding issues with the "old system" without a return line.

Do an Advanced search using the terms "Vapor Lock"

As Jim O said, this is not a 100% deal; however, the current design Rotax system with the return line solves many of the flooding, hot soak and vapor lock issues that are somewhat uncool to experience with the old system.

Certainly a person with an experimental can make deviations from plan; however, it's hard to beat doing the installation completely to specification for best reliability.

How well the return system works depends on how closely the installation matches the design. The check valve is critical to prevent unfiltered backflow to the carburetors, where the return line goes back to relates to the potential for recycling volatilized fuel back to the engine; and, the orifice regulates the correct amount of return flow.