Fuel manifold on newer 912 ULS engines

[Cowboy]

what is everyone doing with the nipple fitting on the fuel manifold that says “fuel tank”. I assume they are talking about a fuel return line. I am changing out a 2005 912ULS with a 2019 912ULS and I do not have a return line. Most the people I’ve talked to said they have never heard of a fuel return line on a carburetor engine. Are you blocking this connection off?

Bruce

[efwd]

Look at the Rotax website. They explain why the fuel return line is necessary. I believe its a vapor lock thing.

[Cowboy]

I did read it was essential to the operation of the fuel system, but not sure why. There was no fuel return line on the 912ULS I am taking out.. Don’t know why it would be essential on this new engine.

[jiott]

Just do it. I don't think you want to find out why the hard way. It is very good insurance (not 100%) against vapor lock.

[Cowboy]

Can it just be T’d back into the fuel line before it goes to the fuel pump?

[Dave S]

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.

[rawheels]

Others in this thread (https://www.rotax-owner.com/en/rotax-forum/3-4-stroke-technical-questions/5335-fuel-return-using-banjo-bolt-and-restriction-jet) sent their return line to the top port of their gascolator. I was planning to do the same.

[jiott]

I know several have returned into the gascolator, but in my opinion this is not the best way to do it. It sort of defeats the whole idea of the return, which is to bring a small amount of the coolest fuel (from the header tank) to replace the hot soaked fuel from the engine compartment. If you tee in the return anywhere after the header tank you are not getting full advantage of the return system. As my daddy used to say "if a job is worth doing, it is worth doing well".

[Cowboy]

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.

Thanks Dave and thanks to everyone who replied. I talked to one of the techs at Lockwood today. He said yes you need to run the line. He did say it was alright to tee it into the header tank. I put a 3 way tee in the line between the fuel tank and the header tank. So, I think I am good to go now.. Appreciate the feeb back

Bruce