N

n38139

Well Known Member
I have an RV4 that is an old airplane and it has a return line from the carb back to the right tank. I have a carb and not fuel injection. The real question is that in the line to the tank it has a shutoff valve and I don't know why. Anybody have an explanation?

thanks
 
This is just a guess;
A fuel return line from the fuel admission point on the carb is a way to purge air or hot fuel vapor from the line between the fuel pump and the carb. It would keep the fuel flowing in this line even at low idle, which would prevent the fuel from absorbing too much engine heat. This heated fuel is returned to the right tank on your plane, so cooler fuel can flow into the fuel line/carb.
The builder may have been concerned with possible vapor lock.
The valve would be open for ground operations on hot days, where heat soaking might be expected, but closed in flight to prevent the steady flow of fuel to the right tank?
 
Exactly what Shipchief said.
Having built 2 airplanes, both with return lines and return fuel selectors, I am quite certain that Shipchief is right on the money.
While some return lines have no selector or shut off, they simply return to one tank. Having a shut off or selector valve adds flexibility for fuel management.
A little complexity too, but the benefits out weigh the added complexity IMHO.
 
That really makes sense, I didn't really state the proper thing though it actually comes off the pressure side fuel pump back to the valve then to the tank. The fuel pump has a dual outlet fitting one side going to the carb and the other side coming back into the airplane. I have never opened it since I bought the airplane.

thanks

Steve
 
Done right!

So does mine.
Like Shipchief explained, this return line allows for a constant trickle of cool fuel to come through the fuel pump and thereby minimizing the onset of vapor lock during the most critical time when you taxi around in 100F and idle engine operation consumes a mere couple of GPH. Vapor lock rears its ugly head on take off when you need fuel pressure the most but manifests itself on the ground when you boil the fuel in the fuel pump. I had the very same set up in my RV-8 and I never bothered to turn it off. You just need to keep track of where the fuel is and have accurate fuel level indicators. Mine was an IO-360 but same concept.
The idea of a return line has been around for some time and just recently got some renewed traction in the forums.
Alternative methods of vapor lock prevention has somehow replaced this simple idea.
 
Thanks for the replies. There are alot of little things like that the original builder included in my airplane. It is a 1986 model with 1700 hours on it and I am basically re-wiring and fixing stuff.

When I had my RV8, a friend of mine had a large tank with auto fuel we put into one of my tanks because of a favor I did for him, and taxiing out, it quit and wouldn't start. I pushed back off the taxi way and when it cooled down taxied back in drained all of the fuel and put avgas in it and had no problems. There was no water or trash, I believe it was vapor lock.

Steve
 
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Fuel System Diagram

Would one of you guys have a fuel system diagram showing the return line? I was planning on a Bendix FI I0-360 which doesn't require return fuel, but the cooling feature would be great for N. Texas summers! Thanks!
 
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