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  #21  
Old 11-26-2022, 07:55 PM
cgarts1 cgarts1 is offline
 
Join Date: Mar 2020
Location: Lyons CO
Posts: 70
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Quote:
Originally Posted by DanH View Post
Before we go, return to the photo from post #4



Is the circled fitting an elbow, or a tee?

If an elbow, and connected to a return line, all the return flow would be passing through the Bendix inlet screen. Trash tends to be cumulative based on volume, so the inlet screen would collect garbage at a higher rate than one which only screens the fuel burned. There are owners out there who fail to clean inlet screens, and...

If a tee, it would be connected to both the supply line from the pump and to the return. If so, it would supply fuel to the wrong side of the screen.
It's an elbow and I capped it.
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RV-14A flying 140600
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  #22  
Old 11-27-2022, 10:04 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 8,032
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Quote:
Originally Posted by theduff View Post
Im not familiar with using the inlet fuel pressure tap on an RSA-5 injector as a return line fitting but it stands to reason if you are feeding the servo with a -6 inlet fuel fitting and you use a -4 to return the fuel to the tank your just going to pump the fuel in a continuous by-pass and never develop enough inlet pressure to run the engine. I suspect the restricter fitting was to choke off some of the by-pass fuel so it would develop the proper inlet pressure.

I would like to hear from someone who is familiar with plumbing an RSA system in this manor and how it helps with Hot-Starts.
I use the -4 ORB tap on the RSA to feed a block that has both a transducer and a -4 return line to the tank. The return line has a valve to open and close the circuit. I use this to purge hot fuel from the lines up front after a heat soak and it works very well for this purpose. On really hot days, I will leave the return open until just before takeoff to insure no stumbles as I go WOT. This is a big issue for mogas, but not as much for 100LL with it's higher/lower vapor pressure.

While a wide open line would allow faster purging, it also creates the risk of starving the engine if you forget to close it before going WOT. I used a #60 drill bit for the restrictor in the return line fitting. This limits flow via the return to about 10 GPH @ 25 PSI. The mechanical pump delivers close to 45 GPH (boost is higher I believe) and the servo consumes less than 20, so there is no negative consequence if you accidentally leave it open.

I suspect that a wide open -4 return line to the tank would prevent the servo internals from seeing anything close to 25 PSI, though I have never done the calculations, as it seems like a bad idea. With my restrictor, I DO NOT see any pressure drop with the return open. Only downside is that it takes longer to fully purge the hot fuel.

I consider it a must have if running mogas in hot temps.

Larry
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Last edited by lr172 : 11-27-2022 at 10:18 AM.
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