Fuel return Line for a 912UL

What you have in the photos looks about right. 

0.3mm (0.012") to 0.7mm (0.028) have been used sucessfully.

The larger sizes would be more appropriate for the lower fuel pressures seen in the UL engines.

The smaller sizes would be more appropriate for the iS engines with 45 psi fuel rails.

Once you get it installed, Test it!

An unrestricted fuel system can pump ~30 gal per Hour.

You want to bypass ~5 gal per hour. Not too much more.

5 gph is one quart every 3 minutes.

Run the bypass hose to a quart jar.

It should be about half-full in 90 seconds.

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Be cautious of those thin walled barbed brass fittings.

I had a whole bag of them that leaked through the brass walls right from the get go.

They looked nice and the price was right but in the end they were scrapped.

Thank you Bill,

I hear you about the poor quality cheap parts at attractive prices.  It has become very difficult to find parts at reasonable prices out there.  I'm not at all opposed to paying a fair price for something that will do the job.  The certified aircraft parts or just aircraft-grade hardware, in general, is not just expensive but outrageously expensive.  Case in point a coolant system cap from Rotax is $82.  On a side note has anyone found a reasonably priced substitute for the expansion tank cap?  I might not mind my lawnmower catching fire in the middle of the yard, but the airplane catching fire in the middle of the sky is something very different.  

This airplane is the one I continue to chase my tail on.  It originally came to me for an annual and a 5-year rubber replacement.  The owner admitted that the carbs overflowed fuel out the vent tubes at idle and about up to 2800 rpm.  It would run rough, of course, and then smooth out at higher rpm when the engine was using more fuel.  It did not at this time have a return line.  I know the excessive vibration can cause the float bowl needles to unseat and cause flooding.  I know Rotax wants a return line to help eliminate vapor lock during a hot restart.  I know the floats get saturated with fuel and sink.  I have discovered many things about Bing carbs and aircraft fuel systems in general during the process.  But the airplane needs to start acting right and fly away.  

I even jokingly suggested to the owner that we install a fuel injection system from Edge or someone like that.  Troubleshooting goes on.  It is just a machine after all, right?  

Hi Bill,

Okay, the jet that is currently in the return line is a Rotax part # 963 143 and is described as an Idler Jet 35.  The same jet that screws into the carb body next to the main jet.  Page 9 in section 73-00-00 of the Installation Manual calls for that size restrictor.  I am assuming 35 is 0.35mm.  I disconnected the return line at the header tank and used the electric backup pump which gave me 2.8psi and got 4oz in one minute.  So 4 x 60 = 240 and 240 / 128 comes out to 1.875 gal /hr.  If I am looking for an approximate bypass of 5 gals/hr then 1.875 is way too little and the fuel is backing up in the manifold and overpressurizing the float bowls.  I need to test your suggestion of a larger orfice.  Is my thinking on centerline or am I still going off the side of the runway? 

Thanks again for your advice.    

 Fuel is not backing up and overpressuring anything.  The pressure is regulated at 2.8psi.

Other than the 912ULS, all the other installation manuals only have very vague recommendations.  No dimensions.

It is interesting that the fuel injected engines were the first to have the bypass hose with the only intention of de-pressurizing the fuel lines while stopped.

I have the feeling that the 0.35mm dimension was a Cut and Paste from the earlier 45psi engine manuals.

Based on your current flow rates, passing a #74 drill bit through your orifice should put you real close to 5 gph.

Try a #77 bit first incase I miscalculated.  You are currently real close to a #79 bit size.

Dear Bill

How is the pressure regulated to 2.8 bar?