Fuel return line Rotax 912

[Arnulf]

Hi

In the standard Rotax 912 installation is a return fuel line, that branches off the line between the fuel pump and the carbs. Via this return line surplus fuel flows from the fuel pump back to the tank. Most cars with carbs have a similar installation.
This return line serves to avoid the formation of vapour locks.
Are vapour locks a problem on an installation where the motor is in the open, like a trike?
Are there any safety implications if I block off the return line?

Regards,
Arnulf

[Mogas]

This reply also in response to a pm from Morph regarding the routing for the return line. It is the same layout I have used for all my microlights and homebuilds and works for me.

[Morph]

OK,

This setup will still give you accurate fuel usage readings through the fuel flow.

So all you are doing is providing a return line via the carb side of the pump, via a restrictor to the tank side of the pump.. What exactly does this achieve?

Secondly, you have an electric fuel pump. Does this pump have a one-way valve in it? Otherwise fuel will push back through it instead of the restrictor

Where does one get a restrictor

[Mogas]

The restrictor allows excess fuel pressure to return to the line and not try to overcome the float levels in the carbs.
I can not tell you what size my restrictor is unless I strip the system. My restrictor came with the 2nd hand engine I bought. I recall it looks home-made, about the size of a main jet, made out of a piece of drilled brass pushed into the restrictor line. I suppose the size will depend on your pump output.
Yes, the electric pump I have also has a one-way valve. BTW, this pump is purely used for priming the carbs if required and as a standby pressure source. I normally have no use for it in flight even for takeoff and landing, emergency use only. In a normal situation gravity feed from the tanks would be sufficient if the mechanical pump packed up, provided filters were not restricted.
Do you have a means of measuring the fuel pressure? if so you could experiment on the restrictor size to get the correct pressure at your carbs.
Hope this helps.

[Arnulf]

Hi Mogas,

I like your fuel system. Redundancy, what a clever concept. Gravity feed, mechanical fuel pump, emergency fuel pump. I like!
Do you know off hand what electrical fuel pump you have installed? Make and model No?

Regards,
Arnulf

[Morph]

Only one change I would make here, I would put the filter before the Fual pumps, to stop any crud from going through them

[Morph]

Next question,

where do you get this fuel restrictor?

I spoke to Aviation Engines and they told me to take a t-piece. Block the leg that is to be used as the fuel return with alluminium or lead? Then drill a .35mm hole through it. Any other ideas guys?

[Arnulf]

Hi Morph,

when I needed a restrictor, I scratched around in my collection of odd nuts and bolts, and found some brass "screws" with a head for a flat screw driver, a smooth hollow shaft with a thread inside. I can't recall where these screws come from, possibly stripped out of some electrical component. They cannot be too scarce, as I had a couple of them. The diameter of the shaft is 6.5mm, and it fits snuggly into a 6 mm fuel line. The required size hole can be drilled into the head. The head I filed to the size of the shaft. If anybody recognizes these screws, let me know what they are. Also where do I get a 0.35 mm drill bit

[Mogas]

Hi Arnulf
I am not sure what make and model of fuel pump it is, came with the motor, tested it and installed. I will check next time I have the cowling off. The pump is not the pulseing Facet type, it has a motor and impeller.
Hi Morph
I fit my filters just before the carbs as there is always the small possibility of the pumps themselves generating a bit of loose stuff.

[Pumba]

Guys

Have a look at this website: www.800-airwolf.com, item number 8511. It's a fuel pressure regulator for US$29.95. I'm sure they will do mail order.

Rather than find the required orifice size by trial and error, this allows you to set your fuel pressure to the specified value.

Regards
Barry

[Arnulf]

Hi Pumba,

unfortunately the link does not work.

Regards,
Arnulf

[Morph]

Hi Morph
I fit my filters just before the carbs as there is always the small possibility of the pumps themselves generating a bit of loose stuff.

Yes but what's stopping grit from getting between the diaphrams of the fuel pump and impairing the operation?

A .35mm drill should be available from any electronic supply shop like Communicate.

[Mogas]

Hi Morph
Quote: Yes but what's stopping grit from getting between the diaphrams of the fuel pump and impairing the operation?
With the sump in the header tank I have never even had any crud reach my filters, I use the glass purolators, inspect every preflight, and they are sqeaky clean.
BTW I syphon with a modified jiggle pipe, its got a filter on the outlet end of it so nothing gets into the fuel sustem that way.

[John Young]

I use the glass purolators.

THIS (shown below)

SURELY not

Regards
John ZU-CIB

[Mogas]

Was waitin for that!
I should have said purolator type.