ROTAX 912 - New style fuel pump/fuel pressure question

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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FastEddieB
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Sat Apr 28, 2018 4:41 pm

3Dreaming wrote:I had one airplane that the new fuel pump was to much for the carb. It would overpower the needle at idle and the engine would run rough. After trying a few things myself I removed the carb and sent it to Leading Edge Airfoils and they worked some magic on it and the problem went away.

Interesting.

So, in that case the extra pressure just uncovered an existing issue.

Otherwise, it seems it would be trivial to install a metered orifice at the pump’s output to slightly lower the pressure.

As an aside, BMW airheads were/are notorious for carb float bowls overflowing due to needle/seat problems, even with just gravity feed. Has led to a lot of gas-soaked boots over the years!
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby drseti » Sat Apr 28, 2018 7:56 pm

FastEddieB wrote:As an aside, BMW airheads were/are notorious for carb float bowls overflowing due to needle/seat problems, even with just gravity feed. Has led to a lot of gas-soaked boots over the years!


As an aside to your aside, those BMW airheads used the same Bing 64 carbs we all know and love. (The only difference is, when mine leak they don't soak my boots.)
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Sat Apr 28, 2018 8:35 pm

drseti wrote:As an aside to your aside, those BMW airheads used the same Bing 64 carbs we all know and love. (The only difference is, when mine leak they don't soak my boots.)


As an aside to your aside of my aside...

...I think I’ve posted this before, but here I am at a BMW Rally in Missoula, MT in the 1990’s:

Image

I was replacing my BING floats and float bowls with a new style, recently designed for the BINGs on ROTAX airplane engines. The old style had the two floats connected to each other, which I guess might have been a problem with an airplane in uncoordinated flight. And in a case of positive transfer, the new style also worked better when retrofitted to older airheads.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Sun Apr 29, 2018 3:22 pm

I flew the plane today on a short crosscountry.

I was surprised to see this before I began my runup:

Image

Ran normally, but remained high in the climb:

Image

In cruise is gradually dropped back to 6 psi indicated:

Image

On the flight back home, this in the climb:

Image

Went up a bit in cruise.

I'm wondering, why all the variability? It was a bit warmer in the afternoon, but not by all that much. I remember some complaining about high pressure alarms from their Dynons or whatever with the new pumps, and wonder how that was handled.

Another curiosity - before starting, the boost pump used to slowly bring up the fuel pressure until it barely touched the green. Now it goes well into the green. Which is fine, but I'm trying to visualize how change in the engine-driven fuel pump could affect the boost pump pressure before engine start.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby dstclair » Sun Apr 29, 2018 5:11 pm

I found an anomaly in the operation of the 'new' style pump and the Facet aux pump (3-4.5psi). Documented it in viewtopic.php?f=19&t=4915.

I had a chance to prove my theory. Replaced my 40106 (up to 7psi) with a used 40105 and my fuel pressure behavior went back to unstable. With the 40106 I have steady in the green pressure, whether the pump is on or off. I do have steady pressure with the used 40105 'on' but when it's off I'll drift into the low yellow, then creep back into the low green.

The 40106 is around $41 at aircraft spruce so you might want to give it a try.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby drseti » Sun Apr 29, 2018 9:30 pm

Make sure the vent line for your new pump is in undisturbed air (preferably inside the cowling). Should cure instability.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Mon Apr 30, 2018 7:38 pm

drseti wrote:Make sure the vent line for your new pump is in undisturbed air (preferably inside the cowling). Should cure instability.


I thought that vent line was just to vent fuel to a more benign spot if the pump failed in a specific way.

Do I have that wrong? Can positive or negative pressure to the hose make a difference?

In any case, I really want to plumb mine to here:

Image

That's my gascolator, and a bunch of engine drains leading to a funnel-type deal that has a hose leading to the belly.

But it was impossible for me to snake it by itself, so for now its just routed to the cowling:

Image

Remember, that's the back of the cowling. Next time I have help I'll go to more effort to reroute it where I want it.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby drseti » Mon Apr 30, 2018 7:45 pm

Yes, Eddie, it can matter. If the end of the hose is in a low pressure area, it can pull a vaccuum, warp the pump diaphragm, and affect the fuel pressure. Given where you have it routed now, just try trimming 3 or 4 inches off the end, to make sure it's in a neutral pressure location inside the cowling - and then do a test flight. (If it makes a difference, you can pay me later.)
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Mon Apr 30, 2018 7:46 pm

Additional weirdness.

Flew up and back to the TN property.

Nothing was changed, and in essentially the same flight conditions as yesterday, instead of seeing the fuel pressure maxed out, I saw this:

Image

Good news is the pressure was somewhere in the green for all 1.5 hours. But in level flight, I could see 3 psi or 6 psi or somewhere in-between with no rhyme or reason.

Its not bouncing around - it just very gradually settles in at one spot or another.

Like I said, weird.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Mon Apr 30, 2018 7:48 pm

drseti wrote:Given where you have it routed now, just try trimming 3 or 4 inches off the end, to make sure it's in a neutral pressure location inside the cowling - and then do a test flight.


I will try that.

(If it makes a difference, you can pay me later.)


Later sounds good. Maybe next eclipse?
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TimTaylor
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby TimTaylor » Mon Apr 30, 2018 8:10 pm

Looks like it might be creating a negative pressure where it's mounted now. I suppose that might cause a higher fuel pressure reading, IDK?
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby drseti » Mon Apr 30, 2018 8:16 pm

FastEddieB wrote:Later sounds good. Maybe next eclipse?


No way, Eddie. You'll pay me during totality, so I can't Count de Monet!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby proemer » Wed May 02, 2018 10:12 am

Eddie -- On Sky Arrow sn 019, the "new style" fuel pump has no hose at all on the "vent line"... don't know if it came from the factory this way, or was a replacement. Anyway, in flight, any vented fuel would go out pretty well to the rear, on the ground, some might inside the cowl. Fuel pressure seems to be "normal"... which is to say "pretty variable, but in the green".

Pete

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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby FastEddieB » Fri May 04, 2018 6:55 pm

As an update, I went on a 3.8 hr cross-country yesterday.

Good news, the pressure stayed in the green the whole time.

But weirdly, it still varies. In level flight it could start out just over 2 psi, then over time inch up to sit in the middle of the green, about 4 psi.

I find it hard to attribute that to dynamic affects on the vent hose, since the flight conditions did not change at all.

But, to keep Mr. Bernoulli at bay, I came up with this kluge:

Image

Just a larger diameter hose with small holes slipped over the vent line to serve as a "baffle", plus shortened the hose to be more out of the direct airflow.

I also cleaned and applied Stabilant-22 to the terminals of the fuel pressure sender, to be sure there was no issue there.

Will report results next time I fly.
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Re: ROTAX 912 - New style fuel pump/fuel pressure question

Postby roger lee » Sat May 05, 2018 8:09 am

All that was needed was to adjust the float arm height which would change the amount of pressure the floats applied to the armature and then to the needle valve.
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