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frans(at)privatepilots.nl Guest
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Posted: Sat Aug 25, 2012 12:05 pm Post subject: Vapour lock? Why? |
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On 07/31/2012 10:57 PM, Frans Veldman wrote:
Quote: | I will check the system thoroughly and see if I can find any abnormalities.
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An update about this problem.
(For those who didn't follow the conversation; it is about a sudden fuel
pressure drop on a Rotax 914 when applying take off power).
1) The waste gate appears to function normally. It is not binding, it
cycles normally when applying power to the TCU.
2) The fuel flow exceeds 120 liters per hour. So it has nothing to do
with the fuel filters or fuel pumps.
After confirming this, I changed my focus on the fuel pressure
regulator. I disconnected the airbox pressure connector and put a small
hose and syringe on it. With the syringe I could imitate a rising
pressure in the airbox (like the turbo spooling up during TO power).
What I found was this:
1) When I increase the reference pressure, I hear the fuel pump
labouring for a short while, while it has to cope with the higher back
pressure.
2) Whatever pressure I apply to the reference port, it quickly
disappears. There is no external leak according to a "soap" test. The
only possibility is that the air escapes via the fuel return line. (I
did check the syringe and the hoses, this is not where the pressure
escapes). I think that the fuel pressur regulator should not bleed away
the reference pressure.
3) When the reference pressure increases, the fuel pressure lags behind.
It comes up after a short while. It looks like is not a fuel pump
problem, it is the regulator that responds slowly.
4) If I pull the syringe (instead of pushing), there is not so much lag.
It also looks like the fuel pressure regulator holds an underpressur
better than an overpressure. This could mean there is a pinhole leak in
the diaphragm, it lets air more easily out than fuel in.
My conclusion is that the fuel pressure regulator is faulty, binding
and/or internally leaking. Any comments about this?
Should I replace the diaphragm or the whole regulator?
Thanks,
Frans
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Gilles.Thesee(at)ac-greno Guest
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Posted: Sat Aug 25, 2012 1:59 pm Post subject: Vapour lock? Why? |
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Frans Veldman a �crit :
Quote: | My conclusion is that the fuel pressure regulator is faulty, binding
and/or internally leaking. Any comments about this?
Frans,
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Before concluding about the fuel pressure regulator, I would make sure
that the test procedure is adequate.
From numerous experiments in the past, I came to the conclusion that a
syringe is a poor means of applying a calibrated pressure to a dynamic
system such as the fuel pressure regulator.
What we did some years ago to ascertain the proper functionning of the
regulator, is apply a *calibrated head* of water by means of a clear
plastic line hoisted up to a certain height in our hangar. Of course we
provided an adequate 'S' routing so that no water would enter the
regulator port.
We found no discrepancy (using a usual fuel pressure gauge).
The resulting fuel pressure could be measured in much the same way, but
with a T fitting in the fuel line. Since we are awaiting a 0.25 bar
difference, the difference in the level in the two measuring tubes is
expected to be about 2.5 m, divided by 0.7, the density of fuel.
You could do that even with no head of water, just to make sure that the
regulator maintains Pairbox (= ambiant) + 0.25 bar, and with no leak.
Took some pictures, but failed to muster the courage to complete the
corresponding webpage on Contrails !
http://contrails.free.fr/engine_regul_en.php
BTW, we did not exactly use water, but a special scientific liquid with
better visibility : the remaining wine in the bottle left from the meal
we just had in the hangar on that particular Sunday...
FWIW,
Best regards,
--
Gilles
http://contrails.free.fr
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klinefelter.kevin(at)gmai Guest
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Posted: Sat Aug 25, 2012 3:10 pm Post subject: Vapour lock? Why? |
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Hi Frans,
I was having a similar problem with my 914. Mine was running rough and stumbling at high power, but only at altitudes above 10,000'.
The fuel differential pressure gauge would drop and engine run worse with more power applied.
I replaced the carb bowl gaskets with new after inspecting the floats and now it runs perfectly again. I think one of the gaskets leaked at high manifold pressure, low external pressure, causing a fluctuation in the fuel/air box gauge and very rough due to the imbalance.
I wonder if you might be experiencing the same problem? Seems more likely than a problem with the regulator. Have you already ruled this out?
Kevin
On Aug 25, 2012, at 1:57 PM, Frans Veldman <frans(at)privatepilots.nl> wrote:
Quote: |
On 07/31/2012 10:57 PM, Frans Veldman wrote:
> I will check the system thoroughly and see if I can find any abnormalities.
An update about this problem.
(For those who didn't follow the conversation; it is about a sudden fuel
pressure drop on a Rotax 914 when applying take off power).
1) The waste gate appears to function normally. It is not binding, it
cycles normally when applying power to the TCU.
2) The fuel flow exceeds 120 liters per hour. So it has nothing to do
with the fuel filters or fuel pumps.
After confirming this, I changed my focus on the fuel pressure
regulator. I disconnected the airbox pressure connector and put a small
hose and syringe on it. With the syringe I could imitate a rising
pressure in the airbox (like the turbo spooling up during TO power).
What I found was this:
1) When I increase the reference pressure, I hear the fuel pump
labouring for a short while, while it has to cope with the higher back
pressure.
2) Whatever pressure I apply to the reference port, it quickly
disappears. There is no external leak according to a "soap" test. The
only possibility is that the air escapes via the fuel return line. (I
did check the syringe and the hoses, this is not where the pressure
escapes). I think that the fuel pressur regulator should not bleed away
the reference pressure.
3) When the reference pressure increases, the fuel pressure lags behind.
It comes up after a short while. It looks like is not a fuel pump
problem, it is the regulator that responds slowly.
4) If I pull the syringe (instead of pushing), there is not so much lag.
It also looks like the fuel pressure regulator holds an underpressur
better than an overpressure. This could mean there is a pinhole leak in
the diaphragm, it lets air more easily out than fuel in.
My conclusion is that the fuel pressure regulator is faulty, binding
and/or internally leaking. Any comments about this?
Should I replace the diaphragm or the whole regulator?
Thanks,
Frans
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frans(at)privatepilots.nl Guest
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Posted: Sun Aug 26, 2012 12:37 am Post subject: Vapour lock? Why? |
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Hi Kevin,
Quote: | I was having a similar problem with my 914. Mine was running rough and stumbling at high power, but only at altitudes above 10,000'.
The fuel differential pressure gauge would drop and engine run worse with more power applied.
|
I think my problem is different. The weird thing is that I'm looking at
a *momentary* drop in fuel pressure, during the first few seconds at
take off power. The fuel pressure then comes back up and the engine runs
fine. Now I'm 100% sure that there is sufficient fuel flow (pump and
filters are ok) it is a mystery why the fuel pressure drops. Note here
that I'm referring to the differential fuel pressure. So it looks like
when the turbo pressure goes up, the fuel pressure lags behind, to catch
up a few seconds later. That's why I suspect the fuel pressure
regulator. This is what you could expect when the regulator valve is
"sticky".
I'm also suspecting the carbs (including the carb bowl gaskets). However
it is a bit strange that the engine runs fine after a few seconds. From
your description of leaking carb bowl gaskets I get the impression that
your symptoms were of a more constant nature.
A carb slide with some friction could be an explanation for the
temporary nature of the problem, but I can't see a relation with the
fuel pressure here. If the slide misbehaves, you should't see a drop in
fuel pressure.
I have observed an overall slightly higher fuel consumption, based on
the fuel flow gauge. Having said that, it is a differential flow meter,
substracting the return flow from the main flow. As the main flow is
over 120 liters per hour, it is hard to say whether a measured fuel
consumpion increase of about 1 to 2 liters per hour is real or just a
small deviation in one of the two flow sensors. Also the carbs or entire
engine could be aging and the whole problem might be unrelated to the
power surge in the first few seconds of the take off.
Back to the fuel pressure regulator. I think that it should held
pressure in its reference port (the small hose connector in the lower
shell) but the air is leaking away rapidly, mostl likely internally,
i.e. via the fuel return hose. This would indicate a leaking diaphragm.
Can anyone confirm that this leak in the reference port is indeed a
fault? (This is not so obvious as I can imagine that the designers
choosed to deliberately incorporate a small leak to cope with a
potentially plugged reference port and to use ambiant pressure in that
case as a backup reference).
Frans
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klinefelter.kevin(at)gmai Guest
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Posted: Sun Aug 26, 2012 5:11 am Post subject: Vapour lock? Why? |
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Frans,
My engine symptoms were not constant. Only at high power. Your symptoms sound similar. Momentary drop in fuel differential pressure. I only changed the float bowl gaskets and that fixed it.
Could be when the turbo pressure goes up, the fuel pressure leaks out of a carb bowl gasket.
In the Rotax classes I took (Rotech in Canada who also run Rotax-owner.com) they emphasized always suspecting and ruling out carburetor problems first.
They also told us that the cork gaskets are better than the paper ones. Bob Borger sent an eBay link a while back for nitrile gaskets. I bought a pair but have not installed them.
I replaced the gaskets without pulling the carbs off the engine, which is faster but a bit awkward. A wee bit of grease on the gasket will help it stay put upside-down in the carb body till you replace the bowl. Over-torque of the retaining bolt (that has an o-ring to inspect) can distort the bowl and cause float problems.
The drip trays are such a pain with the little nuts and washers that I modified them with nut plates. Much easier.
You must be about tired of this by now!
Best of luck,
Kevin
On Aug 26, 2012, at 2:29 AM, Frans Veldman <frans(at)privatepilots.nl> wrote:
Quote: |
Hi Kevin,
> I was having a similar problem with my 914. Mine was running rough and stumbling at high power, but only at altitudes above 10,000'.
> The fuel differential pressure gauge would drop and engine run worse with more power applied.
I think my problem is different. The weird thing is that I'm looking at
a *momentary* drop in fuel pressure, during the first few seconds at
take off power. The fuel pressure then comes back up and the engine runs
fine. Now I'm 100% sure that there is sufficient fuel flow (pump and
filters are ok) it is a mystery why the fuel pressure drops. Note here
that I'm referring to the differential fuel pressure. So it looks like
when the turbo pressure goes up, the fuel pressure lags behind, to catch
up a few seconds later. That's why I suspect the fuel pressure
regulator. This is what you could expect when the regulator valve is
"sticky".
I'm also suspecting the carbs (including the carb bowl gaskets). However
it is a bit strange that the engine runs fine after a few seconds. From
your description of leaking carb bowl gaskets I get the impression that
your symptoms were of a more constant nature.
A carb slide with some friction could be an explanation for the
temporary nature of the problem, but I can't see a relation with the
fuel pressure here. If the slide misbehaves, you should't see a drop in
fuel pressure.
I have observed an overall slightly higher fuel consumption, based on
the fuel flow gauge. Having said that, it is a differential flow meter,
substracting the return flow from the main flow. As the main flow is
over 120 liters per hour, it is hard to say whether a measured fuel
consumpion increase of about 1 to 2 liters per hour is real or just a
small deviation in one of the two flow sensors. Also the carbs or entire
engine could be aging and the whole problem might be unrelated to the
power surge in the first few seconds of the take off.
Back to the fuel pressure regulator. I think that it should held
pressure in its reference port (the small hose connector in the lower
shell) but the air is leaking away rapidly, mostl likely internally,
i.e. via the fuel return hose. This would indicate a leaking diaphragm.
Can anyone confirm that this leak in the reference port is indeed a
fault? (This is not so obvious as I can imagine that the designers
choosed to deliberately incorporate a small leak to cope with a
potentially plugged reference port and to use ambiant pressure in that
case as a backup reference).
Frans
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frans(at)privatepilots.nl Guest
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Posted: Sun Aug 26, 2012 11:28 pm Post subject: Vapour lock? Why? |
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Hi Kevin,
Quote: | Frans, My engine symptoms were not constant. Only at high power.
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Mine are not even constant at high power. Only temporary. After a few
seconds the engine runs fine at full power, even at altitude.
Quote: | Your
symptoms sound similar. Momentary drop in fuel differential pressure.
|
Ok, this momentary drop in fuel pressure is interesting and sounds
similar indeed.
Quote: | I only changed the float bowl gaskets and that fixed it.
|
All right. I'm about to take the carbs out for examination anyway, so I
will replace the float bowl gaskets and see what that does for me.
Quote: | Could be when the turbo pressure goes up, the fuel pressure leaks out
of a carb bowl gasket.
|
Yes, but why only temporary? This is what nags me. You may be right
about the gaskets, I will try it, but I also like to understand what is
happening and why it is happening the way it is.
Quote: | In the Rotax classes I took (Rotech in Canada who also run
Rotax-owner.com) they emphasized always suspecting and ruling out
carburetor problems first. They also told us that the cork gaskets
are better than the paper ones. Bob Borger sent an eBay link a while
back for nitrile gaskets. I bought a pair but have not installed
them.
|
If I remember correctly I have the cork gaskets. I will replace them
with nitrile gaskets if I can get them.
Thanks,
Frans
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frans(at)privatepilots.nl Guest
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Posted: Mon Aug 27, 2012 2:27 am Post subject: Vapour lock? Why? |
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Hi Ivor,
Quote: | A couple of month�s ago I noticed fuel staining from my float bowls, so
decided to replace the gaskets with the nitrile type,
My experience were they are next to useless,
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Great. I just ordered 4 of them.
Has anyone else tried these gaskets as well? I know some of us ordered
them, but has anyone actually tried to use them?
If they are indeed too soft, maybe we should apply less torque? The
recommended torque is obviously determined for best seal with the
original gaskets. I think I will just apply enough torque to get it
correct by feel and look (and still use a torque spanner to make sure I
at least don't overtorque.)
Quote: | 40C takeoff from Croatia last week
|
Hey, we have been to Croatia recently as well. Which field(s) did you
do? We've done Brac, Trogir (Split) as a diversion because Brac was
closed, Unije, Pula (for customs only). We certainly want to go back to
Brac and Unije. We found Landshut in Germany a very friendly EU-entry
airport... with an excellent Croatian restaurant.
Frans
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klinefelter.kevin(at)gmai Guest
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Posted: Mon Aug 27, 2012 4:42 am Post subject: Vapour lock? Why? |
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"Mine are not even constant at high power. Only temporary. After a few
seconds the engine runs fine at full power, even at altitude."
That is different. I hope your carb inspections reveal something.
Kevin
On Aug 27, 2012, at 1:20 AM, Frans Veldman <frans(at)privatepilots.nl> wrote:
Quote: | Mine are not even constant at high power. Only temporary. After a few
seconds the engine runs fine at full power, even at altitude.
|
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Ivor Phillips
Joined: 16 Jan 2006 Posts: 253 Location: London UK
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Posted: Mon Aug 27, 2012 5:34 am Post subject: Vapour lock? Why? |
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Hi Fran's
Hope you fair better with the nitrile gaskets than me, �but I would also order a pair of paper gaskets Just in case,
Bob Harrison and I Landed at Losinj a beautiful Island south of Paula,
Then down to Dubrovnik, On the way back we flew into Portoroz Slovenia to get back in the EU to cut down the hassle flying into Italy,
It was a great trip just a fair bit hotter than I would like,
Hope to see you at the LAA rally this weekend,
Regards
ivor
On 27 August 2012 12:19, Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)> wrote:
[quote] --> Europa-List message posted by: Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)>
Hi Ivor,
> A couple of month�s ago I noticed fuel staining from my float bowls, so
> decided to replace the gaskets with the nitrile type,
>
> �My experience were they are next to useless,
Great. I just ordered 4 of them.
Has anyone else tried these gaskets as well? I know some of us ordered
them, but has anyone actually tried to use them?
If they are indeed too soft, maybe we should apply less torque? The
recommended torque is obviously determined for best seal with the
original gaskets. I think I will just apply enough torque to get it
correct by feel and look (and still use a torque spanner to make sure I
at least don't overtorque.)
> 40C takeoff �from Croatia last week
Hey, we have been to Croatia recently as well. Which field(s) did you
do? We've done Brac, Trogir (Split) as a diversion because Brac was
closed, Unije, Pula (for customs only). We certainly want to go back to
Brac and Unije. We found Landshut in Germany a very friendly EU-entry
airport... with an excellent Croatian restaurant.
Frans
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