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@privatepilots.nl> wrote:
>
> 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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