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@privatepilots.nl> wrote:
>
> 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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