On 08/13/2011 01:14 PM, Kingsley Hurst wrote:
> The fuel regulator on top of the airbox is bad. As the engine RPM is
> going to full power, at about 4400 RPM the missing starts.,the fuel
> regulator on top of the airbox, is supposed to shut off the return fuel,
> so that all fuel is avaiable for the turbo.
No that is not how it works. It just maintains proper fuel pressure over
the airbox pressure. It is just a pressure valve, opening the return if
the pressure gets higher, closing it a bit more when the pressure gets
lower.
Even during full power plenty of fuel returns to the tank. This is
intentional to overcome filter losses and to prevent vapor lock.
This situation again shows the importance of a differential fuel
pressure gauge. Why not install one now permanently, and see what is
going on, now you have to hook up a gauge anyway? It is a must have for
the 914. Without such a gauge you can't tell whether your regulator is
working correctly, or that your filters are clogging, or whatever. If a
differential fuel pressure gauge shows a healthy value you can pretty
much rule out the majority of the fuel related problems.
I would place my bets on the carb slides, just like Trev said. It is
common that after some months out of use the slides get a very small
layer of oxidation due to lack of fuel, and they get a bit "sticky". You
might not notice it on the ground, but temperature changes might be
enough to get the slides stuck during flight. There have been a couple
of crashes exactly due to this phenomenon. If they get sticky they
usually limit the power, or get stuck in the full power configuration
and flood the engine when the pilot reduces power. In both instances the
engine cuts, typically in an awkward situation.
> I am however convinced the problem IS the
> fuel pressure regulator
Without a differential gauge you really can't tell.
> Now that I have read your full posting, with hindsight and as a result
> of the steep learning curve I have been on regarding the 914, maybe if I
> had clamped off the fuel return line somewhat, we may have at least seen
> some change in the running to confirm we might be on the right track.
It wouldn't tell anything, even if the engine runs better. You might
overcome the improper mixture due to a stuck slide by changing the fuel
pressure outside the normal range, but for sure this fix wouldn't be
airworthy. It would be similar like intentionally clogging the fuel
filter to keep your engine running with the choke stuck in the activated
position. Clogging the fuel filter would make the engine run better in
this situation, but it wouldn't be an airworthy fix either. Never mess
with the fuel return of the 914, or you risk flooding the carbs. Proper
fuel pressure is of utter importance for the 914.
Frans
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