Frans,
If vapour is formed anywhere in the fuel line, the bubbles will move towards
the carbs as soon as a pump is switched on. I do not know the length of the
fuel line from the pump to the carbs, but let us for argument's sake assume
3 m. This mean an internal volume of 0.15 liter, plus say 0.05 liter for
one filter and a gascolator (if fitted). This means that when a total
volume of 0.2 liter has been pumped through the line, there should be no
more vapour bubbles (with one exception, see below).
I do not know how much fuel is returned to the tank in a 914 installation
during ground operation. Engine consumption is also unknown, but I am sure
it is at least "some liters" per hour. At consumption of 1 liter/hour it
takes 12 minutes for 0,2 liter to flow through; at 5 liter/hour it takes 2.4
minutes. Any fuel return flow reduces this time. With some idling for
warm-up, taxi to holding, run-up and line-up, I believe that much more than
0.2 liter is pumped through the line before you begin the take-off roll.
Theoretically, bubbles may get "stuck" in the filter or gascolator (if
fitted) at low flow, and then "torn loose" when the flow increases at full
throttle. My 912ULS consumes abt. 33 liters/hour at full take-off
RPM/power. At this flow, it would take 24 seconds for a bubble to pass
through the entire 3 meter long system, less if it had been trapped
somewhere closer to the engine.
What does the transient pressure drop that you observed indicate? There
will be a pressure drop just when a bubble coming from upstream of the pump
passes through it, and when a bubble passes through the pressure regulator
and into the fuel return line (I do not know the details of the 914
installation, so I may be mistaken here). Both drops will be of very short
duration, however.
A pressure drop in itself will not cause any change in engine performance,
but reduced pressure may cause too low fuel flow to the carb bowls. A fuel
vapour bubble reaching a carb has of course the same effect - too little
fuel.
What speaks against vapour lock as the cause is your observation that the
engine afterwards ran fine at low/medium throttle but not at high, and that
the fuel pressure then dropped. This would indicate that the fuel flow is
restricted (as already suggested on this forum). Later on all was fine, you
report. Has the restriction migrated to the filter, where it does not do
so much harm? Hmmmmm...
Your question regarding AVGAS vs. MOGAS: Wikipedia states that "Avgas has
a lower and more uniform vapor pressure than automotive gasoline so it
remains in the liquid state despite the reduced atmospheric pressure at high
altitude, thus preventing vapor lock."
Regards,
Svein
LN-SKJ
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