I suffered heavy in-flight vibration the other day. By the time I had
checked t's and p's and voltage, and before I had time to switch on the
fuel pump everything was back to normal. So I left everything as it was
and returned to base with no further trouble. I checked engine
mountings, propellor and fuel filter and carb float chambers and found
quite severe felting of fine fibres round the filter, which I changed,
and a minute amount of dirt in the bottom on one float chamber. The
nature of the vibration felt as though the cylinders on one side of the
engine had stopped firing; it did not feel like misfiring due to,eg,
carb ice, and if it had been a blocked filter I think the engine would
have stopped, and not started again until I changed tanks. If so that
could only be a carburettor problem and I wondered if there had been a
momentary vapour lock on one side, or possibly a sticking float valve
stopping fuel flow to that side, perhaps cured by the vibration.
Neither the OAT nor the engine were hot enough to make vapour a likely
answer. I also wondered if I had picked up a child's helium balloon in
the prop which blew off after a second or two. The next day I climbed
straight up to 6,000' with no hint of trouble, so I have stopped
worrying about it.
Frank Mycroft G-BWYD
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