Recently I posted something on Matronics entitled "Carburettor icing with heated
carbs" and now I must put my hand up and admit that the diagnosis was probably
wrong, although I still find it difficult to let that one go!
Today's weather was about as perfect as it gets in North East Scotland, so three
flights were undertaken by a sharing pilot friend and the first two flights
went without a hitch. But on the third climb-out passing 800' AGL, the engine
misfired briefly which prompted my friend to retard the throttle before
cautiously
advancing it while he levelled the aircraft. Thereafter it ran as smoothly
as anyone could wish until he landed.
Having already found one sunken floatafter the previously reportedoccurrence of
brief 1 - 2 second violent shaking due to one bank of cylinders failing
momentarily,
I removed both float bowls again and at first all 4 floats seemed to be
buoyant. Then I commenced to press each float down in turn to see whether they
bobbed back up to regain their half-submerged levels in the fuel bowl. It
quickly
became apparent that one float could only be depressed slightly with my
finger, as though it was already almost resting on the bottom of the bowl.
Here it comes, now wait for it, wait for it, the smoking gun! The brass sleeve
which lines the vertical holes through older floats, had unstuck and slid down
the pin which guides the float vertically, until more than half of the sleeve
was projecting beneath the float. Of course this constitutes an obstruction to
prevent downward movement of the float when the fuel level is dropping,
particularly
during high fuel demand such as during the climb-out after take off.
So when fuel is most needed, the float chamber needle valve can't open enough
to let more fuel into the bowl at a sufficient rate.
Recently I acquired some replacement floats after one of my old ones had sunk
and
all of the replacement floats were supplied without brass sleeves lining the
vertical holes. I wonder why (not)! Clearly this phenomenon of insecure brass
sleeves must have become known to Rotax/Bing, or why else have they discontinued
installing them? If anyone else was made aware of this, it certainly passed
me by.
You won't be surprised to know that all 4 floats in my carburettors now are of
the new type without the inserted brass sleeves.
Hopefully this isn't old news to you.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=497585#497585
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