Graham 2 wrote detailing problems with fuel venting, and recommended venting
the fuel filler cap.
We reply:-
It is correct that when the tank is full or very nearly full, and
subsequently subjected to some form of heating, the expansion of the fuel
and more particularly of the air in the top of the tank will cause the fuel
to spill out of the vent.
However, although at first sight your proposed method of curing the problem
seems simple, it is in fact dangerous.
If the fuel cap is vented, the pressure in the tank will no longer be set by
the forward facing tank vent, but will be set by the pressure at the cap,
which will probably be lower - but in any case will be different from the
pressure on top of the fuel sight gauge, and therefore the sight gauge will
read wrongly. It would not in any case solve the problem because even though
the fuel filler hose would then be vented, the air trapped in the triangular
cross section of the top of the tank forward of the vent fitting (tail down)
would still not be vented.
Additionally, if you remove the O-ring, rain could get in.
We have carried out an experimental modification on G-ODTI which comprises
an aluminium tube bonded into the back of the F09B vent fitting. This tube
is arranged to bend slightly upwards and to finish close to the front of the
tank right at the top. This way the tank is vented from the highest point,
and the air is not trapped. It has the added advantage that the tank can be
filled fully without having to lift the tail.
Even with this mod, when the tank is filled right to the top of the filler
hose, there will be a small quantity of fuel in the vent pipe up to the
level of the fuel filler cap. When heated, a small amount can still spill
out, but the quantity is very much less, since there is virtually no air in
the system, and the expansion of the liquid is quite small.
Regards
Roger Bull
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