Sorry, I thought you had said the carbs were H4s but I guess you are talking
about HS carbs. I deleted your original post. Spitfire, right? Anyway,
this is what the Weber, Z-S, SU manual has to say:
There is a brass valve with steel needle as on types H and HD, or a plastic
bodied spring loaded type which was introduced to overcome the effects of
engine vibration which tended to affect the seating of the valve. This
does cover fuel being "ejected forcibly".
This is the Haynes Techbook page 19-2
Quite frankly, I have never been able to get my arms around a vent tube that
vents fuel on the same side of the motor as the exhaust manifold, but that's
me. I have H4s on my 3A. I have Z-S on my 6 and I prefer the Z-S but I am
clearly in the minority.
=== This list supported in part by The Vintage Triumph Register