Its liquid petrol, coming from a jet, evaporating in a stream of air, so the temperature drops. Evaporation of a light liquid. The basic principle behind any refrigeration machine, be it a 25,000 tonne bananaboat (DEEEYYYO) or a 12 sixpack kitchen fridge (OK honey, 11 packs plus your lemonade). I think the icing is likely to happen in any carburettor when ambient is low enough and relative humidity is high enough. Nice find, Tiger.
The last time it happened was at 11pm after a Norton Club meeting in darkest Kent.Unlit narrow lane -2degrees,a few yards up the road,power goes popping and banging, stop wait a few minuits for the engine heat to defrost then all ok, no problem on a 99 dommy but could be lethal on a big 4cylinder with the unexpected engine braking.