A interesting bit of auto fuel info.

A friend of mine experienced an unusual amount of carby ice in his
tiger moth while ferrying it across australia (across the desert)
Had flown lots of time in tiger moths in south africa (roughly similar
climate) and had never experienced the degree of carby ice.
Being a retired Jumbo flight engineer he decided to find out why.
managed eventually to have a chat with a chemist from Shell here in
Australia.

Interesting diagnosis.

It eventuates that many of the local Australian suppliers of regular
unleaded are mixing LPG (liquified petroleum gas) into it. It seems
that useful amounts of LPG can be dissolved into solution with petrol.

Problems only surface in aircraft and it seems that the increased
evaporation in the carby with attendant atmospheric moisture is at the
root of his problem.

now I've heard of many things being mixed with unleaded but never
before LPG. Seems that an ultralight pilot friend of the chemist has
sufferred two forced landings because of similar icing problems.

evidently the premium unleaded fuels dont have LPG mixed into them.

Stealth (I use AVGAS, the magic legacy of WW2 !!) Pilot