I seem to remember on an Austin 7 site, a recommendation of adding a small amount of diesel fuel to the petrol, which altered the burn of modern petrol. Apparently, customs and excise would permit, as the fractions used were insignificant. These additions helped prevent valve seat erosion and helped performance. Unfortunately, I can't remember how much to add, but it was something like an egg cup of diesel to 5 gallons of petrol.
Any comment from those who know ??
BobH

Why bother? Most 2cvs have been running on 'normal' fuel for the last 20 years without adding diesel - it can't be necessary, surely?

You might as well give it a shot of Red X just like the good old days.

Er ? What is normal fuel over the last 20 years. Leaded, un-leaded, methanol added and now who knows what else. The 2CV engine was designed for a very different fuel - it may work OK on present day fuel, but could it possibly work better ??

I've never heard of this. We often used to add a little petrol (10%) to diesel many years ago to stop the diesel waxing up and clogging fuel filters up in winter. My experience (unintentional) is that if petrol is contaminated with even a small amount of diesel it introduces miss-firing and occasional puffs of black smoke.