You dont need to evaporate much fuel from the float bowls to get a fuel composition that will not easily start. A small volume of the fuel, maybe only 5%, provides the bulk of the vapor for the intial start. This component has a high vapor pressure and low flash point. It is designed to evaporate quickly and easily and provide the initial flammable mix for starting the engine from cold. Once this fraction evaporates it is then hard to get good fuel in the carb bowls as whats in the carbs needs to flow through the engine ( potentially wetting the plugs) before enough fresh fuel can enter the carb bowls. You then need to replace the wet plugs to get a start.
The are many variables that control how easily an engine starts from cold and the Elan with its big carbs and dodgy lucas electricals and marginal starter motor and high voltage drop from the battery in the boot suffers from most of them.
On top of this the fuel itself, its source and consistency of blending and its match with ambient conditions also affects ease of starting. Fuel is blended to match the local climate typical at the time of year it is made. How well this is done and how closely the fuel matches the current actual condtions affects ease of starting. Many Elans dont get filled often so the fuel in the tank may not be totally uitable to current conditions often. Fuel that discount service stations may import from other regions may not work as well as fuel blended more locally.
On top of this, modern fuels are designed with lower vapor pressure and higher flash points of the most volatile components compared to fuels when the Elan was designed. This is done to prevent pollution from fuel evaporation and because it is possible in modern fuel injection engines that more easily vaporise the fuel with the high pressure injection spray from the nozzles..
Some cars , some usage and some fuels will have no problems, some will have problems. All that has been described in this link in the various issues described by the contibutors and all are generally true for some or all of the reasons I have listed. If you have problems there are generally a number of ways to fix it and those ways are specific for each individual circumstance. However a well tuned engine, with electricals all in good condition and with an electric fuel pump or priming pump will generally start more easily if it sits for periods.
I would love to be able to say do this one thing and you will never have a problem but it just is not that simple
cheers
Rohan