According to Joseph Meehan :

I acknowledge that this is purely anecdotal, but I figure that the
stuff that turns to gunk is the stuff that is not refined out of
normal gas.

The stuff that turns to gunk IS normal gas. :-)

Indeed.

Gasoline isn't a "pure" chemical. It's a mixture of hydrocarbons of
various weights. Traces as light as propane or even methane, with
traces as heavy as asphalt. Refining is nothing more than forcing the
average to be roughly equivalent to somewhere around C7H16 or C8H18,
reducing the "tails" of the distribution to a "reasonable" level and
specified octane level, and having enough "lights" to give you enough
vapor pressure.

Evaporation acts almost as "fractional distillation". The light
parts evaporate preferentially, leaving the heavy/sticky parts behind.

Which means that "gumming up your carb" is not necessarily
(or even largely) a chemical process, but is in fact a physical
one.

With small engines (especially weed wackers and the like), running
the motor dry and then putting it away is the worst possible
thing you can do. The residue gas solidifies in place. If it's 2-stroke
mixed gas, it's worse because the mixed oil is going to congeal and
evaporate into a sticky obstructive mess.

Think of gasoline like corn or maple syrup. Sticky to begin with,
it gets vastly worse if the water is allowed to evaporate.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.