Sorry - disagree with the previous answer.
There's really just one effect. Molecules generally attract each other, until they come very close. If there's (virtually) no heat, then the molecules will settle at this close distance. You now have a solid.
Heat the solid a bit, and the molecules will start moving, and will no longer be at these closest distances, but they'll generally stay fairly close still - but they'll have enough energy to change neighbors.
Heat the liquid even further, and you'll see some of the molecules fly off entirely, and disappear in the gas phase. They still attract each other, so the speed of each individual molecule is constantly varying.
Now these change-over points are interesting. At these points, we see that with heat added, the average speed of molecules doesn't change. The extra energy goes to increasing the distances between molecules.
What happens in a carburetor is fairly simple. The fuel line is under normal pressure, and the fuel molecules are packed in a fluid. As the fuel enters the carburetor, it encounters a lower pressure which allows the molecules to disperse. As this dispersion increases the distance, the attraction between molecules slows down the escaping molecules - this is evaporative cooling.
Exactly how much cooling you have depends on the attraction as function of distance, which depends on the exact molecules. And aviation fuel is a mixture of many different chemicals, so that's rather complex. But on average, it's predictable enough for engineering purposes. But yes, you might need to heat the carburetor, else you'll get droplets of fuel instead of a vapor.