Why does fuel vaporization in a carburetor cause a temperature decrease rather than a rise?
Good question! There's two parts to the answer.

The first part is the ideal gas law.
The formula of the ideal gas law is $PV = nRT$, where

P = Pressure
V = Volume
n = amount of substance
R = the ideal gas constant, which can be ignored for our purposes
T = Temperature

Since we're ignoring $R$, we're left with $PV = nT$.
Now, say that you send some air through the throat of a venturi. Pressure decreases greatly (that's the point), volume stays close enough that we can leave it alone, the amount of molecules remain unchanged, so to keep the equation in balance, the temperature drops.

The second part is the latent heat of vaporization
In order for a liquid to change state and become a gas, a certain amount of heat is required. Consider a firefighter spraying water on a fire. The liquid water is heated by the fire and evaporates into steam. The energy of the fire is decreased by the amount of energy that went into the steam and which is now floating away into the atmosphere.
Now, replace the fire with your carburetor, and replace the water with fuel. The heat energy in your carburetor is transferred to the fuel in order to cause it to vaporize. The fuel gets hotter (but who cares, it's about to get burned in the cylinders anyway) and the carburetor gets cooler.