Carburetor icing is one of those problems that catches people off-guard because it seems impossible. How does ice form in an engine that's running?

The answer is the Venturi effect and evaporative cooling. Fuel evaporating in the carburetor throat drops the temperature of the surrounding air dramatically — sometimes 20-30 degrees below ambient. On a humid day with ambient temperature in the 40s or low 50s, that's enough to freeze moisture out of the air onto the throttle plates.

The symptom is an engine that stumbles and stalls in cold, humid weather, often after reaching operating temperature. The fix is a carburetor heat plate that draws warm air from around the exhaust manifold. Every fuel-injected car has this. A lot of carbureted classics don't, and their owners wonder why the engine stumbles in March.