Do you know what causes carburetor ice? Many people struggle with understanding how things work when it comes to engines or mechanical things. Rather than get into pictures of a carburetor and go into complicated examples of exactly how a carburetor works, I will make it easier to understand by describing it in simple terms.

To start with, just as you need air and food to live,  an engine needs air and fuel to run. The air and fuel have to get into the engine somehow. There is an opening for that called the carburetor. The carburetor has air directed into it through the holes on the front of the plane which are connected to tubes leading into the opening of the carburetor. Fuel also goes into the same opening, just as your air and food go into your mouth.

When the air and fuel are going into the engine as it should, then everything is fine. Cut off the fuel by pulling the mixture back all the way or block the air holes going into the engine and it stops running, just as you would. Now, I want you to think of the carburetor as a funnel. There’s a big opening on one end tapering off to a small opening at the other end. Air enters the big end of the funnel as it goes into the engine. As air enters into the big opening of the carburetor, it continues into the narrower end before going into the engine. As the air passes through the narrow end, it moves faster. As the air moves faster, it cools down.

Try this as an experiment; hold your hand up to your mouth. With your mouth open as if you were saying “ahhh” at the dentist, blow on your hand. You will notice that blowing on your hand with your mouth open wide produces warm air. Now do the same thing, but this time with your lips close together (restricting the opening like a funnel) like you would when you blow out candles. What you will notice is the air is moving faster and it is cooler. This is the same thing that is happening as air enters the carburetor on its way into the engine. Now wet your hand and do the same thing and it will be even cooler due to evaporation of the wet surface. When the fuel which is also going into the engine vaporizes (turns from a liquid to a gas), it causes the temperature to drop much more. This accounts for the biggest drop in temperature. The difference is that the temperature drop in the carburetor can actually drop to below freezing as the fuel vaporizes!

Now that you know a little about the carburetor and what happens with the air entering the engine through the carburetor, let’s talk about the air. Air holds moisture, just like a sponge. When the sponge gets completely soaked and can’t hold anymore, the water will fall off the sponge. The air does the same thing. When the air gets completely saturated with moisture, it rains. The temperature of the air makes a difference in how much water it can hold. Warm air can hold more water than cold air. When you bring a glass of ice water outside on a hot summer day, what happens? The glass gets wet on the outside. This happens because the ice water is making the air surrounding the glass cooler than the rest of the air. The cooler air can’t hold as much water as the surrounding warmer air. It becomes saturated and will not hold any more water so the water starts running down the side of the glass.

Put both of these together now and you have a setup for carburetor ice. On a summer day, even if it’s 70° outside you can get carburetor ice. Remember, that as the air enters the carburetor, it cools down. If the warm air coming in is holding enough humidity, the moisture will release just as it does on the glass. If the temperature in the carburetor happens to drop to below freezing temperatures, then the moisture that is forming from the temperature drop will  freeze inside the opening of the carburetor. If the moisture freezes and starts to build up, it will begin to restrict the amount of air going into the engine as it blocks the opening and it will start running rough because it no longer has enough air. As the ice builds up and the opening gets smaller, an engine failure will soon follow. I hope this helps you understand things a little better. Sign up for our free newsletter for more good tips and find out more on carburetor ice and what to do if you get it in one of our future email newsletters.