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How Mixture Control Works On Carbureted Engines

Boldmethod

Finely tuning your mixture is crucial to keeping your engine running at the right fuel/air mixture. But how does it work in carbureted engines? Here's what you should know...

But First, How Does A Carburetor Work?

In float-type carburetors, outside air flows through an air filter, usually located right around the intake at the front of the engine cowling. Filtered air flows through the carburetor and through a narrow throat in the carburetor called a venturi.

As air flows through this venturi, pressure drops, and fuel is forced into a fuel jet at the throat of the carburetor. The fuel is then mixed with air, creating a mixture perfect for combustion.

According to the FAA's PHAK, "the float-type carburetor acquires its name from a float, which rests on fuel within the float chamber. A needle attached to the float opens and closes an opening at the bottom of the carburetor bowl. This meters the correct amount of fuel into the carburetor, depending upon the position of the float, which is controlled by the level of fuel in the float chamber. When the level of the fuel forces the float to rise, the needle valve closes the fuel opening and shuts off the fuel flow to the carburetor. The needle valve opens again when the engine requires additional fuel."

Want a deep dive on how carburetors work? Watch our Boldmethod Live on carburetors.

The Mixture Needle

The correct fuel/air ratio is critical for an engine to burn fuel efficiently. In the next section, we'll get into the specifics of why mixture changes with altitude. But first, how is mixture controlled in the carburetor?

Inside the float chamber of the carburetor, you'll find a mixture needle. It controls the amount of fuel sent into the discharge nozzle. The mixture control in your cockpit directly links to this needle. The mixture needle is your way to change the fuel/air ratio entering the engine.

Watch the video below to see how the mixture control adjusts the mixture needle.

Why Do You Adjust Mixture?

Carburetors are normally calibrated at sea-level pressure, where the correct fuel/air ratio is set with the mixture control at or near the "full rich" position. As altitude increases, the density of the air entering the carburetor decreases. If you don't change the amount of fuel entering the carburetor, your mixture will slowly become too rich.

This creates a problem. The mixture will become progressively richer as altitude increases, resulting in engine roughness and a reduction of power output. Rich mixtures slow the speed that the fuel/air mixture burns in the cylinder, which means less power output. Running with an overly rich mixture for extended periods can also lead to spark plug fouling, which creates a weaker spark in the combustion chamber, further reducing your power output.

To maintain a proper fuel/air ratio, lean the mixture. Leaning decreases fuel flow, compensating for the less dense high-altitude air. "Since the process of adjusting the mixture can vary from one aircraft to another, it is important to refer to the airplane flight manual (AFM) or the pilot's operating handbook (POH) to determine the specific procedures for a given aircraft" (FAA PHAK 6-8).

As you descend from high altitudes, you'll have to progressively enrich the mixture to prevent engine roughness. If you leave your mixture leaned as you descend, eventually, the fuel/air will become too lean causing your engine to run rough and not produce sufficient power. It can also in some cases cause detonation, leading to potential engine damage.

Boldmethod

What About Fuel Injected Engines?

Mixture works a little differently on fuel injected engines. But we'll save that for another article soon!

How do you manage the mixture in your airplane? Tell us in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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