What is a choke for and how does it work, on small engines.

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What is a choke for and how does it work, on small engines.

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The Choke is a valve that restricts airflow into the carburetor, causing the fuel/air mix to run rich. More fuel flows into the engine. This is primarily to allow a cold engine to get started and warm up. Once the engine is warmed up, the choke is opened allowing normal operation.

It’s a valve on the air intake that regulates how much air gets injected into the combustion chamber along with the fuel. When the engine is cold you want more fuel and less air because it’s harder for the fuel to turn into an aerosol when it’s cold. As the engine heats up it’s more efficient to use more air.

A gasoline engine generates the force to move the pistons by igniting a mixture of air and fuel. In a car engine, air and fuel are combined in the fuel injection system. In smaller engines, a device called a carburetor is used, which is essentially a big open pipe that sucks in air and mixes it with fuel.

When the engine is cold, the fuel doesn’t evaporate as well as it does when the engine is warm, and it tends to stay as a liquid (condensation) on the walls of the carburetor. So you need to bring in less air from outside to get the engine started. If you brought in the normal amount of air, you wouldn’t have enough gasoline in the cylinder to get the engine started.

The choke is basically a valve that limits how much air is being let into the carburetor. When you close the choke valve, you bring in less air, and then once the engine is warm, you open the choke valve again.

It increases the ratio of petrol to air. When the engine is cold, the petrol doesn’t evaporate as fast so having extra means that enough gets into the vapour state to explode when the spark tries to ignite it.

In an old style carburetor it deploys an extra valve which partially obstructs (chokes) the air intake, creating a higher vacuum as the piston pulls in the charge. This draws more petrol in.

When a carburated engine (as opposed to one with fuel injection) ‘breathes in’ a cylinder full of air, that air picks up some gasoline with it, so that there’s a nice mix in the cylinder to burn. The job of the carburetor is to mix the air and gasoline, so the engine gets the right amount of each.

When you first start an engine, it needs a little more gas. Closing the choke literally ‘chokes off’ the air supply so that the engine is getting a proportionally higher amount of gasoline.

[This is a great illustration of how it works, and is well worth watching](https://www.youtube.com/watch?v=toVfvRhWbj8)

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It’s like the pot lid when you are boiling water for pasta, it allows proper ventilation so things can boil, and you can open and close it to control how much hot air is happening

Always thought it was more air and now realise it’s called a choke for a reason and now feel really stupid. PS. I’m in my 40s.

When you are starting an engine cold (as in at environmental temperature – in this context 30°C / 86°F would be considered cold – operating temperature for my engine is 80°C /176°F) you have to use a fuel rich mixture to effectively move the cold, unlubricated pistons resulting in additional heat to warm the engine up and get the oil to operating temperature. The choke is the mechanism which controls the air-fuel mix. The same thing occurs with fuel injected vehicles only you have the car’s brain (ECU) doing it for you.

Watch a car when it starts up cold and idles for a bit – particularly ones with a big engine capacity (3l, etc.) – you’ll see water pouring out of the exhaust. This is because water is a byproduct of gasoline/petrol/diesel combustion and the fuel/air mix at startup is so rich that the excess water vapor condenses in the cold tailpipe/exhaust.

To run an engine, you want to run a fairly specific balance if fuel and air to get it to combust efficiently.

The problem is that the fuel will evaporate more slowly in a cold engine than in a hot one, so you will want to use a different ratio of fuel to air in a cold engine than you will in a hot one.

The choke is a service that alters the fuel to air ratio, so that you can feed the engine more fuel to get it started and warm up, then once it is at operating temperature you can dial this back to a more efficient setting for normal operation.