Q: What does a carburetor do?

asked by on November 24, 2015

What does a carburetor do?

Carburetor Replacement Get a Quote

In the simplest of terms, carburetors provide atomized fuel to the engine for combustion. By using air pressure and a series of mechanical control devices, carburetors can take liquid fuel from the gas tank and mix it with air as it enters the engine through the intake manifold. Liquid fuel needs to be atomized and finally vaporized to allow for proper burning and combustion in the internal combustion engine. In order for the liquid fuel to change states to a vapor, carburetors were used for decades until the advent of fuel injection. So how exactly does the carburetor perform this task? Think of a carburetor as a control tunnel consisting of several circuits internally. We start with the fuel supply, which is a small bowl area that holds fuel readily for use similar to the tank on the back of a toilet keeping the water on hand. In fact, the float which controls the fuel level operates similar to the float that controls the fill level of water in a toilet tank. The carburetor has a cold start element called the choke that allows for more fuel on startup because a cold engine requires a richer fuel mixture. By restricting air on startup, the fuel mixture becomes richer.

The throttle plates are the main air control element for the carburetor. There is an idle mixture circuit, a high speed fuel circuit, and an accelerator pump to provide additional fuel when depressing the accelerator pedal on acceleration. Each circuit has a specific function to allow for the proper air-fuel ratio, which is 14.7:1. This ratio states that 14.7 parts of air are mixed with one part of fuel for optimal burning. In professional terms this is call the stoichiometric ratio. Proper operation requires clean air entering the top of the carburetor, called the air horn, and then the air speeding through a series of tunnel type structures called venturies. These restrictions cause the air to speed up and allow for a pressure drop which makes the fuel come out of the jets or fuel orifices and mix with the incoming air charge. Air pressure is necessary for this process to work. When you drink from a straw you assume you are sucking the liquid up through the straw. However, the liquid is actually being pushed up through the straw by the air pressure on the surface of the liquid. The pressure on the end of the straw is lower, allowing the greater pressure to force the liquid up the straw. In the same manner, fuel is forced through the carburetor circuits by the air pressure on the fuel in the fuel bowl. Proper adjustment as well as keeping the passages clean of dirt allow for the carburetor to perform these tasks. The fuel pressure on these systems is usually only about 5 psi (pounds per square inch) since we only need to keep the fuel supply at the carburetor bowl filled.

Modern vehicles use fuel injection, which is more precise and runs at much higher pressures. However, air-fuel measurement is done in a similar fashion to carburetors. A MAF, or mass air flow sensor, needs to count or be exposed to the air entering the intake system in order for the computer to add the correct amount of fuel. Just like the old carburetor systems, if air enters through a cracked line or intake with a broken or misaligned gasket, the system will run lean on fuel. The carburetor will only allow fuel for the quantity of air entering through the air horn. Carburetors are on older vehicles and are still used on small engines like lawn equipment, 4 wheelers, chain saws, and alike. If the carburetor fails and needs service or replacement, seek service from a professional technician who is familiar with the proper adjustment procedures.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing


Experienced Mechanic?

Earn up to $70/hr

What others are asking

2007 Honda Accord not going into gear
You seem to be having a hydraulic (fluid transfer) issue here. The leak may be in the slave cylinder, master cylinder or the lines and hoses that move the fluid from each part to the other. Look for leaks under...
Changing timing belt on a 2000 Lexus ES 300
You've said the camshaft pulley but those are the ones on the top inside. I'm assuming that you meant the crankshaft pulley. Those Lexus crankshaft pulleys often will not come off because they are kind of tight and they are...
It lost power and now won't start
It sounds like the battery is dead (https://www.yourmechanic.com/services/battery-is-dead-inspection) from the alternator not charging. This would account for the gradual loss of power and why the battery is dead and you have no lights. Have the battery charged (https://www.yourmechanic.com/article/how-to-charge-a-dead-car-battery) and tested....
Faulty brakes cause unpredictable stopping distance
Hello. Bumpy roads can cause the ABS system to have trouble working correctly. Bumpy roads cause changes in the speed of the wheels which is the main input for the ABS system. If computer is not reading correctly then the...
Cant get my car out of limp mode or "fail safe mode"
The reason "limp mode" has become "unbypassable" is due to the existing codes. The ECT failure (https://www.yourmechanic.com/services/coolant-temperature-switch-sensor-replacement), if it exists, may alone account for that because an engine coolant temperature sensor is essentially a master sensor - the value of...
I have alot of play in my steering wheel .and sometimes it is hard to turn
Hello, thank you for writing in. Hard steering is typically a sign of worn down steering gears or power steering pump. You will want to check the condition of these components and their fluid level. Check for leaks or any...
I'm trying to replace my antenna assy coil from the ignition area inside of the car where the key goes. Do u know of a video on it
The best place to look for videos on automotive repairs would be Youtube.com. I know that there is a guy from 1A Auto who is often on there filming various repairs being done to show people how to do them....
My 2011 chevy Cruze keeps saying check battery charging system and the battery and alternator are both completely fine. Any ideas?
Hello, thank you for writing in. If you are not able to find any faults with the battery or the alternator, you are going to want to focus on their wires and connections. Make sure all of the wires are...
I bought a used 2001 Lincoln LS that needs it's motor replaced. Is there a junkyard I can buy it from and have it put in by you?
The existing motor can be rebuilt. Used motors are available, too, for example from services such as Car-Part.com (http://www.car-part.com). However, used (junkyard) motors carry a risk. If you rebuild what you have, or buy an off the shelf rebuilt motor,...

Related articles

Symptoms of Bad or Failing Fuel Injector O Rings
Common signs include an odor of fuel in the car, fuel leaks, and the Check Engine Light coming on.
How Long Does a Fuel Pressure Regulator Last?
In In order for the fuel system in a car to operate properly, it will need to have the right amount of pressure. The injectors that are mounted in each cylinder of the engine have to disperse the right level...
How to Replace a Fuel Pump Relay
The fuel pump has a relay that fails when there is no audible humming sound as the ignition is turned and when the car cranks after longer than usual.