Small Engines

Published on June 23rd, 2016 | by Brandon


The A to Z of Small Engine Carburetors

Your mower’s engine won’t start. You decide to take a look around online for a possible solution and conclude that there may be a problem with your carburetor.

Then, you realize you have another problem. You don’t understand the terms and parts associated with a carburetor. How are you going to explain or try to fix your problem?

It’s important to be familiar with the details of your equipment for the purposes of both maintenance and operation. Below, you will find a list of some of the basic terms and parts related to small engine carburetors. But first, what exactly does a carburetor do?

Put simply, a carburetor is a device that blends fuel and air into a mixture that is delivered to an internal combustion engine for energy. Such a task requires a complex series of parts and processes. To learn more about the actual operation of a carburetor, check out this article.

Without further ado, the A to Z of small engine carburetors:


  • Accelerator Pump – Liquid gasoline is denser than air and as a result, reacts more slowly to force than air does. When the throttle is opened, airflow is faster than the flow of fuel which leads to an oversupply of air, causing an engine misfire. The accelerator pump forces extra gasoline through a jet to balance the amount of air and fuel.
  • Airflow – The flow of air entering the carburetor to be mixed with fuel.
  • Air-Fuel Mixture – The final product of the carburetor: a precise mixture of fuel and air that is delivered to the engine to be used for combustion.
  • B

  • Butterfly Valve – A type of valve that regulates and controls the airflow coming into the carburetor. It can be rotated to a position that almost completely restricts all airflow, or to a position that barely blocks any airflow at all.
  • C

  • Choke – If the engine is cold, more fuel than air is needed in the mixture to start the engine and run it until it is warm. A choke restricts the flow of air at the opening of the carburetor. This creates a stronger vacuum effect, which will pull extra fuel into the mixture.
  • Cold Start – An attempt to start an engine that is cold. Cold temperatures cause fuel to vaporize less easily and condense on the walls of the intake manifold. The carburetor needs to compensate by creating a richer mixture with more fuel.
  • Combustion – Burning an air-fuel mixture to create heat energy that is used to apply force to a component of the engine.
  • D

  • Detonation – Premature ignition and explosion of the air/fuel mixture. This will interfere with the cycle of the engine and possibly even damage the engine. Also known as knocking.
  • E

  • Engine – Machine that converts energy into mechanical energy that can power something. Internal combustion engines convert fuel into heat energy.
  • Emissions – Air pollutants that introduce harmful chemicals into the environment. Many modern carburetors are required to maintain low amounts of emissions.
  • F

  • Flooded Engine – This occurs when the air-fuel mixture given to the engine is too rich and cannot be ignited.
  • Float Chamber – Also known as a bowl, this component contains an amount of fuel that is ready to be used by the carburetor.
  • Fuel Pump – Replenishes the float chamber with fuel when necessary.
  • G

  • Gas Tank – When the float chamber is reduced to a certain level of fuel, it is refilled with fuel from the main gas tank.
  • Gasoline – The most common liquid fuel that is mixed with air for use in an engine.
  • Gummed Up – Old gas or deteriorated metal/rubber that blocks up or obstructs the carburetor’s processes.
  • H

  • Heat Soak – Engine problems brought on by the fuel in the float chamber being exposed to excessive heat.
  • Heat Deflector – These help to minimize the effect known as “heat soak”.
  • I

  • Intake Manifold – The part of the engine that supplies the air-fuel mixture created by the carburetor to the cylinders of the engine.
  • Idling – When an engine is running only to stay running, not to perform any actions.
  • J

  • Jet – A very precisely sized opening in the fuel path that helps adjust the flow of fuel by introducing more fuel from the float chamber into the air-fuel mixture.
  • K

  • Knocking – Also known as detonation. Premature ignition and explosion of the air/fuel mixture. This will interfere with the cycle of the engine and possibly even damage the engine.
  • L

  • Liquid Fuel – Most commonly gasoline, this is what is stored in the float chamber and mixed with air for use in the engine.
  • Lean Mixture – An imbalanced air-fuel mixture caused by an oversupply of air, which makes the engine misfire.
  • M

  • Metering System – Ensures that an accurate amount of fuel is brought into the mixture for combustion.
  • N

  • Needle Valve – Needle Valve – A small valve that allows flow to be precisely regulated.
  • O

  • Open Throttle Circuit – As the throttle valve is opened more, air speed is increased in the venturi. This creates lower pressure and brings fuel into the airstream.
  • P

  • Priming – Getting the engine ready to start.
  • Piston Pump – Used to either move liquids or compress gases.
  • Q

  • Quantity of Air-Fuel Mixture – The amount of air that enters the engine is controlled by the throttle valve.
  • R

  • Rich Mixture – An air-fuel mixture with an abundance of fuel. This can be easier to ignite and is often necessary for cold starting. Can cause flooding.
  • S

  • Spark Plug – A device that delivers an electric current from the ignition system to ignite the compressed mixture of fuel and air.
  • Scrutinizing Spark Plugs – By removing the spark plugs, you can identify which type of mixture you have. Black and sooty plugs mean your mixture is too rich. White and light gray plugs mean you have a lean mixture.
  • T

  • Throttle Valve – A valve at the bottom of the carburetor that, when open, allows more air and fuel to flow into the engine, and when closed, allows less air and fuel into the engine.
  • U

  • Updraft Carburetor – A type of carburetor in which air enters from the bottom and exits out of the top, typically used with older engines.
  • V

  • Venturi – A narrow kink in the pipe of the carburetor that causes the air flow to speed up and, as a result, a drop in pressure.
  • Valve – A device that regulates and controls the flow of a liquid.
  • Viscosity – How “thick” a liquid is.

Now that you know some of the basic terms associated with carburetors and engines, you’re ready to better understand carburetor maintenance and operation. If you want to know more about carburetors, check out the following:

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3 Responses to The A to Z of Small Engine Carburetors

  1. Larry Holmes says:

    I bought a new 31R976-0016-G1 to install on a 44 In. pull behind swisher mower.Application does not have a battery or starter.Not sure how to wire it.I replaced the anti backfire cellinoid with a bolt to hold the bowel in place.It also has a stator and voltage regulator.I cannot detect spark from the sparkplug wire.The kill wire from coil is not grounded .Cant find a wire diagram.Any ideas

  2. ed says:

    Bottom jet valve screw for a 1976 Merc 4hp 2cycle outboard. other places, cross reference????

  3. mark hansen says:

    “HELP”— I have a craftsman mdl. 917.372270 has a carb. # 632388, when I start this engine (1st pull, real nice, starts right up, set on low speed, it runs fine, under no mow load. When I take it out to lawn and start a mow, when it hits some thick grass, gas will then flow out the carb and it floods out. I have had the float out for inspection and no leakage evident, I was thinking that the bowl was filling to much maybe.
    Any input much appreciated, thank you Mark H.

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