How Does a Small Engine Carburetor Work? (Beginners guide)

By: Author John Cunningham. Published: 2023/07/03 at 7:47 pm

Carburetors are truly amazing bits of kit… they don’t look particularly impressive, but when you understand what’s going on inside a carburetor, I think you’ll agree – they are genius engineering.

A carburetor holds a gas reservoir, mixes air and fuel, and regulates air and fuel flow, ensuring the correct air-fuel mixture for perfect engine combustion.

Index

• What is a carburetor?
• Components of a Small Engine Carburetor
• How Does a Carburetor Work?
• Troubleshooting Common Carburetor Issues
• Carburetor Maintenance and Cleaning
• Replacing a Carburetor Factors to Consider
• Common small engine carburetor FAQs

What is a Carburetor?

Sounds simple enough, right?

A small, insignificant-looking component like a carburetor might have you believe it’s simple, but it’s not.

It’s quite the trick to measure the correct ratio of air and gas and supply just the right quantity for every given engine load.

Total respect for these little guys.

Carburetors are machined to tight tolerance, and no surprise when they wear out or clog with debris; they can cause a ton of problems with varying symptoms. That’s not to say carbs are delicate, not at all; they are as reliable as corduroy trousers, and a good cleaning usually fixes most issues.

And if you need to replace yours, you’ll find replacements are inexpensive, which is surprising given the precision and care required in their manufacture. Both cleaning and fitting are straightforward missions, and we’ll get to both a little later.

For now, let’s look at what’s inside a small engine carburetor.

Components of a Small Engine Carburetor

We’ll briefly describe the various carburetor components, what they do, and where we’ll find them.

Fuel Bowl

Float and Needle Valve

Venturi

Throttle Plate

Emulsion Tube

Main Jet

Idle Jet

Choke

Primer Bulb

How Does a Carburetor Work?

It’s got to be witchcraft… How can something so basic-looking with no control module or electrical parts do something so complex?

So, what’s complex about adding gas to an engine? Well, see, first off, engines like a particular ratio of air (oxygen) to fuel (gas) get this ratio wrong, and the engine will run poorly or, more likely, not at all.

Next, the carburetor must be able to supply a sufficient quantity of this mixed gas but do so instantly for whatever demand the operator places on the engine.

But not only that, the correct ratio and quantity of fuel must be converted into mist (known as atomizing) before the engine can burn it efficiently, but before the fuel can be atomized, it must be emulsified…I know, witchcraft, right?

There are other challenges, too, like idling the engine smoothly, controlling the power, and starting in cold climates.

Let’s dive in a little deeper and look at the main functions of a carburetor and get a feel for how they work together and how a carburetor manages all these challenges.

Fuel Supply

The carburetor is useless without a good fuel supply, so the first challenge is managing gas flow to the carburetor. Typical issues are gas leaks, too much gas supply, insufficient gas supply, and contaminated gas.

Let’s follow the gas flow through the system to see how it works.

Gas tank and cap are basic components, and not much to go wrong here, you might be thinking. But that’s not the case. The gas tank and gas cap do more than store gas…I know, surprising.

These filters help filter out contaminates, but truthfully, stale gas is far more problematic than debris in the gas, and a filter won’t fix that, but more on that later.

The gas cap has some special skills, too; sure, it keeps gas from splashing over the side of the tank, but it also allows the tank to breathe. See, as the fuel level falls inside a gas tank, air must replace it. Otherwise, the fuel stops flowing.

The cap is fitted with a one-way valve; it allows displacement of air in but prevents fuel vapors from leaving.

Getting fuel from the gas tank to the carburetor is a challenge most small engines overcome using fuel-safe rubber hoses and gravity. Gravity feed means the gas tank is positioned higher on the engine than the carburetor, and the gas naturally flows downhill, which works great for most.

Small engine fuel pumps are usually a simple mechanical type—a rubber diaphragm powered by the pulsing of crankcase pressure.

Next in the fuel line is the gas valve, also known as a gas tap. Turning the tap allows the operator to cut fuel supply to the carburetor during longer storage periods and for repairs.

Not all small engines have a tap fitted; some are fitted in the fuel line, and some are carburetor integrated.

We’ve covered fuel tank integrated fuel filters already but haven’t covered the other type of filters, the inline or bottle filters, and gas tap filter bowl. Engines without an integrated system likely have an inline filter or gas tap filter.

Inline bottle filters are, as their name suggests, filters placed directly in the fuel line. Typically, these filters are easier to identify, troubleshoot, and access than the tank-integrated filters.

The gas tap type filters use a small fuel bowl that collects debris and may be removed and cleaned.

At this stage, gas has now arrived at the carburetor fuel inlet, where inside the fuel bowl, the float and attached needle are in the lowered position, meaning the fuel inlet port is open and allows the fuel to fill the bowl.

The petroleum-resistant plastic float floats on the surface of the gas inside the fuel bowl. As the gas level rises inside the bowl, the float and needle lift, causing the needle to seat and restrict fuel flow.

As the engine uses gas, the float and needle will fall, and the process starts over.

The bowl employs an O-ring seal to help prevent gas leaks.

Fuel Ratio, Emulsification & Atomization

At this point, we have gas inside the carburetor bowl; our next set of challenges includes preparing the fuel and delivering it to the combustion chamber.

And since fuel delivery is also an essential driver for the fuel preparation process, we’ll look at the delivery process first.

Fuel delivery describes how fuel is encouraged from the fuel bowl to the cylinder.

Fuel sitting in a carburetor bowl is great, but we need to get it into the cylinder, where it’s compressed by the piston into the combustion chamber and then ignited by the spark plug.

We are describing the stages of the four-stroke engine cycle. And it’s the first stroke we are particularly interested in – The Intake Stroke.

That’s where the piston draws downwards inside the cylinder with the intake valve open; the vacuum it creates moves gas from the carburetor to the engine, and that’s the main driver behind fuel preparation and delivery.

But it doesn’t do it alone; it has some help from the venturi effect – another essential ingredient.

I’m surprised a carburetor isn’t called a venturi, as carburetor operation isn’t possible without the venturi effect.

The venturi is a tapered area inside the airflow section of the carburetor, and when incoming air passes through this tapered section, it speeds up considerably.

The increase in speed causes air pressure to drop – that’s the venturi effect. The difference in air pressure causes the gas from the gas bowl (normal air pressure) to draw upwards into the venturi (low air pressure) and into the airstream to the engine cylinder.

The venturi is a critical carburetor ingredient; it isn’t a separate component; instead, the venturi (tapered shape) is machined or cast into the carburetor body.

Both the intake stroke and the venturi effect are essential for fuel delivery, but also fuel preparation.

Surely gas is gas; what preparation does it require? Thing is, an engine won’t burn gas all by itself; it must be mixed with oxygen, and it’s particular about the ratio of air (oxygen) to gas.

The carb must add the correct amount of gas proportionately to the volume of incoming air, and to do that, the carb is fitted with a main jet, and we’ll look at that next.

The main jet is submerged in gas, and its small orifice restricts gas flow to the emulsion tube. The jet is specifically sized to match the airflow of the carburetor. And as you know, the jet allows gas to flow proportionally to airflow passing through the airstream.

As the regulated gas is drawn up through the jet, it enters the emulsion tube, a brass tube designed to mix a small amount of air with gas; that’s what we’ll look at next.

The tube is positioned directly below the venturi in the main post of the carburetor and stretches downwards and into the gas bowl. Typically, they are made from brass, but in the latest carburetors, they are plastic (often main jet integrated).

The function of the emulsion tube is to allow air and fuel to mix before arriving at the venturi.

The carburetor includes an air passage to atmosphere to allow the bowl to breathe.

Mixing air with fuel at this stage is known as emulsification and is essential for the next step – atomization.

When the mix arrives at the venturi, the high-speed incoming air collides with the fuel and causes it to atomize or mist, which is the final stage of fuel preparation.

Airflow Control

So far, we’ve mixed air with the gas to the correct ratio, converted it to a mist, and delivered it to the cylinder. Now what? Now, we need to harness all that power… you know, dial it up and down on our command.

And to do that smoothly, we’ll need a little help from a throttle plate and an idle speed control jet.

The throttle plate is a simple concept; it’s a plate located in the airstream of the carburetor that the operator can open and close (usually), thereby controlling air and, as a result, fuel flow to the engine.

Opening the throttle plate allows more air and gas in, increasing engine speed and power. (Engine speed, also known as RPM (Revolutions Per Minute)). 

Some small engines have an adjustable throttle, but some are controlled automatically by an engine governor, so those machines won’t have throttle controls.

Because closing the throttle plate shuts off air and fuel flow, our engine needs alternative access to both, and that’s where our idle circuit comes into play.

The idle control comprises an independent air and fuel supply and a fuel metering jet known as the idle jet.

The jet’s contribution to air/fuel supply depends on the throttle plate position. With the throttle plate open, the idle circuit and jet do no work, but when the plate is all but closed, the engine vacuum sucks air and gas through the metering idle jet.

Not all carburetors locate or configure the idle jet the same. Some locate the jet for easy access in the outer body of the carb; as per the illustration above, however, on other models, the jet is machined into the carburetor and isn’t accessible without removing the carburetor.

Some idle jet circuits are fitted with an air adjustment screw; however, many aren’t, as the manufacturers want to comply with EPA (Environmental Protection Agency) emission standards. More on adjusting later.

Cold Start Control

Cold start controls come in two flavors: choke plate and primer bulb. But wait, what do we need cold start controls for?

Cold air is denser in oxygen than warm air, and if you remember, our engine likes 14.7 parts oxygen to 1 part gas. That means on a cold morning start, our engine is attempting to start with too much oxygen in proportion to gas.

Adding choke, causes the choke plate to close and restricts airflow, and simultaneously increases fuel to the engine.

Not all carburetors employ a choke, some employ a primer bulb instead, and some equipment, like snowblowers, need both a choke and a primer bulb.

The primer bulb may be carburetor-integrated or fitted remotely. A manually activated vacuum or manual air pump squirts gas directly into the venturi.

Adding gas using the primer is great for quick starts; however, a primer-only carburetor will struggle to stay running in freezing temperatures, so cold-weather engines are fitted with a choke. We need the consistency of an adjustable choke plate for cold climate starts.

Some chokes are thermostatically activated, meaning it’s auto-choke – you don’t need to do anything, yank the cord or hit the start button.

And that’s it. You are now ready to troubleshoot some common carburetor symptoms.

Troubleshooting Common Carburetor Issues

Carburetors are durable. Well, why, then do they cause so many problems? And the answer is they generally don’t. It’s the gas that flows through them that causes the most issues.

I’ve been a mechanic for over twenty years, and by far, bad gas is the number one issue with small engine performance issues.

Let’s now look at some of the most common carburetor-related symptoms; I’ll add a link to the stepped fix where appropriate.

Before presuming we have a fuel system issue, it’s good practice to verify our spark plug is good and that our air filter is clean.

Engine Not Starting

An engine that won’t start is typically caused by old gas; this is especially true if the small engine has been lying idle for an extended period. Typically, the gas goes stale, and depending on how stale it is will dictate the level of repairs needed.

In worst cases, the gas turns to sticky sludge and gums up the carburetor; a thorough cleaning is required, but without the proper ultrasonic carburetor cleaning kit it may be necessary to replace the carburetor.

Adding a gas stabilizer will prevent carburetor gumming.

But before condemning the gas, we must run a test or two. A no-start engine is typically caused by a fuel system or ignition system fault.

Testing for spark is the go-to test as it’s easy to execute, and whatever the result, you’ve learned an important clue.

If you don’t have a spark, try cleaning, gapping, or swapping the spark plug; still no spark, you are likely looking at a failed coil.

You may find these links helpful:

• Spark testing
• Spark plug replacement
• Spark plug cleaning
• Spark plug gapping
• Ignition system test

If, on the other hand, you do have a spark, then it’s likely a fueling issue is causing the no-start, and we’ll need to examine the fuel system.

Check out the following links:

• Gas removal
• Gas tank cleaning
• Carburetor bowl cleaning
• Gas bowl draining (video)
• Carburetor cleaning

A faulty carburetor is also possible but harder to diagnose. So if you have eliminated an ignition system fault, your engine compression is good, and your carburetor is clean, then pull the trigger on a new carburetor.

Remember to clean that fuel tank before fitting the new carb.

Poor Idle or Stalling

A bad idle or stalling sounds like your RPM needs to be adjusted a little or your idle jet is dirty.

Rough or Surging Engine

Rough running and surging sounds like a dirty carburetor. Try cleaning the idle jet, and as we learned earlier, the idle jets on some carburetors are easy to access and clean.

Check out the Snowblower engine surging post.

Fuel Leakage

Fuel leaks are somewhat common; they typically happen for three reasons: worn or damaged gas bowl seals, worn-out needle valves, and worn-out fuel lines.

The bowl seals give in over time and commonly get pinched when refitting the gas bowl after cleaning. A new bowl O-ring gasket solves that issue.

A leaking needle valve is a more serious issue, not because it’s expensive or difficult to repair but because of the downstream issue it can cause if not caught or ignored.

And that can be a terminal for an engine if not fixed ASAP. Check out White smoke from engine post.

And finally, worn out or loose fuel hoses and clamps. The fix here is obvious: replace these fuel lines.

Some less likely causes of fuel leaks include damaged fuel taps and split gas tanks (plastic), rusted gas tanks (metal), and damaged fuel filters.

Rich or Lean Fuel Mixture

When referring to carburetors, a mechanic may refer to the engine running rich or lean. This, as you know, refers to the ratio of air to fuel in the mix.

Symptoms of a rich-running engine include sputtering, black smoke, flooding, rough running, plug fouling, hard starting, and the list goes on. Common causes are dirty air filters, bad gas, bad plugs, dirty carburetors, and bad needle valves.

Symptoms of a lean condition include coughing hard starting, surging, hanging idle, overheating, and lacking power. Typical causes include bad gas, wrong gas type, mechanical issues, and vacuum leaks.

Small two-stroke engines (oil+ gas mix) such as chainsaws and weed eaters have adjustable carburetors, but most four-stroke engine fuel systems, lawnmowers, generators, power washers, etc., are not adjustable.

They are installed and set from the factory, and the adjuster screw is broken off to prevent further adjustment. The EPA doesn’t trust folks to adjust correctly, so the temptation is removed.

Anyhow, if yours has an air mix screw, you can see how to adjust it like a pro here.

Carburetor Maintenance and Cleaning

If correctly maintained, carburetors aren’t problematic. You likely know the issue with ethanol gas, also known as blended gas. Small engines don’t like it; more precisely, they don’t like laying idle with ethanol in the fuel system.

Fuel Stabilizer

Ethanol gas goes stale and gums up the fuel system. It turns into a sticky sludge if it isn’t treated with a fuel stabilizer. It is really important to treat all your small engines if you expect them to lay up for more than a month.

I’ve covered mixing and adding gas stabilizers here.

Regular Inspection

Inspecting your small engine kit before every use is a great habit. Check the oil level and check for leaks around the gas cap, gas tank, fuel lines, and carburetor bowl.

Remember to pay particular attention to an over-full oil level and the stink of gas from your engine; it is a common symptom of carburetor needle failure. Check out the White smoke from snowblower post.

Cleaning the Carburetor

Carburetor cleaning is a chore we all have to do at some stage, and it isn’t too challenging, but like all tasks, good preparation and some insider know-how will make the chore move like butter.

I have you covered; check out the beginner’s step-by-step guide to carburetor cleaning with pictures.

I’ve also made some videos that cover the procedure for popular small engines, and you can check them out here:

• Briggs & Stratton (Classic) carburetor cleaning (video)
• Briggs & Stratton (Intek & Quantum) carburetor cleaning (video)
• Briggs & Stratton (e-Series) carburetor cleaning (video)
• Honda carburetor cleaning (video)
• Kohler carburetor cleaning (video)

Adjusting Carburetor Settings

Adjusting your small four-stroke engine may or may not be possible; it all depends if your air mix screw adjuster is accessible.

Many manufacturers fit a fixed air-fuel mix screw. It means the engine is factory-adjusted and then rendered inaccessible to prevent further adjustment by either the operator or the shop. They do so as they attempt to comply with the EPA (Environmental Protection Agency) emission standards.

Note an adjuster located on the engine side of the throttle plate (Butterfly) means it’s an air mix screw, and an air screw works in reverse to a fuel mix screw.

Meaning:

Anyhow, if you have one fitted, this is how we adjust it.

The engine needs to be running to adjust the air screw. Keep your digits and toes away from engines with spinning and cutting parts.

Locate the air-mix screw and have an appropriate driver

Engine running, need a helper or improvise

• Adjust the screw clockwise until the engine runs rough
• Now adjust the screw counterclockwise until the engine runs rough (count the turns or part turns)
• Now turn the screw clockwise again by half the number of turns you counted.

The engine should now run sweet, but if you need to tweak it on either side, go for it.

Replacing a Carburetor Factors to Consider

When replacing a carburetor, it is important to make sure you fit the correct model carburetor. While a carburetor may look identical to your old one, it could be different, and remember, carburetors are calibrated and matched to the size of your mower engine.

Fitting a carburetor designed and calibrated for a larger engine will cause your engine to run rich, if at all. Conversely, fitting a carburetor that’s too small for your engine could damage your engine by running lean and hot.

Selecting the Right Carburetor

Clean the body of your old carburetor and check for the make and model code stamped into the body. Use that code to verify you’re buying the correct replacement.

Engine manufacturers generally don’t build their own carburetors, so your carburetor may be branded by another maker.

Check out the carburetor page for links to common carburetor models.

Installation Tips and Precautions

I’ve been swinging wrenches professionally for twenty-plus years; here are my top tips for swapping out a small engine carburetor:

• Select work location – a bright sealed surface, preferably outdoors
• Remove the plug wire and turn the gas valve off
• Tilt the engine over for ease of access, if needed, but with the carburetor facing upwards
• Verify new carburetor is the same as the old unit
• Use a container for small parts
• Take a ton of photographs of carburetor linkages and springs before removing
• Drain the gas tank entirely and clean
• Fit new air filter
• Be sure the gas refill can is clean and the gas is less than a month old
• After fitting, turn the gas on and check for leaks

Frequently Asked Questions

Here are the fuel system questions I’m asked every year without fail.

How often should I clean my carburetor?

The carburetor should be cleaned every five years. However, I advise cleaning the carburetor bowl and fuel filter every year before the season begins.

Can I use ethanol-blended fuels with a carburetor?

For most of us, we don’t have a choice. We simply have to use blended gas. Most manufacturers are OK with E10, but if you are particular, you can buy pure gas in a can from your local outdoor powered equipment shop. Gas in a can is pure; it’s high octane and is treated so that it will last about a year.

When using ethanol gas, we’ll need to use a gas stabilizer; I know it’s a pain in the jacksie, but if we don’t, the blended gas will gum up our small engine kit as it lays idle.

Do not use E15, E20, E30, E50, or E85; it is outlawed by the EPA (Environmental Protection Agency), and besides, it will kill small engines, and worse, the manufacturer’s warranty will not cover the damage.

Why is my carburetor flooding?

Carburetors commonly flood for a few reasons:

The engine was tipped over incorrectly – An engine should be tipped over with the carburetor facing upwards. Check out – White smoke from mower engine

Float needle sticking in the lower position after hibernation – This can happen after a machine has been lying up with the gas tank drained. A giggle of the float usually gets it back working again.

Worn-out needle valve – The needle valve regulates the gas bowl fuel level, and when the seat seal wears, it allows gas to sneak into the carburetor, eventually flooding the engine.

What are the signs of a clogged jet?

Small engines typically have two fuel jets; one is called the main jet, and the second is known as the idle jet.

The main jet supplies fuel to the engine whenever the throttle plate is open.

The idle jet supplies gas whenever the throttle plate is closed. It’s responsible for keeping the engine idling when the throttle is set to the low position.

When the main jet is blocked or partially blocked, the following symptoms are typical:

• Engine won’t start
• Engine starts then stalls
• Engie lacks power

When the idle jet is blocked or partially blocked, the following symptoms are typical:

• Engine surges on idle (revving up and down)
• Engine fails to idle
• Engine starts then stops

You may find these posts helpful:

• Carburetor bowl cleaning
• Carburetor cleaning
• Carburetor cleaning (video)

Should I rebuild or replace my carburetor?

Carburetors eventually wear out and need some love, but they’re finely tuned bits of kit, so unless your carburetor is particularly rare or expensive, I’d advise you replace it with a new one.

New carburetors are inexpensive, and quality parts will come tested and guaranteed.

As well as that, rebuilding a carburetor that’s worn out will have you chasing your tail; you’ll nail it back together, and you may very well have the same fault symptoms you began with.

But worse, now you may be thinking – Well… I know my carburetor is good! When, in fact, it’s not, and you go on to throw unneeded parts at it.

You get what I’m saying: a carburetor isn’t that complex, and you could rebuild it, but rebuilding won’t fix worn-out metal carburetor components. Risk reward just isn’t there.

And that’s it. You are now up to speed on how a small engine (four-stroke) carburetor works.

If you want to get up to speed on small engine ignition systems, check out – How small engine ignition systems work.

You may find the following links helpful: