How 2-Stroke Carburetor Works – seriously clever

By: Author John Cunningham. Published: 2023/07/22 at 4:27 pm

I love carburetors….seriously clever engineering. It’s astonishing that something so compact can handle such a remarkable range of tasks. Once you learn their secrets, I think you’ll be in love too.

A 2-stroke carburetor pumps, meters, mixes, and atomizes air and gas in preparation for combustion chamber ignition.

In this post, we’ll look at the components of a carburetor and how they work; we’ll look at common problems and cover mechanic maintenance tips and carburetor FAQs. Let’s dive in!

What Does a Carburetor Do?

A fuel system is tasked with managing the fuel supply for an engine. It does boring stuff like containing the fuel, filtering the fuel, and transporting the fuel, but it does more complex tasks too, like pumping, metering, mixing, atomizing, delivery, idle control, throttle control, cold weather enrichment, and allows for manual fuel ratio adjustment.

And it’s our carburetor that does all the really hard stuff.

See, an engine won’t run on gas alone. It needs a quantity of air (oxygen) mixed with the gas; it’s called an air-fuel-ratio (AFR), and one of the carburetor’s main tasks is to add just the correct amount of gas proportionally to the volume of air that passes through it.

The complexity doesn’t end there; an engine prefers its air-fuel mix atomized or converted into a mist – Yep, our carburetor takes care of that, too, and a few other tasks.

See, I told you they were clever!

2-Stroke Fuel System Components

Before we get to how a 2-stroke carburetor works, we’d better introduce the other fuel system components a carburetor relies on.

They include:

Next, let’s look at the carburetor in more detail and look at its components and what function they perform.

Components of 2-Stroke Carburetor

The main components of a carburetor are as follows:

How 2-Stroke Carburetor Works

Since an engine won’t run without gas, it makes sense to begin our story with how the gas gets to the carburetor.

The Fuel Pump

A 2-stroke engine is fitted with a fuel pump, surprising, right?

It’s true. A small fuel pump moves gas from the gas tank through the fuel line to the carburetor, and it does so using little more than a rubber gasket and the pulsing crankcase pressure created by the piston.

And it’s all powered by channeling crankcase pressure to a port on the carburetor either by interfacing manifold and carburetor ports or by using an impulse feed hose from the crankcase to the carburetor, depending on the carburetor type.

It works like this:

The vacuum opens the inlet reed valve, allowing fuel to flood in and fill the pump chamber, but at the same time, the vacuum closes the reed valve on the exit side of the fuel pump.

Now the pump has the opposite effect; it forces closed the reed valve on the fuel inlet side but pushes open the reed valve on the exit side of the fuel pump chamber, which in turn forces fuel to flow through to the metering valve fuel feed channel.

The Metering Valve

As the metering valve feed channel receives the gas, it filters it through the Microscreen to remove debris that could easily block tiny jets.

A lever arm (with a return spring) unseats the needle valve, allowing fuel through, but only when the carburetor’s intake vacuum pulls the metering diaphragm downward onto the metering lever.

High-speed circuit and Main Jet

The high-speed circuit (main circuit) and jet carry fuel from the diaphragm chamber and deliver it directly into the venturi. The jet’s orifice is brass and very precisely drilled to allow gas to flow proportional to the airflow through the carburetor.

The low-speed circuit (Idle circuit) and jet (L-Screw) control the fuel supply from idle to part throttle. The high-speed circuit (Main circuit) and jet (H-screw) control the fuel supply from part throttle to full throttle.

The Throttle Plate

The throttle plate is fitted in the intake airflow that the operator controls using linkages and throttle levers. The throttle plate controls airflow and fuel flow to the engine.

The Low-speed circuit and Idle Jet

The low-speed, also known as the idle circuit and jet, are employed to prevent a small engine from stalling when at idle; remember, the throttle plate is closed, so little to no air or fuel can pass through the carburetor by the main intake. Instead, a bypass circuit is used.

Engine vacuum sucks fuel through this alternate route when the throttle plate is closed; typically, it’s a channel drilled through the carburetor body. The circuit terminates with three (usually) drillings that straddle the throttle plate, one on the engine side and two on the intake side, just where the throttle plate begins to open.

The low-speed circuit controls the fuel flow from idle to part throttle, and then the high-speed jet takes over.

The Venturi

The venturi is a tapered section of the carburetor’s intake. The narrowing shape of a venturi speeds air up, which causes air pressure to fall. This is known as the venturi effect, and it encourages fuel to move from a higher pressure area – through the jets and into the intake, where the large fuel droplets collide with the incoming air, causing the fuel droplets to atomize.

If you’d like to see how ignition works, you can check that out here – How a small engine ignition system works. (it’s a four-stroke engine, but the ignition system is very similar)

The Choke

The choke plate is positioned at the carburetor’s entrance, which, when activated, closes and restricts airflow.

Choke is great for helping small engines idle smoothly, but some need a little extra help to start initially, and for those engines, a primer bulb is also fitted.

The Primer Bulb

The primer bulb uses a simple one-way valve, a rubber bulb, and a fuel line to move gas from the gas tank to the metering chamber.

That’s it; you’re a pro!

Next, we’ll look at some common 2-stroke carburetor problems and the fix.

Ethanol Fuel & Small Engines

I’ve been a mechanic now for more than twenty years, and in recent years, ethanol gas has been the single biggest driver of repair work in my workshop.

What is ethanol?

Ethanol is a renewable fuel made from plant materials. Basically, it’s alcohol, and it’s blended with pretty much all gas sold today. The most common grade sold is E10, meaning the fuel contains 10% ethanol and 90% gas.

There are other grades too, E15, E20, E30, E50, and E85; the EPA outlaws these for use in powered outdoor equipment. But you wouldn’t want to run your small engine on it anyway; ethanol attacks all the plastic and rubber components of the fuel system.

But not only that, small engine manufacturers won’t cover damage caused by using these outlawed fuels.

More ethanol equals more damage.

As ethanol is a solvent, it causes plastics and seals to swell and degrade, which is not great since small engine fuel systems use lots of plastic and rubbers.

What’s the problem with ethanol blended gas?

The biggest problem with ethanol is it doesn’t store very well. Because it’s an alcohol, it attracts water, and water inside a fuel system is bad news.

But the news gets worse – when left to sit in a gas tank or refill can, the ethanol and gas/oil mix separate, and some of the alcohol evaporates, leaving a water/alcohol mix at the bottom of our gas tank with the gas/oil mix floating on top.

And since carburetors suck gas from the bottom of the tank, our small engine now gets a drink of alcohol and water; it may or may not start on this mix, and honestly, it would be better if it didn’t since it doesn’t contain our 2-stroke oil in the mix. Either way, performance is compromised.

Top Tips when using ethanol

• Only buy as much gas as you can use in thirty days
• Use a gas stabilizer
• Keep gas tanks full
• Use spill-proof refill cans
• Older machines are under major threat from ethanol and will need draining at the end of the season.

Spill-proof containers help keep ethanol fresh; the image below links to Amazon.com.

What grade gas is recommended for small engines?

1. Regular gas (lowest octane fuel 87)
2. Midgrade gas (octane fuel 89–90)
3. Premium gas (highest octane fuel 91–94)

What does octane mean?

Octane measures how stable gas is, in other words, how predictable it will ignite at a given pressure. The more predictable a fuel is, the better it is – Higher is better.

Common 2-Stroke Carburetor Issues

Here, we’ll cover all the most common 2-stroke carburetor-related symptoms. Most are easily fixed, and where available, I’ve added a link to the stepped repair process.

Before commencing any carburetor diagnosis, it is always worth checking that your fuel is fresh and that the spark plug is the correct type, clean, and gapped correctly.

You’ll find all the tools I use here on the carburetor tools page.

Engine won’t start?

There may be lots of reasons a 2-stroke engine won’t start, but in my shop, the root causes are typically fuel/carburetor-related.

Most issues relate to old gas gumming up the carburetor, and the usual fix is carburetor removal and cleaning, but when you have the carburetor stripped, it makes sense to replace the carburetor gaskets, too. (metering diaphragm, fuel pump diaphragm, and needle and arm)

The problem is, as you likely know, the ethanol blended gas doesn’t store well; it evaporates and attracts moisture.

You may find this link helpful:

• Saw won’t start after sitting
• Adding gas stabilizer (video)

Engine starts but dies?

Starts and then dies, which is a symptom of an engine that’s running out of gas. And that could be for several reasons. Common among them are:

• Carburetor idle needs adjustment
• Carburetor mix screws need adjustment
• Stale gas
• Fuel lines leaking
• Worn-out carburetor gaskets
• Dirty carburetor
• Metering valve arm needs adjustment
• Vacuum leaks

Always worth checking that your fuel is fresh and that the spark plug is the correct type, clean, and gapped correctly before commencing any carburetor diagnosis.

You may find these links helpful:

• Spark plug testing
• Carburetor adjustment

Engine bogs down?

This is the symptom of a saw that needs some carburetor adjustments, but it could be a saw with a more significant fuel starvation issue.

Typical issues are:

• Carburetor needs adjustment
• Leaking fuel lines
• Worn-out carburetor gaskets
• Metering valve arm needs adjustment
• Stale gas
• Blocked gas filter
• Bad gas cap vent
• Vacuum leaks

You may find this link helpful:

Carburetor adjustment

Engine won’t Idle

A saw that won’t idle may simply need a tweak of the idle screw adjuster. You can check that out right here – Idle screw – Adjusting.

It may also mean we have an issue with the idle circuit or jet, a blockage, or maybe some adjustment of the L-screw would help. You can check that out right here – Carburetor adjustment.

As always, ensure your gas is fresh and your spark plug is the correct type, clean, and gapped correctly. A bad plug can cause a wide array of symptoms.

Engine won’t start when hot

A hot start issue could be fuel-related but also ignition-related; not helpful, right? On the fuel side, hot start issues are usually caused by fuel vapors. The fuel evaporates and creates air pockets in the carburetor and fuel lines, which prevents the fuel pump from moving the gas.

It’s common in saws that get too hot, which is often a symptom of another issue. Running a saw too lean or using gas with too much ethanol will cause your saw to run hot.

I wrote a post all about troubleshooting such an issue, and you can check it out here – Chainsaw overheats

Engine won’t rev up

When an engine doesn’t rev up, it is likely caused by a blocked exhaust spark arrester screen. Another possibility is a faulty coil, I wrote a post about this exact issue, and you can follow along while I troubleshoot it.

Chainsaw won’t rev up

Engine revs up by itself

This typically means our engine is sucking air from somewhere it shouldn’t be. It’s a vacuum leak and can be difficult to find and fix. Anyhow, here are the most common reasons a saw sees fit to rev by itself.

• Crankshaft seals leaking
• Base gasket
• Manifold gasket loose
• Manifold gasket cracked
• Carburetor loose
• Carburetor throttle shaft worn
• Adjuster screws leaking

To help diagnose, try idling the saw and spraying brake cleaner around the carburetor and manifold; try spraying around the crank seals, too. The saw will stall, or the engine note will change if there’s a vacuum leak.

Primer bulb won’t fill

When the primer bulb doesn’t fill, it’s worth checking the following components.

• Gas filter – Check the gas filter in the gas tank for restriction.
• Primer bulb – Check the primer bulb for holes or cracks.
• Fuel lines – Check for cracked or perished fuel lines.

In the shop, we’ll pull the gas filter from the gas tank and fit a hand pump, adding about 10 lbs of pressure and checking if it holds pressure. Pressure loss means our lines or carburetor gaskets are leaking, and we’ll need a rebuild kit.

2-Stroke Carburetor Maintenance Repairs and Cleaning

Maintenance pays off; even a little maintenance here and there will extend the life of your machine. So, what type of maintenance, repairs, and cleaning are required?

Cleaning air filter

Most other 2-stroke equipment may only need an air filter cleaning once or twice a season. That said, if the conditions are dusty, the air filter should be checked and cleaned as needed. We never run a 2-stroke engine without an air filter; the dust particles will eat away at the intake side of the piston-kind of like a sandblaster.

Chainsaw air filters are available in three grades: mesh nylon for normal conditions – fabric filter for winter use, and a paper or fleece filter for dusty conditions.

All these filters are cleanable. However, we don’t clean them all the same way.

The air filter removal process is as follows:

The nylon mesh, fabric, and foam filters may be cleaned as follows:

The paper filter may be cleaned, but they’re designed for single use and should ideally be replaced when soiled. You can, of course, clean them, and here’s how we go about that:

Remember, a damaged air filter is useless and will kill your engine. If your clamshell filter is fitted with a rubber O-ring seal, make sure it’s present and in good condition. Clean the top cover’s underside and the air filter mount before replacing the filter.

Clean gas tank

A gas tank clean-out once a year will help prevent your carburetor from sucking crap from the bottom of the tank. Even with a filter, tank crap makes it to the carburetor.

Replacing fuel filter

With a clean fuel tank, it seems a shame to spoil it with a dirty fuel filter; I like to change them once a year.

Tools and parts you’ll need:

• New filter
• Pliers
• Container for waste gas
• Fresh gas

The process is as follows:

• Use a piece of wire to fish out the filter.
• Empty out the old gas and rinse out the tank if you haven’t already.
• Pull the filter retaining clip back and remove the filter by twisting and pulling.

Replacing gas lines

The process varies by machine; saws are the most challenging as the engine sits so close to the top of the tank where the pick-up goes in. That said, loosening a few vibration mounts usually gives enough access.

Best make sure your lines are the correct size; typical sizes include:

Inner diameter x Outer diameter

• 3/32″ x 3/16″
• 1/8″ x 3/16″
• 1/8″ x 1/4″
• .080″ x .140″

The fuel line replacement process looks like this:

• Loosen the gas cap to release tank pressure; best to empty the gas too.
• Remove the top cover to gain access to the lines.
• Note where they go (important if you have two lines (feed and primer line)). Replace one line at a time (if two lines).
• Clean where tank and lines meet.
• Remove the line from the carburetor side.

Gas lines are fitted into the tank in one of two ways: they pass through the tank using precisely drilled holes that match the fuel line, or a large rubber grommet is fitted with holes for the fuel lines.

• Use the old lines as a template for new.
• Rinse out gas tank with fresh gas.

• Make sure filter has sufficient length to sit and the bottom of the tank.

Replacing primer bulb

The primer bulb is like all the plastic and rubber fuel system components – under constant attack from ethanol fuel, and the bulb is a common casualty.

Here’s how we go about replacing both:

To fit the all-in-one unit:

If you mix up the lines, not to worry; the vacuum line goes to the metering diaphragm (carburetor), and the pressure line goes back to the gas tank.

To fit the primer bulb element type:

• Run fasteners in until they seat, then tighten in opposing fashion.
• Check bulb works and doesn’t leak.

Replacing the bulb isn’t the same as replacing the valve, and if you have trouble priming, the valve itself may be at fault or a faulty carburetor gasket. Pressure testing the line will help identify the issue.

Replacing carburetor gaskets

Ethanol is hard on carburetor plastics and rubbers.

For most 2-stroke carburetors, there are four critical internal gaskets, they are:

1. Metering diaphragm
2. Metering diaphragm base gasket
3. Fuel pump diaphragm
4. Fuel pump diaphragm base gasket

There are critical external carburetor gaskets also: two gaskets if your carburetor mounts to a manifold and just one gasket if your carburetor mounts directly to the engine.

These gaskets are critical because they make an air-tight seal between the carb and engine, allowing air to sneak in and cause erratic running or no start at all.

When fitting these gaskets, top tip – make sure the interfaces are smooth and oil-free; while I don’t like using sealer in these areas, it is possible to use a gasket maker such as TB1184 to help make a good seal but be super sparing around the impulse port, if this blocks up, your fuel pump won’t pump gas to the metering chamber.

I’ve covered replacing the carburetor gaskets previously, and you can check that out right here – Carburetor removal and rebuild process.

Cleaning or replacing the microscreen

Your carburetor has an internal gause filter (micro screen) located at the fuel pump end of the carburetor at the entrance of the metering diaphragm fuel feed channel.

The screen is designed to catch whatever our gas tank filter missed. Typically, we change the screen when replacing the carburetor gaskets.

The process looks like this:

• Remove the carburetor from the engine
• Remove pump cover
• Remove the diaphragm gasket
• Use a pick to remove the microscreen
• Clean carburetor
• Use the appropriate size tool to seat the new microscreen
• Replace gaskets as appropriate

Adjusting metering lever

The metering lever regulates fuel inside the diaphragm chamber; a maladjusted lever can either cause a lack of gas or flood the carburetor with gas.

The metering lever is adjustable, but you will need a special tool, and you’ll likely need to remove the carburetor to adjust it.

A new carburetor gasket kit includes a new needle valve and lever and should be correct, but it is always worth checking before fully assembling and fitting the carburetor.

The metering lever adjusting process is as follows:

Replacing Welch plugs

A welch plug seals off small chambers of the carburetor the manufacturer needed to expose to gain access to make precise drillings etc. Manufacturers then seal off the small chambers with a welch plug.

Here’s how we do that:

Replacing a 2-Stroke Carburetor – Factors to Consider

Love that new carb smell, but before you pull the trigger on a new carburetor, take one minute to read my top tips for buying and fitting a new 2-stroke carb.

OEM – Original Equipment Manufacturer, I know what you are thinking – Spendy…right? Yes, they can be expensive, but I like to use OEM if possible.

With OEM, you know it’s going to fit, and you know it’s going to perform. There are a lot of cheap Chinese knockoffs out there, and I understand the reasons for using them. Just saying, in my experience, the buyer of these cheap parts tends to be the quality control officer, if you know what I mean.

Use Carb Code – All carburetors are etched with a maker and a code. It is normal for branded equipment makers to buy carburetors from carburetor manufacturers.

So, shopping for your carburetor by the carb manufacturer brand is usually cheaper than shopping by the equipment maker brand.

Bottom line – your new carburetor code must match your old one.

Clean Gas Tank – Drain the gas tank and rinse with fresh gas before starting work. If your refill gas is older than one month and you don’t use a fuel stabilizer, I’d use it for cleaning paintbrushes and getting fresh. It’s not worth contaminating our new carburetor with old stale gas.

Replace Fuel Filter – We need to order a new fuel filter as the carburetor likely won’t come with a new one.

Check Fuel Lines – They become brittle, crack, and leak. And leaking fuel lines will leak air into the carburetor and compromise fuel flow to the engine. If you need new lines, now is the time to fit them.

Order new Manifold Gaskets – new manifold gaskets don’t always come with a new carburetor, and really, your new carb should get a new carb gasket. Using old gaskets risks vacuum leaks, and you’ll recognize a leak as the engine will run erratically. It’s a right pain in the jacksie, and you’ll need to strip again and replace that gasket.

While there, check the manifold gasket or boot for damage and cracks, and if you have an impulse line, make sure it’s in great shape too.

Take Pictures – If you don’t fit carburetors every day, it can be tricky to remember where all the bits go, especially if there’s a lag between stripping and refitting. Taking a few pictures helps, especially with orientating gaskets, throttle linkages, etc.

Tune Carburetor – Your carburetor should be close to perfect right out of the box (OEM), but we may need to adjust it a little.

This link will help you with that chore – Carburetor adjustment.

Use Fuel Stabilizer – Add a stabilizer to your gas to help prevent gumming. I’d put it into my gas can and mix a big batch enough for all my small engine kit mowers, snowblowers, generators, saws, and trimmers. The stabilizer is not a substitute for a 2-stroke oil mix; you’ll add 2-stroke oil to the stabilized gas in the usual way.

Check out gas stabilizer here – Adding gas stabilizer (video)

You’re all tooled up now.

Common 2-Stroke Carburetor FAQs

Should I run my engine dry? I prefer to use a gas stabilizer to protect all my small engine kit.

Is blended gas OK? Most manufacturers say using E10 is OK, but the EPA outlaws using anything greater than E10 in outdoor power equipment.

How often to clean 2-stroke carburetor? Ideally, the gas tank and filter should be cleaned and replaced annually, and the carburetor should be cleaned as needed, about every four years, but it does depend on usage and how the engine is performing.

Should I rebuild or replace my 2-stroke carburetor? If your carburetor is over ten years old and has many hours on it, I would treat your kit to a new carburetor; otherwise, a carburetor rebuild kit will leave it as good as new.