Finding the carburetor you need for your motorized bicycle engine can be tricky. You’ll see all kinds of carburetors out there that tout all sorts of benefits. But how can you be sure which is the best one for your engine kit?
There’s a carburetor for every need, whether you’re running a 2-stroke motorized bike or 4-stroke motorized bike. To know which one you need, though, it’s important to get to know carburetors inside and out. That’s where this guide comes in handy.
In this guide, we’ll detail everything you need to know about bike engine carburetors to find the best one for you. Whether you’re looking for better acceleration and top end speed, better torque, a smoother ride, or you just want to get your bike up and running, we’ll help you figure out what carb’s best for you — and we’ll even show you how to tune it!
Table of Contents
- Bike Engine Carburetors 101
- Carburetors: A Piece-by-Piece Breakdown
- Types of Motorized Bike Carburetors
- Choosing Your Next Carburetor
- What the Experts Are Riding With
- Best Upgrade for Stock 2-Stroke Carburetors
- Best Upgrade for Stock 4-Stroke Carburetors
- Best Carburetor for More Torque
- Best Carburetor for Top End Speed
- Best 2-Stroke Racing Carburetor
- Tuning Your Motorized Bike Carburetor
- Carburetor Maintenance
Before we get ahead of ourselves, let’s dive into what motorized bicycle engine carburetors are and what they do.
Bike Engine Carburetors 101
In its simplest terms, a carburetor is the part of an engine that regulates the amount of fuel and air that gets into a cylinder. It’s what connects an engine (specifically the cylinder) to its fuel and air source. Without a carb there would be no way for an engine to receive the chemicals it needs to create the combustion necessary for the piston to move and propel you.
It’s important to note that in order for gas to combust, the cylinder needs to be at a certain temperature.
This is universal from engine to engine, whether you’re running a motorcycle, a car, or in our case, a motorized bicycle.
How Does a Carburetor Work?
Like we mentioned, carburetors are responsible for air and fuel entering at the right mixture that’s best able to combust and keep your bicycle engine running. To do that, though, your carburetor has to perform some important functions:
Fuel comes into the carburetor from the fuel valve and enters the float bowl.
- At the same time, air is drawn into the carburetor through the air filter.
- As air enters the carburetor, it creates a vacuum that draws fuel.
- The fuel and air mixes together and is then sent through the intake manifold and into your cylinder.
- At the right temperature, the spark will cause the vaporized fuel to combust. This combustion is a small explosion that will cause your piston to travel up and down the cylinder again.
- As the piston travels, it draws in more vaporized gas via the carburetor and repeats the process until the engine is turned off.
So while carburetors are responsible for the air/fuel getting into your cylinder, they also play a big role in the warming of your cylinder — and subsequently fuel — in order for combustion to occur.
A Quick Note on Combustion
Engines work by cranking a system of pistons that draw in air and fuel via the carburetor, which is maintained through a process called combustion. In combustion, chemicals react to a heat source by igniting and creating fire.
In a cylinder, the vaporized gas (chemical) it receives is pressurized, and when a heat source (the spark) is introduced, the vaporized gas ignites. When it ignites (or combusts), it causes a small explosion that rotates the crank and sends the piston down the cylinder to draw in more air and fuel.
Here’s a look at what that looks like in real time:
As the piston travels down the cylinder, it draws gas and air into the cylinder.
- The CDI and magneto send a spark to the spark plug, and that spark is given off into the cylinder.
- The air in the cylinder becomes pressurized with air, and when the vaporized gas meets the spark, it combusts, igniting and causing an explosion that sends the piston back down the cylinder.
- When the piston is forced back down the cylinder, it will draw on more fuel and air from the carburetor.
- As the piston travels up the cylinder, the CDI and magneto send out another spark which then ignites the vaporized fuel, exploding and sending it back down.
- Note: Momentum is what keeps your piston traveling up and down the cylinder. So long as your cylinder is warm enough and able to keep the air pressure it needs to facilitate proper combustion, you’ll get those small explosions that create the momentum that cranks your piston and keeps your engine running.
The combustion process may be more involved with 4-stroke engines compared to 2-stroke engines, but the common denominator is their carburetor. Eliminate it and your motor won’t be able to get the fuel and air it needs, let alone keep the cylinder at an optimal temperature for combustion.
Now that you know how carburetors work, let’s talk about the parts of the carburetor and how they function. It’s important to know the individual parts of a carburetor to know how they function together to have your engine running at top performance.
This next part is what you’re going to focus on when it comes to tuning and eventually upgrading your carburetor, so let’s get to it.
Carburetors: A Piece-by-Piece Breakdown
Carburetors have lots of moving pieces in them, and each piece helps various functions of the carburetor’s performance. One mistuned or damaged part and you can cause some serious damage to your engine. At the very least, carbs with bad or untuned parts won’t run as efficiently as you need them to, causing undo stress on your 4- or 2-stroke motorized bike engine overall.
Here, we’ll go over the various parts of a bike engine carburetor to give you a more detailed idea of how a carburetor works. There are a lot of parts, though, so to make things easier to understand we’ll categorize these components into three parts: fuel, air, and throttle components.
The first thing we’ll focus on are the fuel components, which are the parts of the carburetor that are responsible for bringing in fuel. These include the fuel inlet, float bowl, the float assembly, and the fuel jet.
Fuel’s delivered via the fuel inlet into the float bowl.
- The float bowl is where fuel is stored and regulated by the float assembly.
- The float assembly is what assures only the right amount of fuel comes into your carburetor’s float bowl and subsequently out of your fuel jet. The float consists of two parts: the float and the float bracket.
- The hollow plastic part of the float assembly is commonly referred to as the float. This part is buoyant and sits in the fuel, floating as the fuel level in the float bowl reaches its filling point.
- As the float bowl fills, the float pushes against the float bracket. This bracket has a needle attached to it, and that needle sits inside of the fuel valve, slowing letting fuel in from the fuel inlet. Once the float bowl is full, the float will push against the bracket, which will press the needle into the inlet and block fuel from coming in altogether.
- When your float is damaged, you’ll notice your engine will flood. That’s because fuel is being dumped into the fuel jet unregulated, which your motor won’t be able to handle so it will clog up.
- Once the fuel is in the bowl and regulated, it will be drawn up to be mixed with air via the needle jet. That needle jet has its own fuel jet that regulates how much fuel is allowed to pass through the needle jet on its way to be mixed.
Airflow through your carburetor is vital to ensuring its function. It’s what draws in fuel from the float bowl and pushes it into the cylinder, so let’s take a look at how air makes that happen. This includes the air filter, the choke valve, and the Venturi pipe.
When you start your engine (usually by pedaling), the piston starts pulling in air. That air comes in through the air filter.
- In front of the filter is a choke valve, which restricts the amount of air that’s allowed into the carburetor. Choking the carburetor allows the cylinder to warm up to a temperature where it can combust pressurized air and vaporized gas. Once it’s warm enough to take in 100% of the air your carburetor receives, the choke is then opened.
- As air enters the carburetor it goes through the Venturi pipe, which is an hourglass bend that creates a vacuum to pull fuel in from the needle jet.
- Once the air pulls the fuel out of the jet, that air and fuel are sent through the intake manifold and on through to the cylinder.
In order for your engine to receive fuel and air, it needs to be told to do so. The components that do that are what make up the throttle assembly, so let’s see what it takes for fuel and air to get into your cylinder. There are essentially two components associated with the throttle assembly: the throttle valve, jet needle, and the idle adjustment screw.
The throttle valve has two important functions within your carburetor.
- The first thing it does is allow air to enter your cylinder. When you pull the throttle cable it lifts the valve up and in doing so opens the port attached to the intake manifold that goes to the cylinder. The slide has a slight curve to it, so at idle that curve allows a small bit of air to pass through .
- The second thing a throttle valve does is house and lift the jet needle. Whether the throttle’s being pulled or your engine’s idling, the needle valve is what holds the jet needle in place so it can let fuel into your motor when needed.
- The jet needle is held into place inside of the valve with a c-clip and a spring. When the throttle is pulled, the valve pulls the jet needle up to let the vacuum of air created in the Venturi pipe to bring in more fuel.
The jet needle is your main source of getting fuel into your cylinder. It’s a tapered needle that sits inside of the needle jet and allows fuel to enter the Venturi tube and go to the cylinder. At idle, the needle sits deep in the needle jet and lets just enough fuel to keep the motor running. When you throttle, the jet needle lifts out of the jet and lets in the amount of fuel you need to propel yourself forward.
A c-clip on the jet needle regulates how high it sits inside of the needle valve.
- The closer the c-clip sits toward the flat end of the needle, the deeper the needle will sit in the needle jet. This will allow less fuel to enter the Venturi tube that will be pushed into the cylinder when idling.
- The closer the c-clip sits to the pointed end of the needle, the further it will stick out from the needle jet. This will let more fuel to enter the Venturi tube when idling.
- Note: Idling is essentially any time the engine is not being throttled. Some think of idling as when the bike is not in motion, but whether it’s moving or not, it is idling if the throttle isn’t being actively pulled.
The c-clip is vital in ensuring the right amount of fuel and air enter your cylinder at the same time. When it’s on the wrong setting, the amount of air and fuel entering your engine won’t be sufficient to create the conditions needed to combust.
Idle Adjustment Screw
The idle screw is located on the outside of the carburetor and adjusts the RPM of your engine at an idle. When you adjust the RPM your engine idles at, you’re adjusting the amount of air and fuel it receives while idling.
By turning the screw, you adjust the height of the needle valve. This will help determine how much air’s allowed into the cylinder. The height of the needle valve also determines how much fuel is sucked into the Venturi tube at an idle.
Adjusting the amount of air and fuel with the idle screw is another way to help your motor get the right amount of fuel and pressurized air to combust and propel you forward.
- The higher the idle is raised, the less air and fuel are allowed into the cylinder.
- The lower the idle, the more air and fuel are given to your cylinder.
Now that you know about the different components of a carburetor, let’s talk about the types of motorized bike carburetors out there.
Types of Motorized Bike Carburetors
While there may be a lot of carburetors out on the market, there are three classifications all carbs fall into: standard, performance, and racing carburetors. Here we’ll explain each carb and the benefits they bring to your motorized bike.
As the name suggests, standard carburetors are the stock carburetors most 2-stroke bike engines and 4-stroke motorized bikes come with. These are meant to get your bike up and running, and usually don’t require any tuning.
Standard bike engine carburetors are much easier and more dependable to work with that higher performing carbs. The .70mm to .72mm fuel jets these come with are actually perfect for any engine, letting in plenty of air and fuel without bogging down the engine. Larger jets will let in too much air and fuel for your cylinder, and smaller jets wouldn’t let enough in. These 70mm-.72mm jets let in enough to get your bike started and running smoothly.
We will admit, though, that most standard carburetors have limited tuning capabilities. The NT carburetor, for example, can only adjust the idle screw and the c-clip on the jet needle to run with more or less air and fuel. Moreover, most 4-stroke carburetors are sealed other than their idle screw, so they don’t have options for much- if any- tuning.
This is important to know because if you want more performance out of an engine, you’ll want something you can swap the jets out on, or a carb that lets you fine tune more aspects than the choke and c-clip. Pushing your motor’s power with a standard carburetor will be difficult.
In times you need your engine to get up and running reliably, you’ll want a standard carburetor. By repairing or replacing your standard carburetor, you’ll be riding easy in no time.
Standard carburetors include the NT carburetors, the 38cc carb, and the 49cc 4-stroke carburetor.
Also known as high-performance carburetors, these types of carburetors are meant for high-performance bicycle engines. As you upgrade various parts of your engine — like the cylinder head and the exhaust — you’ll naturally need more air and fuel to run properly. A performance carburetor will make sure that happens, whether you need performance for the track or you need more power on rougher terrain.
These carburetors usually come with .78mm jets, which are on the larger end of the fuel jet spectrum. If you’re asking your motorized bicycle to work hard, it will need to burn a lot of fuel. To burn higher amounts of fuel, you’ll need plenty of fuel and air entering your cylinder, and without large carburetor jets, your air and fuel intake is limited.
With performance carburetors, you have the ability to dial in your performance to whatever you need it to be. Say, for example, you live in the mountains where you need much less air and fuel entering your carburetor to run. These carbs allow you to swap out the fuel jet for a different size to help your bike motor run at peak performance, and maybe even a little faster. Of course, you can also dial in your performance with idle adjustments and c-clip jet needle adjustments to get your performance just right.
Performance carbs can be a bit complex to tune, though. The 66/80cc High Performance Carburetor, for example, will need to be fine-tuned by re-jetting, adjusting the jet needle height (i.e., the c-clip), and adjusting your idle due to the amount of air and fuel it can handle. Compared to standard carburetors that you rarely need to tune, these will need some mechanical know-how to tune. However, once you do, you’ll feel your engine running stronger and even a little smoother depending on how well it’s tuned.
Racing carburetors are just that: carburetors designed to allow for high amounts of fuel and air in order to reach high speeds. They’re made to be tuned and adjusted far beyond the ability of a high-performance carburetor. That said, when tuned properly, these carbs will unlock the maximum potential of your bike engine’s speed. This is due to four key elements:
- The air filters and velocity stacks on racing carbs are at least 2 times the size of standard and performance carbs. This allows for the maximum amount of air to enter your cylinder.
- The .65mm to .68mm jets that come with them force your engine to run leaner (more air than fuel). Thankfully, with a leaner mix from medium-sized carb jets, your engine can stand more throttling than richer mixtures that would normally send too much fuel to sustain high speeds.
- The pilot jet (or the idle jet) helps your carburetor run more efficiently. With a pilot jet, racing carburetors are designed to allow fuel and air to enter your motor through a different compartment, putting less stress on your motor.
- Note: Standard and high-performance carbs give your motor small amounts of fuel and air by holding back your needle valve and jet needle and letting them pass through. This can cause some excessive wear on your engine over time, as you’re essentially always giving your motor some amount of throttle.
- The upgraded adjustments allow you to tune your carburetor in ways you’ve never been able to. This includes a more effective air/fuel screw, a whole different choke assembly for smoother choking, and a longer, more critically adjusting idle screw that raises the needle valve higher than ever.
With this type of carburetor, you’ll be forcing your motorized bike engine to receive tons of air and reduce the gas it receives, which, when regulated, will allow you to burn more fuel. Doing that moves your piston faster than before, resulting in higher speeds. However, to use most racing carburetors, your motorized bicycle engine will need a few extra parts.
Racing carburetors like the BBR Tuning Racing Series High Performance 2-Stroke OKO Style Carburetor can supercharge a 2-stroke bike engine, but you can’t just slap one on. Your engine requires extra parts to fit one of these carburetors onto it, including:
- A windowed piston.
- A reed valve.
- An oversized intake manifold.
- A specialized cylinder with an intake spacer for the oversized intake.
- A baffle-less exhaust system capable of expelling lots of exhaust.
Once your engine is set up with the parts and tuning it needs, these carburetors are capable of unlocking speeds of around 50 MPH and faster. That’s why it’s important to be mechanically inclined when you’re running a racing carburetor on your motorized bike: Reaching those speeds takes a good amount of work. If you’ve got the skills, though, you can smoke nearly any racer on the track.
Choosing Your Next Carburetor
When it comes to choosing a carburetor for your 2-stroke or 4-stroke motorized bike, it’s important to understand what you need out of it. As you can see, there are lots of components to keep in mind and power capabilities to choose from.
So how are you supposed to choose the right carb upgrade for your motorized bike? We like to think of it this way:
- Standard carburetors are perfect for riders that want their engines up and running smoothly. While they may not be able to unlock huge speeds, they’re more than capable of getting you where you need to go with ease. They require little to no tuning and can be used with most any high-performance parts, including high torque cylinder heads and torque-boosting exhaust systems.
- High performance carburetors are for the mechanically inclined who want some more power out of their bike engine. This is possible by fine-tuning these carburetors to work with the high-performance parts you may have. Even with stock components, these carburetors can give you an extra boost of top end speed and bottom end pull with a little bit of tuning.
- Racing carburetors are for the hardcore mechanics out there that want to supercharge their engines. With a super lean fuel mixture and all the tuning capabilities you can think of, these carburetors are made for mechanics that already have a highly tuned racing engine and need the carb to unlock its true potential as a racing machine.
What the Experts are Riding With
Still not sure what carburetor to go with?
Picking the best carburetor for your motorized bike can be a daunting experience. Even with all the knowledge you have about carburetors and what’s best for various bike engines, it can still be hard pulling the trigger on a carburetor for your motorized bike.
That’s why we asked expert 2-stroke and 4-stroke motorized bike riders what they’re running on their motorized bikes. We tested out their carbs and now present you with the 5 best carburetors for bicycle engines.
Best Upgrade for Stock 2-Stroke Carburetors: 2-Stroke Speed Carburetor
If you’re running a stock 2-stroke engine like the 66/80cc BBR Tuning Angle Fire Bicycle Engine Kit, upgrading to the 2-Stroke Speed Carburetor will pick the performance up on your engine. This carb will give you a boost of top end speed with a hint of bottom end torque.
This performance increase is due to the larger, interchangeable fuel jets. While the stock NT Carburetor has .70mm jets, the Speed Carburetor has .72mm jets, with the ability to change them out as needed. The .72mm jet is perfect for letting in more air and fuel into the cylinder without overloading it, leading to a lack of combustion. Compared to a stock engine that can go around 25 MPH, this carburetor can push your engine up to 30 MPH.
Better yet, this is the only carburetor on this list that won’t need extra parts or much tuning. Your throttle response will be a little faster and your top-end speed will slightly increase as soon as you slap the carburetor on your engine.
So when you need to get to where you’re going faster without pushing your motor to the max, the 2-Stroke Speed Carburetor will give you the power you want at a rate your motor can handle.
Our Price: $24.95 | Get This Carburetor Upgrade Here
Best Upgrade for Stock 4-Stroke Carburetors: NT Carburetor with BBR Tuning Billet 4-Stroke Intake Manifold
A 2-stroke carburetor for a 4-stroke engine can’t be worth it, can it? It is, actually!
You see, stock 49cc 4-stroke carburetors are significantly smaller than 2-stroke carbs despite the larger motors they need to power. When you add a stock 2-stroke NT Carburetor to a 4-stroke engine like the 49cc BBR Tuning 5G Pull Start Bicycle Engine Kit, you unlock a whole new level of airflow your 4-stroke has never felt.
This is going to improve your 4-stroke engine’s performance in 3 important ways:
- The Venturi pipe (and the body in general) is much larger than the stock 4-stroke carburetor. That means the NT Carburetor is naturally able to pull in more fuel and air for your engine to burn. When properly tuned, you’ll be able to utilize this extra air and fuel to your advantage for a slight increase in speed and better throttle response.
- The .70mm jets of the NT carburetor help give your 4-stroke more fuel than the stock 49cc carburetor jets. More fuel to burn means increased power, both top and bottom end.
- You’ll have the ability to tune the c-clip and idle screw to give your motor more or less air/fuel into your engine. This will give you way more control over how your motor runs compared to a stock 4-stroke engine’s carburetor.
Be aware that you can’t just throw an NT carburetor on a stock 4-stroke stock. Even then, all you need is a simple bolt-on adapter like the BBR Tuning Billet 4-Stroke Intake Manifold, and in seconds you’ll be able to hook up an NT Carburetor to your 4-stroke’s intake system.
With just a little bit of tuning, this carburetor will have your 4-stroke engine running smoother than before with a little more top end speed, too.
Best Carburetor for More Torque: 66/80cc High Performance Carburetor
Bogging up hills and having a hard time keeping your speed on rough terrain? Enter the 66/80cc High Performance Carburetor. With a larger body, bigger air filter, and larger jets than stock carbs, you’re going to be able to get plenty of bottom end torque with this carburetor.
What gives this carburetor its appeal is that it’s larger than standard carburetors in every way.
- A larger air filter allows more air into your carburetor than standard and performance carbs.
- Its larger float bowl and float assembly allows you to draw more fuel at an idle, to then be spent when throttling.
- Big .78mm fuel jets allow for more fuel to enter your cylinder.
- The bigger Venturi pipe and body allow for a stronger vacuum that will pull more air and fuel to be pushed into your cylinder.
Though this larger assembly is great at giving you more torque, it may need a bit of tuning to ensure you don’t bog your motor down. That can be as simple as swapping out the fuel jet and adjusting the c-clip on the jet needle, or it could be as complicated as adding more parts to your engine and dialing them in to work properly.
Note: If a High-Performance Carburetor is not tuned properly, it could flood your engine. That said, if you’re ready to do a little bit of tuning, swapping out jets, and really working with a carb, when this carburetor is dialed in there’s nothing it can’t do.
Our Price: $59.95 | Get This Carburetor Upgrade Here
Best Carb for Top End Speed: Cone CNS High Performance Carburetor
This classic performance carburetor brings more speed to the table than any other carburetor out there. That’s because compared to other carbs — from simple to performance — the Cone CNS High Performance Carburetor naturally runs lean (i.e., with as little fuel and air as possible).
To get this lean mixture of air and fuel, here’s what the CNS Carburetor offers:
- Its low-profile filter and small velocity stack allows for a limited amount of air to enter your motor, making sure your cylinder utilizes the little air it receives quickly.
- The .68mm jets allow less fuel to enter your cylinder when drawn in at a time, forcing your motor to run on less gas.
- The Venturi pipe is about the size of the NT, which allows for the perfect vacuum to draw air and fuel given the air and fuel components.
Now, this all may seem counterintuitive to a fast speed. It’s important to remember, though, that the faster you want an engine to go, the faster it will need to burn air and fuel. The faster it’s able to burn and process fuel, the more throttle you’re able to give your motor.
Provided your motor’s tuned up and ready to burn and expel lots of air and fuel, the CNS Carb will make sure your engine gets the right mixture to combust quickly.
Like any performance carburetor, this one has a couple of considerations to keep in mind:
- Tuning — When tuning these carburetor, you'll want to focus on the c-clip, idle screw, and an air/fuel screw (which may or may not be sealed depending on the production year).
- Manifold — The manifold ports on this carb are oversized, so you'll need a custom intake manifold or a spacer to ensure a proper seal around the manifold.
Thankfully, outside of some tuning and a new manifold, these carburetors are simple to use and just as effective on your engine. Even better, they have a choke cable installed so you don’t have to reach down to choke your engine every time you start. Add a bit of convenience and top end speed to your build with this carburetor.
Our Price: $59.95 | Get This Carburetor Upgrade Here
Best 2-Stroke Racing Carburetor: BBR Tuning Racing Series High Performance 2-Stroke OKO Style Carburetor
The BBR Tuning Racing Series High Performance 2-Stroke OKO Style Carburetor is one of the best, if not the best carburetor for racing engines. The size of these carbs allow for a lot of air and fuel to flow through, while the fuel management system keeps your engine running more efficiently than ever. This helps increase power on the track in terms of both speed and torque.
Like the High Performance Carburetor, this carb has a large float assembly, Venturi pipe, and air filter that allow for a maximum amount of airflow into your engine. This carburetor, however, offers a tall velocity stack that helps increase the amount of air that comes into your engine. It also helps smooth out the velocity at which it enters your engine, giving you better throttle response over other carbs.
Just like the CNS Carburetor, this carburetor also has premium jets made to work with little to no extra tuning. However, these jets give you a super lean mixture, as they come stock with .38mm jets — which are tunable, of course — made to allow the least amount of fuel and the most amount of air as possible.
The D-slide on this carburetor helps mitigate both fuel and air. With this simple slide, this carburetor runs with virtually no tuning, though there are c-clip, idle, and air/fuel adjustments to dial in your engine as needed.
You should note, though, that this carb does require a bit of reinforcement in terms of parts. Due to the massive amount of air and regulated fuel this carburetor can issue, it will require a few special parts to help run at peak performance:
- A reed valve (usually part of a much larger intake manifold). This smooths out the massive amount of air that’s given to your cylinder — otherwise it will bog down your engine.
- A cylinder with a modified intake system (preferably ported). The DIO reed valve is oversized and can't be installed on a cylinder as is. That’s why cylinders like the BBR Tuning Racing Series DIO Cylinder Body are modded with large blocks to help mount the reed. They also have a specialized intake port to feed your cylinder more efficiently than ever.
- A windowed piston. This component will allow more airflow throughout your engine as it passes into your cylinder.
When you’re ready to supercharge your engine, the BBR Tuning Racing Series High Performance 2-Stroke OKO Style Carburetor will give you all the fuel and air you need to rocket on the track. You’ll need a bit of mechanical know-how to work with this carb, but once installed and dialed in, it will give you the torque and speed you need to beat the competition.
Our Price: $59.95 | Get This Carburetor Upgrade Here
Tuning Your Motorized Bike Carburetor
Once you’ve chosen the carburetor that’s best for your motor, it’s time to start getting down to business and install it. Depending on the carburetor you choose, you may need to do a little tuning. From the choke down to the fuel jet, let’s go through some basic tuning procedures to get your new carburetor running.
The choke will limit the amount of air that enters your engine through the air filter. This will increase the temperature inside of your cylinder to reach a level that gas will vaporize and combust at. Once your engine is warmed up enough, you can give it more air until it’s eventually warm enough to combust on its own.
Ensuring your choke is properly managed is vital. Without warming up your motor properly, the air and fuel you push into your cylinder won’t be able to combust and will eventually stop your engine. We can’t tell you how many people get a brand new carburetor and forget to warm their engine up before riding.
Remember: Before riding, turn the choke on and let your idle even out. When your engine’s warmed up, you can open your choke all the way and the engine won't bog down. You’ll also notice that when you give your engine throttle, it will raise your RPM and come back down to a comfortable idling speed.
If you can get your engine to start but the idle won’t stay at a consistent rate when it’s idling (either too high or too low), then it’s time to adjust your fuel jets.
Fuel Jet Adjustment
Fuel jets ensure your engine stays running at peak performance by regulating the amount of fuel being pulled into the Venturi pipe. Too wide of a jet and you’ll give your motor too much fuel, both at idle and while throttliing. Too small and your motor will be starved for fuel while running.
Remember: Improper jetting will lead to engine failure, so it’s important to recognize when you need to swap them out.
Before you start, determine whether or not you can adjust the fuel jet. Most carburetors will let you, but some carburetors (like the NT Carburetor) don’t have an adjustable fuel jet.
Provided you can swap out your fuel jet, the first thing you’ll need to do is figure out what size you’re running. Most jets will be marked in the front with a 2-digit number, which is the millimeter (mm) diameter of the jet you’re currently running.
From there, you’ll want to pay close attention to your engine’s performance:
- If you backfire or bog down when opening up the throttle, you’re giving your engine too much fuel. When that’s the case, you’ll want to use jets that are smaller than what you’re currently running.
- If your engine can run with the throttle open but is lacking the overall power it should be getting, chances are you’re giving your motor too much air. In those instances, you’ll need to even that mixture out with more fuel and larger fuel jets.
As a quick example, let’s talk about the High Performance Carburetor, which comes with .78mm fuel jets. When most riders install this carburetor, they say that the carburetor either won’t give them the speed they want, or the motor will bog out once you start giving it throttle. These are signs that you’ll need to adjust the fuel jets:
- When a bike engine is running with this carburetor but it bogs out while throttling, that’s a sign that it’s getting too much fuel. In this case, you’ll want to lower the size of the fuel jet — usually, .71mm-.75mm fuel jets should do the trick.
- When a bike engine allows you to run but you lack overall power, it’s a sign you’re getting too much air. In this case, you’ll want to enlarge the size of your fuel jets — .80-.90mm and larger will get your motor running correctly.
Usually, swapping out your fuel jets will take care of the issues you’re experiencing. Even then, just because your engine’s running doesn’t mean it’s running at peak performance.
If you’re able to give your motor gas but it won’t idle right, it’s a sign that you’re letting a little too much or slightly not enough air in. To adjust that, you’ll want to adjust your idle via the idle screw.
When your engine’s warmed up and jetted but the idle’s just not where it needs to be, take a look at the side of your carburetor — here you’ll find the idle screw. As we mentioned earlier, this will raise and lower the needle valve to let more or less air into your cylinder at idle, which will adjust the rate of your idle.
You’ll want to adjust your idle with the choke wide open to ensure it’s running at max efficiency when adjusting.
- If your idle is running too high, that means your motor’s starving for air, so you’ll want to back your idle screw in to raise the valve.
- If, on the other hand, your idle runs too low, turning the screw out will lower the slide and restrict the amount of air coming in to help the cylinder reach the temperatures it needs.
If, however, you adjust your settings and it’s still not helping, it’s time to turn your attention to the c-clip on your jet needle.
Jet Needle Adjustment
The position of the c-clip on the notches of your jet needle will determine how far your jet needle sits inside of the needle jet. This controls the amount of fuel that’s allowed to mix with air while your engine idles — the higher it sits, the more fuel you’ll be giving your engine overall.
The higher you set the clip, the less fuel will be pulled into your cylinder at idle. The lower you set it, the more fuel will leave the float bowl.
- If you’re idling too high even with adjustments, lowering the c-clip will let you draw in more fuel to even out the air it’s receiving.
- If you’re idling too low, raising your c-clip will restrict the amount of fuel coming into your cylinder, evening out the gas and fuel going into your cylinder.
After this adjustment, your carburetor should be all dialed in and ready to ride. All you need to do is pay attention to your motor and use the adjustments outlined here to determine when and how to tune your carburetor.
The great thing about carburetors is that, with a little maintenance, not only will they last indefinitely, they’ll be interchangeable between different engines. Even better, maintenance is pretty simple when it comes to carburetors.
When your carburetor begins running rougher than before, you'll want to inspect the following components:
- Fuel and Air Jets — When jets are clogged, it makes it hard for your carburetor to take in and send air and fuel into your cylinder. This will result in poor engine performance, if not stopping the engine altogether.
- Use pressurized air to clear all jets of debris if your tuned carburetor is starting to run rough.
- Air Filters — Inspect the filters and ensure they’re intact. While you can run your engine without one, doing so will lead to much more debris entering your carb and, subsequently, your engine. If your air filter is torn or damaged, swap it out for a new one.
- The UNI Flex Carburetor Air Filter is a great alternative for those who want the maximum amount of air possible in their engine.
- The BBR Tuning Speed Carburetor Air Filter is perfect for replacing high performance filters or upgrading Speed Carb and NT Carb filters.
- Jet Valve and Needle — If your jet valve is damaged, it won’t be able to slide in and out properly, leaving you with too much or not enough fuel. The same goes for the jet needle — if it's bent, it won't sit in the needle jet correctly, which spits tons of fuel into your cylinder.
- If either the slide or jet needle are damaged, source these parts or replace them.
- Float Assembly — When your float is damaged, your motor can’t regulate the amount of fuel that enters the float bowl. Too much fuel and you’ll flood the engine. so if you’re finding that you’re flooding your engine (i.e., you can’t start it because there’s too much unburned fuel going to your cylinder), chances are the float assembly is damaged.
- Check and replace any damaged float components.
- After reinstalling your entire float assembly, make sure the springs and fuel needle are functioning properly. You can do this by pressing them against the float — if properly installed, they should spring back into place. This will ensure your float is doing its job and cutting off entry of fuel into your float bowl.