Motorized Bicycle Carburetor Buyer's Guide
Finding the right carburetor for your motorized bike 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 motorized bicycle with a 2-stroke engine or 4-stroke engine. 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.
Here we’ll detail everything you need to know about bike engine carburetors to find the best one for you. Regardless of 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!
But before we get ahead of ourselves, let’s dive into what 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 carburetor, there would be no way for an engine to receive the chemicals it needs to create the combustion it needs in order 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 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 your 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 create 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-strokes compared to 2-strokes, 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 a lot of moving parts in them, and each part 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 stress on your 4-stroke or 2-stroke motorized bike engine.
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, float assembly, and fuel jet.
Fuel is 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 ensures 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, slowly 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, resulting in it becoming clogged 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, choke valve, and 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 percent 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 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. There are essentially three components associated with the throttle assembly: the throttle valve, jet needle, and 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 — in doing so, it 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 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 is 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 that idle raises, 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 Carburetors
While there may be tons of carburetors out on the market, there are three classifications all carbs fall into: standard, performance, and racing carburetors. Here we’ll explain each class and the benefits each type of carb brings to your motorized bike.
As the name suggests, standard carburetors are the stock carburetors most 2-stroke bike engines and 4-stroke bike engines 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 to work with than 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 let in more air and fuel for your cylinder, and smaller jets let in much less. These .70mm to .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 carburetor, 38cc 4-stroke carburetor, and 49cc 4-stroke carburetor.
Also known as “high performance carburetors,” these types of carburetors are meant for high performance bike engines. As you upgrade various parts of your engine — like the cylinder head and the exhaust, for example — 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 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 want your bike to work hard, it will need to burn a lot of fuel. To burn lots of fuel, you’ll need a good amount of fuel and air entering your cylinder — and without large 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 rejetting, 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 rarely need tuning, these will need some mechanical know-how to tune. However, after successfully tuning a high performance carb, you'll feel your engine running stronger and even a little smoother depending on how well it’s tuned.
Racing carburetors are exactly what you'd expect based on their name: 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, your engine can stand more throttling than richer mixtures, which 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 undue 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 motor to receive large amounts 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 normal, resulting in higher speeds.
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 (like the Dual Window Piston #1 or Dual Window Piston #3).
A reed valve.
A specialized cylinder with an intake spacer for the oversized intake.
A baffle-less exhaust system capable of expelling large amounts 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 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 a lot of components to keep in mind and power capabilities to choose from.
So how are you supposed to choose the right one for your 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 highe speeds, they’re 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 heads and expansion chambers.
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 who 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 a 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 so we could present you with the 5 best carburetors for motorized 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 Silver Angle Fire Bicycle Engine Kit and you want a boost in performance, the 2-Stroke Speed Carburetor is what you’ll want to upgrade to. Compared to a stock engine that can go around 25 MPH, this carburetor can push your engine up to 30 MPH.
This 2-stroke carb will give you an increase in top end speed with a bump in bottom end torque. This is due to the larger, interchangeable fuel jets. While the stock NT Carburetor has .70mm jets, the 2-Stoke 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.
Better yet, this is the only carburetor on this list that won’t need extra parts or much tuning to operate. Your throttle response will be a little faster and your top end speed will slightly increase as soon as you install 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 Speed Carburetor will give you the power you want at a rate your motor can handle.
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 4-stroke carburetors are significantly smaller than 2-stroke carbs despite the larger motors they need power. When you add a stock 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 three 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 engine 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 to your engine. This will give you much more control over how your motor runs compared to your stock 4-stroke bike engine’s carburetor.
As you can probably tell, you can’t just throw an NT Carburetor on a stock 4-stroke stock. Thankfully, 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 carburetors, 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 all ways:
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 adding more parts to your engine and dialing them in to work properly. This is why a lot of people don’t like this type of carb: If not tuned properly, it can tend to 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, there's nothing this carb can't do when it's dialed in.
Best Carb for Top End Speed: Cone CNS High Performance Carburetor
The Cone CNS High Performance Carburetor brings more speed to the table than any other carburetor out there. That’s because compared to other carburetors — from simple to performance — the CNS Carb 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 allow 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 Carb, which allows for the perfect vacuum to draw air and fuel.
Now, if this all seems counterintuitive to a fast speed, it’s important to remember 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. And provided your motor’s tuned up and ready to burn and expel lots of air and fuel, the CNS Carburetor 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. First, you’ll be focusing on tuning this carburetor's c-clip, idle screw, and air/fuel screw (which may or may not be sealed depending on the production year). Second, their manifold ports are oversized, so you'll need a custom intake manifold or a spacer to ensure a proper seal around the manifold.
Thankfully, though, outside of some tuning and a new manifold, this carburetor is simple to use and super effective on your engine. Even better, it has 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.
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 2-stroke racing engines. The size of this carburetor allows for a lot of air and fuel to flow through, while the fuel management system keeps your engine running more efficiently than ever. The improved and more efficient form of fueling will help increase speed on the track in terms of speed and torque.
Much like the 66/80cc 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 carb, however, also 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 compared to other carbs.
Just like the CNS Carburetor, this carb 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 (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 the two. 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.
What it does require, though, is a bit of reinforcement in the parts division. 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 won’t fit onto 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 will help 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 necessary to smoke the competition.
Tuning Your 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. Doing so will increase the temperature inside of your cylinder to reach a temperature 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 doesn’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 are vital in ensuring your engine stays running and runs 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 throttling. Too small and your motor will be starved for fuel while running. Improper jetting will lead to engine failure, so it’s important to recognize when you need to swap your carb's jets 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 the ability to adjust the 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 diameter in millimeters (mm) 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. If 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. If that’s the case, you’ll need to even that mixture out with more fuel and larger fuel jets.
As a quick example, let’s talk about the 66/80cc High Performance Carburetor, which comes with .78mm fuel jets. When most riders install this carburetor, it might not give them the speed they want, or the motor will bog out once the rider starts 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 when giving throttle, that’s a sign it’s getting too much fuel. In this case, you’ll want to lower the size of the fuel jet, usually .71mm to .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 from .80mm to .90mm and larger to get your motor running correctly.
Usually swapping out your fuel jets will take care of the issues you’re experiencing, but 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 in a little too much or slightly not enough air. 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 to the side of your carburetor and 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, here’s a quick rundown of components to inspect:
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 filter and ensure it's intact. While you can run your engine without one, it 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 and NT Carb filters.
Jet Valve and Jet 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 notice that your engine is flooding — 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.
When this happens, check and replace any damaged float parts.
After reinstalling your entire float assembly, make sure the springs and fuel needle are functioning properly by pressing them against the float — when you do this, they should spring back into place. This will ensure your float is doing its job properly and cutting off entry of fuel into your float bowl.
Frequently Asked Questions
What sort of carburetor should I use on my motorized bike?
This is a great question because at the end of the day, all we want to do is making sure our engines are running the way we want them to. One of the key ways we do that is by choosing the right carburetor. So to find which carburetor your motor needs, it's important to ask yourself two important questions:
- What performance do I want out of my engine? If you want your engine up and running smoothly, then you'll want a standard carburetor. On the other hand, if you want more power out of your engine, you're going to want a performance or racing carburetor.
- How much work am I willing to put in with a new carburetor? All carburetors will need some amount of tuning eventually, but some will need more intense tuning than others. Standard carbs, for example, are designed to only need slight adjustments to their jet needles and idle screws for proper tuning. High performance and racing carbs, on the other hand, will require not only more intense tuning like jet replacements and adjustments, but they may also require additional parts like upgrading your exhaust system.
That said, if you're looking for a carburetor to get your engine running so you can ride around town, a standard carburetor will work perfectly. When you're looking for more power and are willing to put in the work (and cost) to dial in your engine's performance, a high performance or racing carburetor is what you need.
How will I know when to tune my carburetor?
When a carburetor is in need of tuning, your engine will either run too high, too low, or not at all. That's why most riders will tell you that if your engine doesn't "feel" right and your spark and compression are good, then you've got to check your carburetor.
Here are some common symptoms of carburetors in need of tuning:
- Your engine starts, but it stops running as soon as you give it gas.
- Your engine won't start despite your compression, fuel, and spark are in order.
- Your engine starts and runs, but it eventually gets sluggish and stops.
- Your engine starts and runs, but it starts to rev really high with the choke off, leading to overheating.
- Your engine is constantly flooding and/or fuel leaking
- Note: If your only issue is flooding, first make sure that your float and float assembly are in proper working order. Damaged floats will cause flooding often.
These all essentially have to do with your engine getting too much or not enough fuel and air. So if your engine's running but not quite running the way it should, there's a good chance it's in need of a tuning.
How do I tune my carburetor?
Once you're ready to start tuning your carburetor, start by diagnosing the issue you're having. No two carburetor issues are fixed the same way, so it's important to know what to do when the time comes.
Now, before you dive into tuning, it's important to know whether that's possible, and to what extent. For example, 4-stroke carburetors have virtually no adjustments that can be made to them. On the other hand, 2-strokes range in their abilities from standard carbs with only c-clip adjustments to racing carburetors with the ability to adjust everything.
Provided your carburetor's tune-able, here's a few troubleshooting measures to get your motor running properly:
- Engine idling too high? This is a sign your engine's starving for air and fuel. The first thing you'll want to do is adjust your idle screw out so it lets more air in. If your engine's still idling high after you've turned the idle screw out, it's a sign that your jet is too small and will need to be swapped out for a larger one.
- Engine idling too low? This is a sign your engine's getting too much fuel and air. Turn the idle screw in to limit the amount of air you're letting in. If that doesn't work, adjust the c-clip on your jet needle to ensure you're getting the correct amount of fuel mixing with your air. If neither of those work, there's a good chance it's time to swap out your fuel jet for a smaller one.
- Engine not idling at all? Provided you have no fuel or air leaks around your carburetor, first ensure you've choked your engine properly, as improper choking will let an engine start but not idle. If your engine's choked properly, then it's likely you're working with the wrong jets. Swap out your jets until your engine idles, and once it runs begin adjusting the idle screw and c-clip.
- Engine not starting? If your engine's electrical components are in order, you're getting fuel, and you have compression but your engine's not starting, it's likely a fuel jet issue. Just like before, swap out your jets until your engine starts, then listen to it idle. Adjust the idle screw and c-clip until your engine is able to idle on its own without the choke.
How can I upgrade my carburetor for better performance?
There are lots of parts and tuning to be done on the majority of carburetors. Whether you want your engine to run better, you want more torque, or you need more top end speed, you can bet there's a way to get the most (if not more) out of your carb. It's just a matter of finding the right tuning or part for the job.
Now before you go buying a new carburetor or a bunch of parts, take stock of your carburetor and its performance.
- How is your engine running?
- If it's running well but can run better, chances are a few parts will give you the parts you want.
- If your motor's running rough but otherwise fine, you'll need to tune it.
- When you can hardly run but you've tuned your carb and even added new parts, you're need of a new carb.
- Have you already tuned your carburetor? Moreover, can your carburetor be tuned?
- If you haven't tuned your carburetor and your engine's running funny, do that before attempting any upgrades.
- If you have and it's not working- or if you're unable to tune it- you'll want to exchange it for another.
Once you've assessed whether you need to tune your carb, upgrade a few of the parts, or swap it out, it's time to get down to work.
Now, if you want to increase the amount of air flow into your carburetor, you're going to want an air filter and a velocity stack.
- A larger air filter will allow more air to enter your carburetor, giving you the ability to increase your engine's torque and top end speed.
- A velocity stack helps direct all of the massive air it receives from a large (or no) air filter into the carburetor for smooth processing.
If you choose to upgrade your carburetor's parts, there's a good chance you're going to need to swap out the fuel jets to assure your engine's running right. That's because you're going to be introducing much more air into your engine.
That means the amount of air that enters your carburetor and engine needs to be matched with a particular amount of fuel. That amount of fuel is regulated by the fuel jets, and we'll explain which ones to get.
When your engine's running but it's feels off, it's time to tune. Click here for a full rundown on how to tune your carburetor.
Keep in mind, though, that not all carburetors can be tuned, so if you have one that can't be tuned, you'll want to think about upgrading your carburetor altogether.
- 49cc 4-strokes, for example, can't be tuned. However, you can use a 4-stroke manifold and a 2-stroke carburetor so you can tune to your specific riding conditions.
- When your 2-stroke's NT or Speed carb isn't giving you the power you want, you'll want something with a larger Venturi tube, like a racing or high performance carburetor.
What fuel jets should I use?
Finding the right fuel jets for a carburetor that just won't run right is a surprisingly easy task. Despite there being at least 30 jet sizes (.65mm - .95mm) The key is to listen to the performance of your motor. How it runs will let you know what size jets to work with.
The first thing you'll want to do is find out what jets you're currently running.
- Most standard and Speed carburetors utilize .70mm jets
- Most high performance or racing carburetors use .75mm - .8mm jets
Once you know what jets you're using, it'll be easy to listen to your engine and figure out what you need to do:
- If your engine's running too rough and bogging down, you're giving your engine too much air and fuel. You'll want to exchange your stock jet with a smaller jet size.
- If your engine's running too high and sounds like it's going to overheat, you're starving your engine of air and fuel. You'll want to exchange your stock jet with a smaller one.
Depending on the performance you're looking for, we suggest getting at least 5-10 jets higher or lower than your stock jet. While it's an easy process to exchange the jet itself, fine tuning to the exact jet size you need can take more than a few tries.