What are carburetors and how do they work?
The carburetor is the device that delivers fuel to your engine for combustion. By design it turns the fuel into an atomize cloud, and mixes it with incoming air that is being drawn into your engine via engine vacuum. Carburetors are engineered for the size of engine they are attached to, with consideration for the amount of fuel that will be required across the entire rpm range of the engine.
There are a few types of carburetors in the motorcycle world. One of the most common ones is the CV (constant velocity). These are easily identifiable because of the large flat cap on the top of the carburetor. These are found on most Japanese motorcycles from the mid-70s on up, and on Harley's from the mid to late 80s, up until fuel injection took over.
The next most common is the mechanical cable pull carburetor and the fully mechanical carburetor. Mechanical cable pull carburetors are found on most imported post war motorcycles until the mid-70s. They are still used on dirt and small displacement single carburetor applications to this day. Fully mechanical carburetors are generally operated by a series of linkages and levers that make all the functions happen. They were used on early 70's inline four cylinders.
Butterfly carburetors are generally found on Harleys. Stock Harley carburetors from the earliest days, until the 80s are considered butterfly carbs, and the S&S Cycle Super E&G carburetors are butterfly style carburetors. This is because the primary mechanical operation of the carburetor is based on the butterfly inside the carburetor throat.
Before we get into the details of how it all works, let’s take a look at the parts. The carburetor is made up of a series of parts that regulate the amount of air and fuel that enters your engine. Those parts are:
Throat: The large opening that runs through the middle of the carburetor. This is where the engine vacuum draws fuel from the float bowl and mixes it with the incoming air. The airflow that is allowed to go through the carburetor is generally regulated by the throttle plate, aka the butterfly. This is what physically opens and closes when you twist the throttle. More grip twist, the further the butterfly opens, more air through the carburetor, and the more fuel goes through the jets into the engine. On mechanical style carburetors, the throttle cable pulls a slide up and down to regulate the amount of air that goes through the throat of the carburetor.
Float and bowl: The bowl is where the fuel is stored after it comes from the fuel tank. From there the jets and passages pick the fuel up and distribute it inside the carburetor at the appropriate time. As the fuel level in the bowl goes down, there is a float that goes down with it. When the float goes down, it opens a port that allows more fuel in. Then when the bowl fills, the float rises, and cuts off the incoming fuel. It works exactly like the tank on your toilet.
Jets: A Carburetor has small brass fittings that are called jets. The jets are identified by their size and they are selected based on the amount of air your engine will draw. These generally do no wear out, and unless you have made changes to your motor such as exhaust, intake, or other performance modifications, you should not have to change your jets.
So where does that engine vacuum come from? When your engine turns over, the intake valve opens, and the piston moves down inside the cylinder. This creates a vacuum that needs to be filled by air. Atmospheric air pressure rushes through the carburetor, goes down the intake, past the intake valve, and into the cylinder. The intake closes, the piston goes up, and compresses the air. Afterwards the spark plug ignites the fuel & air mix, and then combustion happens. The flow of this air creates lower pressure condition inside the throat of the carburetor. So when you have that low pressure condition inside the throat of the carb, the atmospheric air pressure inside the float bowl (which is greater than the air pressure in the throat) pushes fuel up through the jet, and out the port into the carburetor throat. This fuel mixes with the incoming air, and then is drawn into the engine for combustion. This is called the venturi effect. It is important to remember that the fuel is actually pushed by the atmospheric air pressure from the float bowl vent. If this vent is plugged, the engine will not get any fuel. The location of the ports in the carb throat, and the size of the jet, dictate when and how much fuel is drawn into the motor.
At idle, your engine runs off the smallest jet and fuel circuit. As your rpms increase, you will pull a larger amount of air, and you will need a much larger jet to feed the air with enough fuel. This is accomplished by the intermediate and main jets. The slower speed ports are closest to the throttle plate, and the higher speed (main) ports are further away. This prevents the larger fuel circuits from coming on until adequate air flow happens, and that won’t happen until higher rpms, and harder throttle.
If you look inside your carb throat you will see between one and three tiny ports right at the throttle plate. These are your idle ports. They are barely exposed when the throttle plate is closed because your engine only requires a very small amount of fuel at idle. Coincidentally, that means these ports are always the first ones to be clogged by dirt. So, nine times out of ten, if your bike has been sitting, and it won’t idle, it is because these ports are clogged.
After idle, you will begin to twist your wrist, and open the throttle. As the throttle plate opens, more air rushes in to fill the vacuum. This makes the engine rpm increase. Each fuel circuit has a certain capability that ranges from a few hundred rpms to a few thousand rpms. Once that range runs out, another fuel circuit will come into play. It is not uncommon for a carburetor to have multiple intermediate fuel circuits of different sizes. This is especially true on carbureted sport bikes, that have a redline well above 7000 rpms. (some sport bikes go as high at 16,000 rpms). Eventually the intermediate fuel circuit will be maxed out and the main circuit will come into play. Your main circuit is fed by your largest jet.
On CV carburetors, your main jet circuit doubles for the multiple intermediate jets, and works as a variable jet circuit. On your CV carburetor you will see a large cap on the top of the carburetor. Underneath this cap is a piston or rubber diaphragm and this piston is held down by a spring. On the bottom of the piston there is a slide, and on the bottom of that slide there is a needle attached that needle goes down into the main jet. When the bike is off, the needle either partially or fully closes off the main jet. This restricts the amount of fuel that is drawn through the jet by engine vacuum. There is a port inside your carburetor throat that runs from the engine side of the carb, to the top of the piston. As you twist the grip and open the throttle, the butterfly opens and allows more vacuum to pull air through your carburetor throat. As the vacuum level increases, the air pressure on the top of the piston will decrease. The air pressure inside the carburetor throat will push the piston up by overpowering the spring. As the piston/slide goes up, the needle lifts up out of the main jet and allows more fuel to be drawn into the throat of the carburetor, mixed with the air, and drawn into the engine. The sizes and spring tension are all engineered to work together, so that at full throttle the needle is all the way out of the jet, and your engine it is getting a maximum amount of fuel that it needs. When you close the throttle, the vacuum inside the carburetor drops, and the spring pushes the slide or piston back down. This pushes the needle back into the main jet, cutting off the amount the fuel being delivered to the engine.
On mechanical, cable pull carburetors, the throttle cable physically pulls the slide up, this allows air to go into the engine, and lifts the needle out of the main jet at the same time. So, it is the same principal as the CV carburetor, but instead of a vacuum doing the work, the throttle cable is physically attached to the slide and needle. Mechanical cable pull carburetors are the simplest design as an individual carb, but the CV carbs work well when you are using multiple carburetors
The next part to cover is the accelerator pump, and these are generally found on the Harley carburetors. This gives a squirt of fuel into your engine as you move the throttle open, but this only happens when the throttle is being opened. This is required because of the length of the intake and the volume of air going through the single carburetor. They are needed because when you initially open the throttle, the air will rush in to fill the vacuum, and with a longer intake, it will take a few nano seconds for the venturi to happen and the fuel to catch up to the air in the combustion chamber In that brief time the engine will run lean. This means that there will not be enough fuel in the mix coming into the engine. To compensate for this, the squirt of the accelerator pump adds a small amount of fuel to help make up for that brief lean condition. On most of the metric (Japanese) bikes, the intake length is so short that the accelerator pump is not always required. Plus, when running multiple carburetors, there is a smaller amount of air being pulled through each carburetor throat compared to one larger carburetor. That smaller volume of air is less prone to have that initial lean condition when opening the throttle.
Also, if you have a kick start motorcycle, the accelerator pump is a handy feature, because with a single twist of the throttle, your accelerator pump will squirt a small amount of fuel into the intake, and give your engine something to pop to life on, hopefully with the first or second kick.
One of the most misunderstood parts of the carb is the idle circuit or idle bleed circuit. Some carburetors have an idle circuit where there is screw that opens and closes a fuel passage. The further you open the screw, the more fuel the engine gets. But on the majority of metric carburetors there is an Idle Bleed Circuit that regulates the amount of fuel that goes into your engine at idle. This “bleed” is like a small hole in the side of a drinking straw. The fuel circuit is over sized for what the engine needs, and then the bleed is what controls that mount of fuel that actually goes into the engine. As you open the screw, you are making the bleed (hole in the side of the straw) larger. This means less fuel is drawn into the engine. As you turn the screw in, you will close off that bleed, and the engine vacuum will draw more fuel into the engine. It is a little counter intuitive, but it is done this way because the bleed allows for more of a fine-tuned adjustment, and once they are set, it is much more forgiving to temperature changes and altitude changes.
The final part of the carburetor to discuss is the choke. A carburetor is tuned for a motor that is operating at full temperature, which is somewhere between 220 and 350 degrees. When air enters the intake of a full temp motor, it expands and thins out. The amount of fuel that is being drawn into the air is based on the density of this “hot air”. When a motor is cold, the air that comes into the intake is cool and dense. So, there is too much air entering the motor for the amount of fuel the carb has released. This creates either a learn burning condition or a situation where the mix is so lean, that the engine will not run. To compensate for this, carburetors have a “choke”. There are two common types of chokes, Air Restrictive and Enrichment.
Air restrictive chokes are simply a second butterfly, at the entrance to the carburetor that closes and restricts the volume of air going into the intake. As the engine warms, you open this restrictive plate by “turning off” or “opening” the choke.
Enrichment. This is the more common design. When you activate the enrichment choke, you are opening an auxiliary fuel port that allows for additional fuel to flow into the motor. This is done to make up for the volume of dense cold air entering the combustion chamber during initial startup. This is really a better design, because it allows you to lightly operate the motorcycle while the engine is still warming up. This means you don't have to wait for five minutes for the bike to get warm enough to ride, and your neighbors don't have to listen to your bike idle in the morning while it warms up.
It is important to remember that the majority of your carb functions are based on engine vacuum and atmospheric pressure. This reiterates the importance of your carb being clean and having good seals on everything.
Hopefully this has helped to demystify the carburetor a little, allow you to identify your type of carburetor, and give a better understanding of what is going on.
Words and photos by Steve Knoble