Have you ever wondered what the difference is between a constant velocity (CV) motorcycle carburetor and a slide carburetor on a motorcycle? I’ve recently replaced the CV carb on my Yamaha AG200 with a slide carb so that I can learn slow wheelies and boy, did it make a difference!
A constant velocity (CV) carburetor uses vacuum to lift the jet needle to suck fuel through the main jet. On a slide carburetor the throttle cable is directly attached to a slide valve that lifts the jet needle to regulate fuel flow depending on the position of the slide. CV carbs deliver smoother power while slide carbs offer better throttle response.
Both carburetors are made up of similar parts, including a float chamber, a main jet, a needle jet, and some type of slide valve. The float chamber holds a supply of fuel, which is then drawn into the engine through the main jet. The needle jet controls the amount of fuel flowing through the main jet. It is the mechanism that controls the movement of the slide (and therefore fuel) that separates the two types of carbs.
What is the Difference Between a CV Carb and a Slide Carb?
The main difference between a constant velocity (CV) carburetor and a slide carb is the way in which they control the flow of air and fuel into the engine.
In a slide carb (like the ones found on most dirt bikes), the throttle cable is directly connected to a slide valve. As the throttle cable is moved, the slide is lifted to allow more air and fuel to flow into the engine. The amount of fuel is controlled by the position of the needle jet and the size of the main jet.
In contrast, a CV carb works by using a vacuum signal to maintain a constant air velocity through the carburetor. This means that the CV carburetor automatically adjusts the amount of air and fuel entering the engine, according to the engine’s demand, without the need for a separate slide valve.
As the throttle is opened, the vacuum created by the engine is increased and the CV carburetor automatically compensates by allowing more air and fuel to flow into the engine. This results in smoother and more responsive acceleration than a slide carburetor.
Additionally, the CV carburetor is more fuel-efficient than a slide carburetor, as it adjusts the air and fuel mixture to provide the optimal fuel economy for the engine.
How Does a CV Carburettor Work?
A Constant Velocity (CV) carburetor is a type of carburetor that is commonly used on old road bikes (before fuel injection) and dual sport motorcycles. My old Kawasaki KLR 650, BMW F650 Funduro and my Honda XR650L all had CV carbs. It’s the job of the carb to mix air and fuel in the correct proportions (the stoichiometric air–fuel ratio which is about 14.7 to 1 – air to fuel – in weight) before it enters the engine.
Below is a link to an aftermarket CV carb compatible with my Honda XR650L:
A CV carburetor works by regulating the flow of air through a venturi, which is a narrow passage that causes the air to speed up as it passes through. As the air speeds up, it creates a low-pressure area that draws fuel up through a tube from the float bowl at the bottom of the carburetor. The fuel is then mixed with the air in the venturi and atomized into a fine mist that can be easily ignited in the engine’s combustion chamber.
The CV carburetor has three main components: the throttle (or butterfly) valve, the needle valve, and the jet needle. The throttle valve controls the amount of air that enters and flows through the carburetor, while the needle valve controls the amount of fuel that gets sucked into the throat of the carb.
When the rider twists the throttle, the throttle valve opens, allowing more air to enter the carburetor. As the air flows through the venturi, it draws fuel up through the needle valve and into the airstream. The jet needle then adjusts the amount of fuel that is mixed with the air by changing the size of the opening between the needle and the jet. This mixture is then sucked into the engine for combustion through the intake manifold.
The CV carburetor is designed to maintain a constant velocity of air flow through the venturi, hence its name. One of the advantages of the CV carburetor is its ability to provide a smooth, steady flow of fuel and air to the engine. This helps to improve the engine’s performance and fuel efficiency. CV carburetor is relatively easy to tune and adjust by re-jetting.
Check out my video on re-jetting my Honda XR650L:
A properly tuned CV carb will automatically adjust to changes in altitude (within the limits of the jet sizes), something that can’t be said of slide carbs.
How Does a Slide Carburettor Work?
A slide carburetor is a type of carburetor used on most dirt bikes. It is much simpler than a CV carb and arguably more reliable. The main difference on a slide carb is that the throttle cable is directly attached to a slide valve in the carb offering better throttle response and more control by the rider.
When the throttle is opened, the slide valve is lifted by the throttle cable, allowing more air to flow into the engine. As the throttle is pulled, the slide valve is lifted, exposing a small hole in the side of the carburetor, which allows fuel to flow into the engine. The amount of fuel that is allowed to flow is controlled by the needle jet, which is a small, tapered needle that moves up and down in the needle valve. The higher the needle jet is raised, the more fuel is allowed to flow.
The amount of air that is allowed to flow is also controlled by the size of the main jet, which can be adjusted by changing the size of the main jet.
The slide carburetor is a simple and effective device that has been used on mostly dirt bikes for many years. It is relatively easy to maintain and repair, and is a popular choice among dirt bikers. However, it does have its limitations, particularly in terms of fuel efficiency and emissions control. Modern dirt bikes now often use fuel injection systems, which offer greater precision and control over the amount of fuel and air delivered to the engine.
Three advantages of a CV carburettor over a slide carburettor
Improved fuel efficiency
One of the primary advantages of a CV carburetor over a slide carburetor is improved fuel efficiency. The constant velocity design of the CV carburetor allows for a more precise mixture of air and fuel, which can result in better fuel economy.
The precise mixture of air and fuel in a CV carburetor also helps to reduce emissions. By delivering the correct amount of fuel to the engine at all times, a CV carburetor can help to minimize the amount of unburned fuel that is released into the environment.
Another advantage of a CV carburetor is smoother acceleration. Because the CV carburetor maintains a constant velocity of air flow, the mixture of air and fuel delivered to the engine is more consistent, which can result in smoother and more predictable acceleration. In contrast, a slide carburetor can be more difficult to tune, which can result in erratic performance and less predictable acceleration.
Three advantages of a slide carb over a CV carb
Better throttle response
Slide carburetors typically offer better throttle response than CV carburetors. This is because the slide is directly connected to the throttle cable, providing a more direct and immediate response to changes in throttle input.
Slide carburetors have a simpler design than CV carburetors, which makes them easier to service and repair. They also have fewer moving parts, which can make them more reliable and less prone to failure
In some cases, a properly tuned slide carburetor can provide more power than a CV carburetor. This is because slide carburetors typically allow for more air to flow into the engine, which can result in greater power output. However, it’s important to note that this will depend on a variety of factors, including the specific engine and carburetor design, as well as the tuning and maintenance of the carburetor.
Less chance of flooding
I learned this while trying to slow wheelie my Yamaha AG200. Every time I got the bike up at the balancing point, it would flood and die. That’s why dirt bikes all run slide carbs. To learn why your motorcycle keeps flooding, check out this post I wrote.
Carburetors are designed to mix air and fuel in the correct ratios. Both CV carbs and slide carbs do this by drawing fuel from the float bowl into the air stream in the carb throat. The main difference is the way in which the slide valve works.
In a CV carb, the slide is moved by vacuum created in a venturi. The airflow through the venturi is controlled by a throttle valve attached to the throttle cable. The link between the throttle and the slide is not direct. In a slide carb, the throttle is directly attached to the slide valve.
If you intend on riding mostly on the road or at varying altitudes, a CV carb is the way to go. If, on the other hand, you’re racing or riding hard enduro off road, go for a slide carb.