HOW DO MOTORCYCLE CARBURETORS WORK? WHAT EVERY TECHNICIAN NEEDS TO KNOW

(Last Updated On: September 26, 2022)

Despite new motorcycles being almost exclusively fuel injected, countless carbureted motorcycles (as well as dirt bikes, ATVs and more) are still in use today. For many riders and new motorcycle technicians, carburetors are a mystery—however, once we take a deeper look, we learn that they are incredibly simple devices.

Keep reading to learn all about the often-misunderstood device located between the air box and the cylinder head.

What Is A Motorcycle Carburetor?

To gain a better understanding, let’s discuss what exactly a motorcycle carburetor is. In the simplest of terms, a motorcycle ‘carb,’ or series of multiple carburetors, has the job of creating a combustible mix of gasoline and air that will power the engine and create the driving force to propel the vehicle forward. Carburetors have been updated, modernized, and improved over the years; however, the basic premise of mixing fuel and air has not changed.

How Does A Carburetor Work?

So, what does a carburetor do? How does a motorcycle carburetor work? First, we should cover the basics:

  • A carburetor’s job is to supply an internal combustion engine with air/fuel mixture
  • Carburetors regulate the flow of air through their Main bore (Venturi), this flowing air draws in fuel and the mixture enters the engine via the intake valve
  • The Venturi Principle/Effect (discussed below) is critical to successful operation
  • Carburetors consist of a bowl, center bore, passages, jets, vents, a slide, enrichener (aka Choke), idle speed adjustment, air/fuel ratio adjustment and often an accelerator pump

Motorcycle Carburetor Advantages

  • Simple, easy for DIY repairs
  • Do not typically require computer/software for adjustment, making them valuable in racing applications
  • Do not require pressurized fuel lines to operate

Motorcycle Carburetor Challenges

  • Less efficient than fuel injection, especially when the engine is cold or on deceleration
  • Can be finicky after sitting unused for a length of time
  • Multiple carburetor setups on vehicles with multiple cylinders need to be properly synced with one another to work correctly

Now that we’ve covered the basics, let’s take a deeper look into how a motorcycle carburetor works.

Surprisingly, twisting the throttle on a motorcycle handlebar does not directly control how much fuel enters the engine. The throttle controls how much air is allowed to flow through the carburetor, and the flow of air is what controls how much fuel is also fed into the engine.

The Venturi Principle/Effect states that when air flows through a restriction of a tube (narrowing), the air speed increases, and air pressure decreases. This drop in air pressure draws fuel up and out of the carburetor bowl through a series of precisely machined jets and passages, and into the cylinder head.

What causes air to flow through the carburetor and into the engine?

As a piston travels down the cylinder bore of an internal combustion engine, it creates negative air pressure. This negative air pressure, along with an open intake valve, draws air through the carburetor and into the combustion chamber.

As the rider opens the throttle, a butterfly valve or slide allows more air to rush in, and that traveling air picks up a specific amount of fuel from the carburetor bowl. A series of passages, vents and jets regulate the volume of fuel, depending on the engine needs.

Mechanical Slide vs. Constant Velocity Carburetors

Carburetors fall into two categories: mechanical slide or constant velocity (CV). In a mechanical slide carburetor, the rider’s throttle input directly raises or lowers the carburetor slide, which regulates how much air flows through the bore and into the engine. Mechanical slide carbs are popular in racing, off road and other performance applications. For maximum performance, accelerator pumps are often built into mechanical slide carburetors to deliver the fuel needed at any given time.

Constant velocity carburetors allow for better performance at various altitudes and offer smoother throttle response across all ranges of throttle. CV carbs do not have a physical connection between the slide and the handlebar mounted throttle. Instead, the throttle opens a butterfly style valve in the throat of the carburetor, and vacuum pressure will lift the slide as much as needed.

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