All About Motorcycle Carburetors and Fuel Injection

Motorcycle Carburetors and Fuel Injection FAQs

What is a motorcycle carburetor?

A carburetor is a device used to meter fuel and air to any internal combustion engine. When used on a motorcycle, the carburetor is controlled by the rider through the twist-grip throttle, which is connected to the carburetor with a sliding cable. It is not uncommon for a multi-cylinder motorcycle engine to have a carburetor for each cylinder.

The carburetor was developed in the late 19^th century for the first successful internal combustion engines. The earliest carburetor devices were surface carburetors which functioned by moving a stream of air over an open vessel of fuel. Later designs employed a float bowl to hold a reservoir of fuel, which was metered into the flow of air as it entered the engine, a design that would be used on the earliest motorcycles and automobiles and was essentially unchanged for 80 years. It has been said that in 1902 marine engine inventor Ole Evinrude, a friend of William Harley and Arthur Davidson, helped the pair perfect the carburetor for their first motorcycle prototype.

The carburetor remained the most-common fuel metering device on motorcycle engines until the 1980s, when fuel injection began to replace the carburetor on bikes and automobiles, in part to improve performance but also to reduce exhaust emissions.

What is the function of a motorcycle carburetor?

The carburetor is located upstream of the engine’s intake port. On its intake stroke the engine draws air through the carburetor. The carburetor adds a very fine mist of fuel to the airflow, which is then drawn through the intake port into the engine combustion chamber.

The carburetor works on Bernoulli's principle, which states that the static pressure of the intake air reduces as its speed increases. A venturi tube in the throat of the carburetor is connected to the fuel reservoir in the float bowl. As air speed through the carburetor throat increases with engine speed, the pressure in the throat decreases. This draws fuel through the venturi tube and into the stream of air. When a motorcycle rider twists the throttle, they are opening a valve – either a flapper or a slide – that permits more air, and thus more fuel, to enter the engine and power increases. When the rider closes the throttle, less air and fuel can pass through the carburetor and the engine power decreases. Using the throttle, the rider is controlling the flow of air into the engine, and the carburetor reacts accordingly by adding the proper amount of fuel.

Do Harleys still have carburetors?

In 1995 Harley-Davidson offered electronic fuel injection (EFI) for the first time as an option on the Electra Glide® Ultra Classic® model, replacing the carburetor on its V2 Evolution® engine. EFI was soon offered as an option or standard equipment on more models. In 2007 EFI became standard equipment on all Harley-Davidson® models, and carburetors were discontinued.

What is motorcycle fuel injection?

A fuel injection system actively introduces fuel to an internal combustion engine. Modern fuel injection systems are controlled electronically and meter fuel into the flow of air entering the engine, usually into the intake tract very close to the intake port. When the rider opens or closes the motorcycle throttle, they are controlling the flow of air through a throttle body and into the engine. A sensor on the throttle body signals the fuel injection system to add the correct amount of fuel for the prevailing air flow.

The earliest fuel injection systems were devised in the late 19^th century as the internal combustion engine was being developed. Fuel injection technology was advanced for aircraft engines during World War II. These systems were mechanically controlled. Early analog electronic fuel injection systems were developed in the late 1950s and became popular on European autos in the 1970s. In the early 1980s automobile manufacturers developed fully digital EFI systems that could be linked to the engine ignition system and to oxygen sensors in the exhaust stream, a key development in reducing exhaust emissions through the use of an exhaust catalyst. By the 1990s EFI had replaced the carburetor on most automobiles and began to appear on motorcycle engines as manufacturers worked to meet more-demanding exhaust emissions standards.