Harley Carburetor Basics

Harley-Davidson used carburetors on bike engines prior to fuel injection.

Harley-Davidson used a variety of carburetors on motorcycles built prior to 2007, when it switched to fuel injection in order to meet increasingly stringent federal emissions standards. The main carburetor producers for Harley in the past 30 years include Keihin, Mikuni and Bendix, which provided mechanical, side-draft carburetors. Harley went to constant velocity carburetors in 1988 on Sportster models and 1990 on Big-Twin models, and used them until fuel injection became necessary.

How Harley Carburetors Work

Carburetors control the amount of air allowed to enter the combustion chamber of the engine. A butterfly valve controls airflow. The valve opens and closes through a system of cables and springs actuated by the throttle grip position. A precisely metered amount of fuel, delivered and atomized in the intake airstream, provides the appropriate air-fuel ratio for optimal engine performance at any throttle setting or engine rpm. A series of orifices, called jets, meters the fuel and controls the amount of fuel allowed to flow through each circuit in the carburetor.

Mechanical Carburetors

Harley’s older mechanical carburetors have two valve plates within the venturi — or throat — of the carburetor. The throttle plate, placed in the middle of the venturi, controls the amount of air going into the engine. As the throttle plate opens, it uncovers the middle and high-speed jets in the venturi. The increased vacuum inside the venturi draws fuel through the larger jets, increasing the fuel output to match the increase in the volume of air taken in. A second plate in the mouth of the carburetor, called the choke plate, aids cold starting. This plate restricts the amount of air drawn in while increasing the vacuum in the venturi, which draws in more fuel than would normally be used. This causes a “rich” air-fuel ratio to help the engine start.

Constant Velocity Carburetors

Constant velocity carburetors use a throttle plate in the mouth of the carburetor to control the amount of intake air to the engine. As the throttle plate opens, the vacuum increase in the venturi operates a diaphragm in the top of the carburetor, which draws up on a slider in the center of the venturi. This slider increases the inside diameter of the venturi to keep the velocity of the intake stream at a constant rate. As the slider moves up, it draws a tapered needle valve out of the main jet tube in the bottom of the carburetor, increasing the amount of fuel delivered to the intake airstream. Instead of a choke plate to aid in cold starts, CV carburetors use an enriching circuit that acts as a second, smaller carburetor. When activated, the enriching circuit opens a secondary air intake and fuel jet and delivers this rich mixture into the venturi behind the throttle plate and slider.

Other Details

Both types of Harley carburetors come equipped with an accelerator pump. When the throttle opens rapidly, the carburetor vacuum does not react fast enough to meet the fuel demands of the engine. To prevent a temporary “lean” condition, the accelerator pump squirts a short, atomized burst of fuel directly into the intake airstream to help maintain the air-fuel ratio until the carburetor transitions to the new rpm. Additionally, both types of carburetors use a float and needle valve to maintain a specific fuel level within the fuel bowl under the carburetor.

Carb Malfunctions

Damage to the needle valve or float can cause the bowl to overfill, leading to a persistently rich-running condition and fouled plugs. Remove, clean or replace jets in the float bowl as needed. However, do not change the jets to those of a different size, unless it is part of a kit to increase air intake volume and performance, or the bike may no longer meet the necessary emissions standards for the model year. Idle mixture screws are set at the factory. Adjustment of the idle mixture screws would ruin the emissions output profile for the model year.