Harley-Davidson Evolution V-Twin Owner's Bible: All Models 1984-1998 - by Moses Ludel

CARBURETOR TUNE-UP

There have been two types of Evolution carburetors, both of side-draft (horizontal air flow) design. Earlier models use a traditional slide-valve type arrangement, where the throttle cable is connected to a slide in the carburetor. Opening the slide lets in more air and fuel.

Fuel is gravity fed to the carb by a needle-and-seat fuel inlet operated by a float. An accelerator pump circuit squirts fuel when you open the throttle, and a choke valve is used for cold starting.

In addition to the cable controlled cold start choke circuit, these carburetors offer idle, mid-range, high speed and acceleration fuel circuits. This is a fundamental and proven carburetor design, similar in function to traditional Harley-Davidson types.

Later Evolution motorcycles feature a distinctive "constant-velocity" type carburetor. This constant velocity (C.V.) carburetor is in some ways similar to a classic concentric design carburetor with its throttle slide and main jet needle, but the Harley-Davidson C.V. carburetor uses engine vacuum to lift the piston/slide and jet needle.

When you open the throttle on a C.V. carburetor, the throttle cable directly opens a throttle valve within the carburetor's throat. The throttle plate rotates open, and a vacuum diaphragm, a balance spring and atmospheric pressure each work together to smoothly move the piston-and-needle assembly upward. Fuel flows in increasing volume as the slide lifts and air velocity/volume increases.


CARBURETOR ADJUSTMENTS

There are only three adjustments that you can legally make to your carburetor: cold idle speed, warm idle speed and setting the float's height. Cold idle speed setting begins with making sure the choke or enrichment cable is adjusted correctly.

The factory specifications describe how to adjust the two idle speed settings for your particular model. Follow these factory specifications and make sure that throttle and choke or enrichener cable adjustments are correct before setting idle speeds.

Fig. 5-17. Choke valve (non-C.V. carburetors) must open fully with choke knob pushed to its off position. In your Harley-Davidson factory service manual, you will find procedures for replacing and adjusting choke or enrichener cables.

Before adjusting fast idle (cold start "choke on" or fuel enrichener mode) or warm (curb) idle speed, make sure cable works freely and that adjustment is correct.

• Introduction
• Harley-Davidson History: A Shared Ownership Experience
• Buying an Evolution Motorcycle
• Riding Your Harley-Davidson
• Basic Service and Lubrication
• Engine Tune-Up and Electrical Service
• Engine Troubleshooting and Service
• Primary Chain, Clutch, and Transmission
• Wheels, Hubs, Tires, and Brakes
• Chassis, Fork, and Suspension
• Performance Upgrades
• Accessories
• Detailing and Storing Your Harley-Davidson