How to Adjust a Harley CV Carburetor

by Gabriel Scofield
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Harley-Davidson Motor Co. used the constant velocity (CV) carburetor in the 1988 Sportsters, 1990 Big Twins (EVO and TC88) until 2006. Since then, all Harley-Davidson motorcycles make use of fuel injection. The CV carburetor may show signs of needing to be tuned. These signs include a drastic decrease in fuel economy, black smoke appearing from the exhaust, sluggish acceleration, the engine spitting back through the carburetor, or hesitating upon acceleration. A low-speed circuit adjustment to the carburetor can be made in minimal time


Step 1

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Check spark plug for light-brown glassy deposits which may be accompanied by cracks in the insulator or erosion of the electrode, which indicates a lean air-fuel mixture. Dry, fluffy or sooty black deposits indicate a rich carburetor air-fuel mixture.

Step 2

Locate the idle mixture screw on the bottom outlet side of the carburetor, in a passageway next to the float bowl. Turning the idle mixture screw in, or clockwise, will lean the idle mixture and turning it out, or counterclockwise, will enrich it.

Step 3

Drill out the metal plug covering the idle mixture screw. Do not drill too far into the plug; wrap the drill bit with tape or string to prevent you from drilling too far and damaging the screw.

Step 4

Pry out the plug with a small pick.

Step 5

Turn the idle mixture screw in (clockwise) until it stops. Do not over-tighten the idle mixture screw. Damage to the screw and its seat may result.

Step 6

Turn idle mixture screw two turns out (counterclockwise) to an idle speed of 1,000 rpm.

Step 7

Adjust the screw 1/2 turn at a time. After each adjustment, reset the idle speed to 1,000 rpm. The maximum adjustment range is 1/2 to 3-1/2 turns.

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