Harley-Davidson V-twin engine
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
1. CV carburetor mounted to a Harley-Davidson Shovelhead engine
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.
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.
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.
4. Pry out the plug with a small pick.
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.
6. Turn idle mixture screw two turns out (counterclockwise) to an idle speed of 1,000 rpm.
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.