How to Adjust a Keihin CVK40

Carburetors mix air and fuel in gasoline engines. Your constant velocity Keihin carburetor is an English invention that is named for the Japanese manufacturer, Keihin, that first mass produced them. CVK40 means Constant Velocity Keihin carburetor with a 40 mm venturi. A venturi is the air hole in the side of a carburetor and, in your constant velocity carburetor, this opening expands and contracts by way of a vacuum-operated slide that keeps the velocity of the airflow into the carburetor constant. These carburetors are usually mounted on motorcycles.

Instructions

1

Remove the air filter cover on your motorcycle, car or other vehicle to access the carburetor. Car air filter covers are commonly attached with a wing nut. Motorcycle air filter covers usually connect to the air filter housing with one or more Allen bolts or Torx screws that loosen with either Allen or Torx wrenches.

2

Put your vehicle in neutral on a flat surface and start your engine. Wait three minutes for the engine to get hot.

3

Spray aerosol carburetor cleaner or ether into the venturi, the open hole, on the side of the carburetor. Allow the engine to stumble, then repeat.

4

Turn the idle speed adjuster screw on the front of the carburetor next to the two throttle cables clockwise until the engine stumbles. Turn the screw with a flat head screwdriver. Slowly turn the screw counterclockwise to increase idle speed just far enough, usually 1/8 turn or less, that the engine idles without stumbling.

5

Turn the fuel mixture adjustment screw on the bottom of the CVK40 1/8 turn clockwise with a flat head screwdriver. Wait three to six seconds for the engine to stabilize, then repeat until the engine coughs or stalls.

6

Turn the mixture adjustment screw 1/16 turn counterclockwise and restart if the engine stalls. Continue to turn the mixture adjustment screw counterclockwise in very small increments until the engine runs smoothly.

7

Readjust the idle speed adjuster screw by turning it clockwise until the engine runs smoothly on the lowest possible idle speed.

8

Stop the engine. Reattach the air filter cover using your fingers or an Allen wrench or a Torx wrench.