How to Tune Carburetors

When you inspect your spark plugs and discover that one or more appears black, or when the engine is idling roughly, you may need to tune or make an adjustment to the carburetor. On older model vehicles, this process involves turning various screws to adjust the carburetor. Tuning a carburetor can be done by first adjusting the air and fuel mixture. You can then make the needed adjustment to the idle speed.

Things You'll Need

  • Flat-head screwdriver
  • Open the hood of the vehicle.

  • Remove the wing nut that is used to secure the cover for the air filter. Twist the wing nut to the left to loosen it and then keep turning it until it comes off of the bolt.

  • Take off the air filter cover and then pull out the air filter.

  • Locate two screws on the lower section of the carburetor.

  • Tighten the screws with the flat head screwdriver. Turn the screws to the right to tighten them.

  • Loosen each screw 2 ½ turns.

  • Locate the screw that is used for the idle adjustment. The screw is under the fast idle cam that is connected to the throttle cable and looks like a hinge.

  • Start the vehicle and turn the screw a half-turn clockwise or to the right to tighten it. This will increase the idle speed of the vehicle.

  • Wait for about 30 seconds and then see where the rpm indicator is at on the tachometer. The normal reading at idle should be between 850 and 1,000 rpm.

  • Adjust the idle speed by turning the idle screw to the left or right until you are happy with the idle speed.

  • Turn off the vehicle, replace the air filter and the air filter cover.