How to Adjust Air Fuel Ratio


Modern fuel systems are electronically controlled fuel injection systems. A computer takes care of the mixing of air and fuel. However, in older vehicles made prior to 1990, it was common for manufacturers to use a carburetor. The carburetor would mix the air and fuel using vacuum pressure. This system draw air into the carburetor, then draw in gasoline and atomize it while it mixed with the air in the system. This air would then be drawn into the combustion chamber for burning. These carburetors could be adjusted for optimum fuel delivery, giving drivers a great deal of control over how their vehicles ran. If you own an older vehicle that has a carburetor, it's a good idea to know how to adjust the air fuel ratio.

Things You'll Need

  • Flat-head screwdriver
  • Open the hood and remove the air filter. The filter on carbureted vehicles is normally located on the top of the engine. There is a wing nut that holds the cleaning filter to the carburetor. Turn the wing nut counterclockwise and pull the filter off. Do not remove any of the hoses connected to the filter; just set the assembly off to the side.

  • Locate the two screws on the lower front side of the carburetor.

  • Turn the screws clockwise until they are tight and will not turn anymore.

  • Try to start your vehicle. You may have difficulty starting the vehicle. This is OK. Turn each adjustment screw on the carburetor counterclockwise two complete turns and try to start the vehicle again.

  • Repeat step 4 until your vehicle starts normally. Keep adjusting both screws on the carburetor until your idle smooths out and holds steady somewhere between 500 and 800 RPMs on your vehicle's tachometer.

  • Put the air filter back and tighten the wing nut when you are finished adjusting the carburetor.

Tips & Warnings

  • Each vehicle may be slightly different when adjusting the vehicle's carburetor. For specific information on how to adjust your vehicle's carburetor, consult the particular vehicle's manual (see Resources).
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