How To Set Carburetor Settings

Car mechanic photo

The main function of a carburetor is to mix air and gasoline for internal combustion in a car's engine. This mixture needs to be in the proper proportion for the car to operate smoothly. An imbalance where there is more fuel than air results in the car being sluggish and a wasteful consumption of fuel. The other end of the imbalance where there is more air than fuel creates a dangerous effect. In this instance the fuel becomes too hot and is thus ignited prematurely before the spark plug can set it off. This can significantly damage the car's engine. Here are some tips to set carburetor settings.

1. Determine that the other engine components are trouble-free.

The carburetor operates in conjunction with other components of the car's engine such as the ignition system and the valve train. Make sure that the overall condition of the engine is actually good and that there is no malfunction in the other individual parts so that you can safely assume that the problem is isolated in the carburetor. This is an important first step as it will save you a lot of time. You could end up constantly adjusting the carburetor settings and still keep getting negative results because all along it was the ignition tuning that had to be fixed.

2. Check for carburetor mixture imbalances.

One way to do this is to use a gas analyzer to measure the amounts of hydrocarbon, carbon monoxide and oxygen in the exhaust. There are also specially designed spark plugs with glass bodies which will enable you to directly view the color of the igniting fuel in the combustion chamber. A yellow color indicates that there is too much fuel in the mixture; a whitish-blue color means that there is too much air. A proper proportion of air and fuel will show a shade of blue similar to the blue flame on a gas oven's burner. The condition of the spark plug can also indicate any mixture imbalances occurring in the carburetor. With the engine turned off, take out and inspect the spark plugs. If they are blackened by soot then there is too much fuel. If they are peppered with grayish deposits then there is too much air. A correct mixture would leave spark plugs colored brownish-gray.

3. Check the carburetor circuits.

A carburetor is composed of several inlets and valves which are responsible for controlling the amount of fuel and air that will enter the engine's combustion chamber. There are generally three operating conditions that the carburetor must adjust to:

  1. when the car is idle
  2. when the car is in part throttle
  3. when the car is in full throttle

These inlets and valves are grouped into circuits with each one assigned for a particular operating condition. Thus there are idle circuits, part throttle circuits and full throttle circuits. These circuits do not always operate one at a time. For example when a car is in a complete stop, only the idle circuit will be active; when it starts moving the part throttle circuit will join in; and then as it reaches cruising speed, the full throttle circuit will also engage. When the engine stalls or when sudden and inexplicable losses of engine power occur, it's important that you take note under what operating condition the problem manifested. This will help you pinpoint what carburetor circuit you need to inspect and adjust. The specific component you need to carefully look at is the emulsion tube. In this component the air and fuel is pre-mixed before they enter the main chamber of the carburetor. At the bottom of the tube is the main jet which brings in the fuel and on top is the air bleed jet where the air comes in. Along the tube are minute holes that adjust the air-fuel proportion of the mixture. When any of these passages are blocked by dirt, grime or any other engine deposits, the component won't operate correctly and the mixture is unbalanced. Adjusting the carburetor so that it resumes making the proper air-fuel mixture is usually a matter of taking out the emulsion tube and cleaning it.

Carburetors have actually been replaced by fuel injection systems which are more efficient in terms of fuel consumption because they can automatically sense and adjust to the power demands of the engine. However if you own a car that was made before 1990, then you will need to arm yourself with a little knowledge on how carburetors work and how to adjust them.


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