How to adjust a carburetor on a car: step-by-step

adjust a carburetor on a car

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How do you adjust a carburetor on a car? Our step-by-step guide outlines how to adjust single carburetor and multi-carburetor systems.

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An optimal carburetor setting, which has a good effect both in terms of performance and consumption values, can usually not be achieved without a specialist. Experienced carburetor specialists can determine an excellent carburetor setting through driving tests. By the way, if you want to convert to a double carburetor system or multiple carburetor system, it is advisable to use a commercially available system to adjust a carburetor on a car; because the difficult basic setting is already there. 

In practice, it is usually handled in such a way that the carburetors of the series engines are set so that they can achieve the best performance with the lowest consumption. If you choose a richer carburetor setting, this usually has a positive effect on the gain in performance after you learn how to adjust a car carburetor. 

If you are looking for maximum performance, you should not consider the consumption of the vehicle. However, this is very uneconomical for continuous operation. But not only that, it can also have a negative impact on the composition of the exhaust gas. 

Anyone who owns a vehicle that is subject to emissions legislation must also consider the issue of emissions when they want to adjust a carburetor on a car. Because there are some guidelines that influence the setting of the carburetor. 

Increase the amount of gas drawn in

When tuning, a better filling should come about at higher engine speeds. To do this, the amount of gas drawn in must be increased to adjust a carburetor on a car. To achieve this effect, the air funnel must be enlarged. This part is the narrowest cross-section of the carburetor. The diameter of the air funnel is very important for both the performance and the running behavior of the series engine.

 With a large air funnel, the power is shifted upwards. But the air funnel shouldn’t be too big either. Because that has a negative effect on the transitional behavior of the engine. The running characteristics would be suboptimal at low speeds. If you can do without the good running properties, more power can usually be achieved with a large air funnel. 

A smaller air funnel is recommended, which, however, brings with it a low performance, but improves the running behavior. If the air funnel is enlarged, the main nozzle must also be enlarged. Because only in this way does the mixture ultimately have the same composition.

Determine the optimum nozzle values

It is necessary to feel your way to the optimal setting to adjust a carburetor on a car, once you have enlarged the air funnel and the main nozzle. Finding out the optimal values ​​can be achieved either by driving tests or on the test bench. As soon as you have determined which setting allows the highest maximum speed, you still have to find out whether comfortable driving is also possible in the transition area and under partial load. 

There are a few ways to influence the mixture. If the mixture is too rich or too lean under full load, an air correction nozzle can help. A larger air correction nozzle makes the mixture leaner and a smaller air correction nozzle contributes to a richer mixture. After a full ride you should look at the exhaust picture. This gives information about whether the mixture is set too rich or too lean. Please note when you want to know how to adjust car carburetor mixture screws you should judge by this:

  • Whitish to light gray exhaust – engine too hot and mixture too lean
  • Brown to dark gray exhaust – setting roughly fits
  • Black sooty exhaust mixture is too rich

You can use this as a guide to adjust a carburetor on a car and make corrections accordingly. If the correct setting for operation under partial load is to be found out, this is best found out by driving at around three quarters of the maximum speed. In the test bench tests, 75 percent of the maximum speed and half of the maximum power are relevant.

A good alternative to the variants mentioned is to use a CO measuring device to adjust a carburetor on a car. All speeds and load ranges should then be driven in a large gear. If the result is that the mixture is too rich, the main jet should be reduced in size until the engine jerks or the drop in performance can be felt. 

The value that allows trouble-free operation should now be selected. If this value is smaller than the value that was previously determined under full load, the air correction nozzle must be reduced in size. In this way, the mixture is enriched again at high speeds. In the opposite case, i.e. if the mixture was too poor under partial load, the mixture must be corrected again using a larger air correction nozzle.

The size of the main jet also affects the transient behavior of the engine during acceleration. If there are bad transitions, the injection quantity and the injection duration should be corrected. You can quickly see whether the mixture is too lean or too rich. If you notice jerky and hesitant movements when accelerating, this is a sign that the mixture is too lean. If an acceleration hole can be felt and the motor then picks up quickly, the settings are too bold.

Steps to adjust a carburetor on a car

It is of course also possible that there is no basic setting. But there are a few options and tips that can be done without a test bench and specialists. The correct carburetor setting basically depends on the number of cylinders to be supplied.

Four cylinder engine

In normal engines, a carburetor has the task of supplying four cylinders. In this case, the first thing to do is to determine the air funnel diameter that will give the best performance. 

  • The main jet should be a bit larger than actually calculated. 
  • The calculated size should be used for the air correction nozzle. 
  • As soon as the diameter is fixed, the main nozzle can be gradually reduced… Until the performance drops. 
  • Then a nozzle with the better performance should be used. 
  • As already described in the text, the partial load point is found on the test bench at three quarters of the maximum speed or at 75 percent of the maximum speed. 
  • The main jet must be reduced in size until the lowest consumption is achieved with the engine running normally. 
  • The settings of the air correction nozzle must be retained. 
  • Uneven running and engine judder should no longer be felt. If they are, the mixture is now too lean under full load. 
  • To remedy this point, a smaller air correction nozzle must be selected to enrich the mixture. 
  • If low consumption is not so important, as is usually the case with tuned engines, the main jet should be selected a little larger, because this is the only way to achieve optimal performance. 
  • This method works with the carburetors that have four cylinders to supply. If low consumption is not so important, as is usually the case with tuned engines, the main jet should be selected a little larger. 

Four-cylinder performance engine

If it is a sports engine, in which a carburetor also has to supply four cylinders, you should proceed slightly differently. Even with a normal six or eight cylinder engine, the following method is the correct one. 

  • In these engines, a mixture enrichment in the high speed range is usually necessary. 
  • This is done with an accelerator pump. Without this pump, no optimal top speed could be achieved. 
  • With this method, one must first determine the correct air funnel diameter. 
  • Now the main jet and air correction jet have to be determined, which generate an optimal engine run in the partial load range with low consumption. 
  • Usually the result is that the mixture is too lean for full load operation. 
  • Now you have the opportunity to counteract the effect with an enriching acceleration pump and an injection tube. T
  • he injection pipe must end in the narrowest cross-section of the carburetor. This measure enables the mixture to be enriched under full load. 
  • With a pump nozzle, the correct enrichment, which is designed for the maximum speed, must be determined. 
  • If problems arise when accelerating afterwards, the injection quantity should be changed.

In many sports engines, the carburettors supply one, two or three cylinders. It is often the case that the optimal settings for the full load range are too lean in the partial load range. 

  • This negative effect can be remedied with a leaning accelerator pump with an injection tube low. 
  • First you should use a blind pump nozzle for experimental purposes.
  •  If the engine also runs well in the partial load range with the optimal full load setting, a neutral acceleration pump with an injection pipe can also be used.

Adjust idle correctly

Most carburetors have an idle system with mixture regulation. In this area, a finished mixture that is created by the idle nozzle and the idle air nozzle is regulated in terms of quantity at the outlet under the throttle valve. 

  • The idle mixture control screw is responsible for the regulation. 
  • The main air enters the throttle valve. 
  • The air is dosed with an idle adjustment screw. The screw is on the throttle lever. 
  • If the screw is turned in, the result is a leaner mixture. 
  • A richer mixture is therefore created by unscrewing the idle speed adjustment screw.

Idle setting in performance cars

The idling setting is also of great importance in performance carss, at least if they are used more in normal operation. 

  • An idle speed of 800 to 1,000 revolutions per minute is recommended for such engines. 
  • If performance camshafts or other changes have been made, the idle speed should be set higher. 
  • The engine should be warmed up well before the idle speed is set. 
  • The idle speed should be adjusted with the idle speed adjustment screw so that the speed is 1,000 rpm. 
  • A uniform throttle valve position is necessary for engines with multi-carburettor systems. This can be done with the aid of a synchrotest measuring device or a similar vacuum measuring device. 

Single carburetor and multi-carburetor systems

  • If it is a single carburetor system, the idle mixture screw has to be unscrewed, until the idling brings an out-of-round run with it. 
  • As soon as this has been achieved, the screw is turned back in until the idle runs smoothly again. 
  • If it is a multi-carburetor system, the throttle valves must first be brought to the same position. 
  • After this step, every mixture control screw must be used for a perfect idle. So you have to turn the screws out first. 
  • Then you screw the screws back in until you have a smooth run at the highest speed. It makes sense to use a tachometer for this. until the idle runs smoothly again. 

Bypass bores

It is also worth knowing that the bypass bores in an existing carburetor usually cannot be changed. The idle jet primarily determines the transition when accelerating out of idle speed. If there is a bad transition due to an idle mixture that is too lean, a larger idle screw should help. In the winter months, vehicles with tuned engines should generally switch to a larger screw, because this also has a positive effect on the operating behavior when the engine is cold. A smaller idle jet is necessary if there is a bad transition due to the mixture setting being too rich.

If the measure is unsuccessful, the position of the throttle valve in relation to the existing bypass holes should be checked. Maybe there is something wrong with the settings. However, this step should be carried out by a specialist in a workshop.

CO measurement – exact method of carburetor adjustment

The ones mentioned so far without special measuring devices and without a test stand are in part imprecise. An exact method of setting the carburetor correctly is with a CO measurement. The CO component in the exhaust gas is measured. The measured CO content in the exhaust gas indicates whether the carburetor is set too rich, too lean or just right. This involves measuring the CO content in the exhaust gas in full load operation instead of in idle mode. If the engine is to run reasonably economically, the measurement in partial load operation is also necessary.

But how is the CO determination actually carried out in full load operation? At the highest level of the exhaust gas, the output is consistently 3 to 5 percent CO share. That is at least the case with most vehicles at nominal power. 

Motors that are designed for maximum performance can therefore only be operated under lambda 1 under full load. Lambda (from the lambda sensor or oxygen sensor) is the ratio between the amount of oxygen actually present in a combustion chamber vs. the amount that should have been present to obtain perfect combustion. Lambda above 1.0 means lean and lambda below 1 means rich. If such engines are operated with lambda 1, overheating can occur. A drop in performance can also be felt. Motors that require internal cooling have to run even more richly. Mostly with lambda up to 0.75. This is the case with charged engines.

Usually, naturally aspirated engines achieve maximum performance with a CO content of 4%. This applies to almost all engines. There are deviations in engines with a significant loss of filling caused by throttling in the upper area. As soon as the carburetor is set so that the CO value is 4%, the CO measuring device is used. It is started from around 2000 rpm and checked in 500 rpm steps under full load. 

During this process, not only the performance curve of the engine, but also the CO value is displayed without any additional effort. It is usually the case that the CO value deviates on the full load line. Fluctuations between 2 and 8 percent are in the acceptable range as long as the values ​​in the main operating range are between 3 and 5 percent. It is better if the setting is a little too bold. This is how damage from overheating and knocking can be prevented. Carburetor adjustments must now be made again.

It is more difficult to determine the best settings for the partial load range. If you do not drive at full throttle, CO values ​​between 0.5 and 1 percent are sufficient. The goal is to keep the engine running as lean as possible. If there are bad transitions or jerking, corrections should be made. 

  • The engine with the previously set full load power is transported to the roller dynamometer. 
  • On the test bench, the values ​​are determined along the road resistance line with the current settings. It starts at 20 km / h and continues in steps of 20 until 120 km / h is reached. 
  • The CO values ​​of the steps are measured. If the engine runs too lean or too rich in some speed ranges, the carburettors must be adjusted (as already described). 
  • One should be aware that the idling system has a very strong influence on the load and speed range. It is important that the CO value is checked after each setting. If it no longer fits, the full setting must be reset.
  •  If there is no roller dynamometer available, measurements can be carried out with a CO measuring device. There is a measuring device with a measuring tube or a portable CO measuring device. In the system, an exhaust probe is inserted approximately 40 cm into the exhaust and attached from the outside. The inside diameter of the probe is approximately 4mm and is made of copper tubing. 
  • Other metal pipes can also be used. A hose runs from this probe to the measuring device inside the car. The passenger has the task of determining the values ​​under full load or at constant speed.

gerhardt-richter Gerhardt Richter is a writer and a trainer at trade technical colleges, specializing in carpentry, plumbing, mechanics and construction.