Defects in Simple Carburetor | Full Details are Explained.

Defects in the simple carburetor consist of many problems. A carburetor is a mechanical device that mixes air and fuel in an internal combustion engine. There are many difficulties with a simple carburetor. Must read the Faqs also. 
Defects in simple carburetor.
Defects in the simple carburetor.


In this article, I will discuss the defects in simple carburetors.

Defects in simple carburetor-

When the nozzle and venturi sizes of a simple carburetor are set once, it would work satisfactorily at a particular speed and load, providing the mixture, the engine has run at different speeds ranging from idling to racing. The air-fuel ratio must be varied for different speeds. Therefore, certain defects occur in the functioning of the simple carburetor. There are two basic reasons for defects as follows :
  • The correct air-fuel mixture is not supplied at different speeds.  
  • The Coefficient of discharge for air and fuel is not the same. 

If the carburetor is set to work at high speeds at full throttle opening, it will not work at low speeds at part throttle opening, because the suction created at the venturi will not be sufficient to draw fuel from the main nozzle.

Co-efficient with discharge varies with pressure head.
Co-efficient with discharge varies with pressure head.


The coefficient of discharge for air and fuel varies with the pressure head. The coefficient of discharge for air becomes about constant at a certain value of pressure head, but that for fuel increases gradually. Therefore, at higher heads, i.e., higher speeds, the air-fuel mixture will be richer and richer. A low speeds, the mixture will be lean. Thus, if the carburetor is set at one speed, it would not work satisfactorily at different speeds.

Defects in simple carburetor- 

The various difficulties in a carburetor are as follows: 
  1. Starting difficulty.
  2. Idling difficulty.
  3. Running difficulty. 
  4. Acceleration difficulty. 
  5. Weather difficulty. 
  6. Altitude difficulty. 
  7. Icing difficulty. 
All the details are discussed in the following:

1. Starting Difficulty- 

For starting the engine, a very rich mixture is required. The methods of providing the rich mixture at the time of starting are as follows :
  • Tickler.
  • Choke.
  • Adjustable area jet.
  • Separate air passage

1. Tickler- 

A tickler is simply a rod fitted in the float chamber. It can be pressed against the spring. At the time of starting, when it is pressed, it depresses the float, thus providing more fuel to come into the float chamber, and consequently, more fuel comes out from the main jet.

2. Choke Valve- 

The choke valve is fitted at the top of the air horn. It is closed at the time of starting. Thus, it closes the air supply. The whole suction is applied on the nozzle which delivers sufficient fuel making the mixture rich. The choke valve must be opened immediately after starting the engine, otherwise, the cylinder will be flooded with fuel. This is achieved by any of the following methods :
Eccentric mounting of choke.
Eccentric mounting of choke.


  • By mounting the choke eccentrically, the choke is mounted eccentrically. When the engine has started, it opens automatically due to the difference in pressure on the two sides of the choke spindle. 
Strangler on the choke.
Strangler on the choke.


  • By using a strangle valve on the choke. When the engine has started, the air pressure forces the strangler valve to open against the spring and the air enters the carburetor through it making the mixture lean. At the same time, the driver also presses the choke knobs to open the choke valve.

3. Adjustable area jet- 

Adjustable area jet.
Adjustable area jet.


An adjustable screw is fitted in the venturi. Its tapered end projects into the nozzle. Thus, the jet area can be varied by this screw. At the time of starting the engine, the screw is loosened so that the jet area is increased providing more fuel.

4. Separate air passage- 

Separate air passage for starting.
Separate air passage for starting.


separate air passage is sometimes provided just below the throttle valve. At the time of starting, when the throttle valve is closed, the whole suction is applied on this passage which supplies less air and more fuel. thus the mixture is rich.

2. Idling difficulty- 

After starting the engine, the vehicle does not move suddenly but it is kept idle for some time. Idling means keeping the engine running without the movement of the wheels. During the idling, the engine gets heated sufficiently for normal running. Also, the driver and the riders get their seats properly and the vehicle is made ready for moving.
Idling circuit.
Idling circuit.


For idling, the throttle valve is opened slightly. The suction created at the venturi is insufficient to draw fuel from the main nozzle. A less rich mixture is required for idling than for starting, but more quantity of the mixture is required in this case. Thus, for idling another hole or set of holes is provided slightly above the hole for starting. The same air passage is used for this purpose.

3. Running difficulty- 

It has already been stated that the engine requires a richer mixture at high speeds and a weaker mixture at low speeds. The carburetor must provide the correct mixture for different speeds. The process of adjusting the air-fill ratio for different speeds automatically is called compensation. The methods of compensation have already been explained earlier.

4. Acceleration difficulty- 

When sudden acceleration is required, the throttle valve is opened suddenly. This causes the maximum amount of air to come at once and the fuel lags behind it. Thus, the mixture becomes lean instead of being rich for acceleration. In this case, an extra amount of fuel is supplied in the venturi by the accelerating pump. The working of the accelerating pump has already been explained earlier.

5.  Weather difficulty- 

If a simple carburetor is set for a particular weather, it will not supply the correct air-fuel mixture during the other weather. For example, if the carburetor is set for summer, then in winter it will give a weak mixture because air density increases in winter. More air will be supplied and hence the mixture will become weak. Similarly, a carburetor set for winter will give too rich a mixture in summer. Inmodem carburetor, climatic control devices are provided, which control the air-fuel ratio either by varying the air supply or fuel supply.

6. Altitude difficulty- 

At high altitudes, the atmospheric pressure decreases, due to which the air density also decreases. But at higher altitudes, the temperature decreases which increases the air density. However, the net result is an appreciable decrease in air density at higher altitudes. If a simple carburetor is set for lower altitudes, it will not give the correct air-fuel mixture at higher altitudes. The mixture will be richer at higher altitudes due to the decreased density of air.
A method that is used to overcome this difficulty in some Zenith carburetors is explained by means of Fig. Pipe AB connects the float chamber to the bottom of the venturi. Another pipe CD connected the float chamber to the top of the venturi. A valve S is provided in pipe CD, and it is kept closed at low altitudes. The fuel in the float chamber is under atmospheric pressure because of pipe AB. As the altitude increases, valve S is opened gradually, and by doing so, the engine suction is applied to the float chamber. Thus, the pressure is decreased in the float chamber which results in decreased fuel supply making the mixture weaker. 

7. Icing difficulty- 

The fuel is evaporated in the carburetor. Evaporation means the change of state from liquid to vapor. During the evaporation, the fuel absorbers heat, thus cooling the surrounding areas in the carburetor. In cold localities, there is always a chance of ice formation and choking the venturi tube. The ice formation may be prevented by : 
  1. Heating the carburetor with engine exhaust gases.
  2. Providing water circulating passages in the carburetor body to keep the carburetor heated by circulating hot water of the engine cooling system. 


A simple carburetor consists of many parts. It is designed simply but operates mechanically. So the checking of every part from time to time is very necessary. A perfect technician can do this work. So, I will highly recommend verifying your vehicle through a professional automobile dealer. 

Frequently asked questions- 

1. What is Engine Hunting? 
This is a phenomenon in which the engine runs in a series of surges, and the speed of the engine fluctuates intermittently. It can be rectified by adjusting the slow-speed air screw. 
2. What is Backfiring? 
A faulty exhaust valve or spark plug may leave partially burnt or unburnt gas in the exhaust manifold, which when it comes into contact with atmospheric air, bursts in the silencer. Pre-ignition due to excessive overheating of the spark plug and incorrect timing are also reasons for backfiring. 
3. What is an unloader device in a carburetor? 
If at the time of starting, the engine is flooded due to any reason, it becomes necessary to open the choke sufficiently to allow the engine to clear the excessive gasoline. This is done by an arrangement of the throttle lever and chokes linkage whereby depressing the accelerating pedal to the floorboard forces, the choke open sufficiently to allow the engine to clean out the intake manifold. This device is called the unloader device. 
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