The carburetor for all intents and purposes is a tube known as the "throat". At the end of this tube, there is an adjusting plate known as a throttle plate. The purpose of the throttle plate is to regulate how much air is allowed to flow through the tube. Another part of the throttle is a narrowing in the tube which is known as the venturi. The narrowing of the tube creates a pressure difference which creates a vacuum in the tube. At the venturi, there is a hole known as a jet which allows the vacuum to draw in the fuel. This part of the carburetor is connected to the gas tank. One other jet in the carburetor is a hole behind the throttle plate which is known as the idler jet. This allows fuel to be drawn when the throttle is closed. There is also a choke plate which covers the venturi when the engine is first started to allow the engine to run slowly. Air is drawn into the carburetor through a connecting tube to the area under the piston and the mixture exits the carburetor and into the combustion chamber. These are the main parts that make up the carburetor.
The fuel flow is dependent on the varying air velocity in the venturi. This is the type seen on the Briggs and Stratton Engine 91200.
Variable Venturi or Constant Depression
The air flow varies the jet opening which alters the fuel flow.
Recap of Main Parts:
How it works
The carburetor works on what is known as Bernoulli's Principle. The principle says that as the velocity of an ideal gas increases, the pressure drops. As the piston compresses, air is pushed out of the chamber, through a pipe, and into the throat of the carburetor known as the venturi. The air must accelerate from rest to some desired speed which is specified by the engine speed and throttle. The air flowing through the venturi is at a pressure less than atmospheric.
At this low pressure area, a hole is drilled into the venturi and this is where the main jet comes into play. The main jet rests in the gasoline tank or the float bowl. Since the gas tank is between the low pressure area and the atmosphere, a vacuum is created. The vacuum pulls gasoline into the air stream flowing through the venturi. The more air the engine pulls through the venturi, the greater the pressure drop and the more fuel which is pulled into the air stream.
The venturi/jet arrangement can only meter fuel accurately over a certain range of flow rates. As the flow rate begins to increase, the venturi or jet or even both will begin to choke. This means that no matter how hard the engine tries to increase the flow rate, it will not increase. On the other extreme, at times such as startup or idle, the velocity of the air in the venturi is very low and therefore the pressure drop is almost non-existent.
This is where the idle jet comes into play. The idle jet is located just downstream of the throttle plate which during idle is almost completely closed. This jet is tiny because there is a very high pressure difference across the throttle plate and there is not a lot of fuel required. As the throttle is opened, the amount of fuel that flows through this jet is very small and therefore almost constant which means that it is easily compensated for in the midrange and higher mixtures.
During startup, the amount of air flowing through the venturi is still very small until the engine begins to run on its own. This means that the pressure difference across the jets is not significant. At startup, the engine is cold and needs an extra-rich mixture of air and fuel to compensate for the amount of fuel which precipitates when it hits the cold engine walls.
Choke Plate/Throttle Plate
This is where the choke comes into play. The choke is a plate which can be moved so that it completely or very nearly closes off the throat of the carburetor at its' entrance. The main and idle jets are downstream of the choke plate. The choke plate does not allow any air to get into the throat of the carburetor. The only way to get air into the venturi is through the jets. Since there is very minimal air coming through the carburetor, a very rich mixture of fuel results. To maximize the effect of the choke plate, the throttle plate should be wide open otherwise the engine does not recognize there is even a choke plate, it only sees that the throttle is closed. The engine will pull as much fuel as it can through the idle jet but this is not very much. Therefore to get a rich mixture, at startup the throttle should be wide open. As the engine starts, the throttle should be closed to cut down on the effects of the choke. This is not enough to simply close the throttle and therefore many chokes are designed so that as soon as there is sufficient airflow, they slightly open so the engine does not flood. To summarize the choke, they cut off air supply to the carb which increases the pressure difference across the jets at low speeds and enriches the mixture.
The carburetor was invented in 1893 by Hungarian scientists Donat Banki and Janos Csonka. The term carburetor comes from the French word carbure meaning carbide. In 1900, the carburetor was successfuly incorporated on an engine in a petroleum driven car created by Frederick William Lanchester from Birmingham, England. In 1902, George Kingston built his version of the carburetor known as the Kingston carburetor. Older cars used what was called an updraft carburetor which meant the air entered the carburetor from below and exited out of the top. The advantage of this type of carburetor was that it never "flooded" since fuel would drop out of the bottom of the carburetor and not into the intake manifold of the engine.
In the early 1930's, catalytic carburetors were made commerically available. These faded after the advantage of using kerosene over gasoline was no longer advantageous. Also in the 1930's, downdraft carburetors were the most popular type used in the automotive industry in the United States. In Europe, the sidedraft carburetor was the most popular due to the decreasing space in the engine bay.
In the 1960's it became standard on most cars to have an automatic choke which was connected to a thermostat using a bimetallic strip. The bimetallic strip was exposed to the engine heat and a sensor detected the heat. Acting along a bimetallic spring, the sensor controls the tension and therefore controls the choke.
Fuel injection was introduced in the late 1950's and was commercialized in the 1970's. In the 1980's, many American-market vehicles used a feedback carburetor which could change the base mixture in response to exhaust gas oxygen sensors. These fell after fuel injection was made standard.
Two barrel carburetors were seen on many high performance American V8's and four barrel carburetors are seen on very high performance engines still today.
|Part Name||# Req'd||Function||Mfg Process||Material||CAD File||Image|
|Pump Plate Bolt||4||N/A||Machined||Steel||Pump Plate Bolt|
|Pump Plate||1||N/A||Machined||Steel||Pump Plate|
|Pump Ring||1||Ensures an air-tight seal||Machined||Steel||Pump Ring|
|Pump Spring||1||N/A||Machined||Steel||Pump Spring|
|Pump Assembly||1||Vacuum created from intake stroke of piston compresses srping and pump plate and pulls gas into the intake valve.||N/A||Steel||Pump Assembly|
|Throttle Plate Connecter||1||N/A||Machined||Copper||Throttle Plate Connector|
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|Throttle Spring||1||N/A||Machined||Steel||Throttle Spring|
|Throttle Screw||1||N/A||Machined||Steel||Throttle Screw|
|Throttle Plate||1||N/A||Machined||Aluminum||Throttle Plate|
|Throttle Plate Screw||1||N/A||Machined||Steel||Throttle Plate Screw|
|Throttle Plate Assembly||1||Regulates how much air is allowed to flow into the carb.||N/A||Copper/Steel||Throttle Plate Assembly|
|Choke Plate||1||Restricts airflow during start-up.||Machined||Aluminum||Choke Plate|
|Choke Plate Shaft||1||Connected to choke and holds choke plate.||Molded||Plastic||Choke Plate Shaft|
|Choke Plate Assembly||1||Restricts airflow during start-up.||N/A||Plastic/Aluminum||Choke Plate Assembly|
|Engine Block Gasket||1||Creates air seal between carb and block.||N/A||Rubber||Engine Block Gasket|
|Carburetor Bracket||1||N/A||Machined||Steel||Carburetor Bracket|
|Carburetor Screw 1||1||N/A||Machined||Steel||Carburetor Screw 1|
|Carburetor Body||1||N/A||Molded/Machined||Aluminum||Carburetor Body|
|Carburetor Screw 2||1||N/A||Machined||Steel||Carburetor Screw 2|
|Carburetor Spring||1||N/A||Machined||Steel||Carburetor Spring|
|Carburetor Body Assembly||1||Houses components to attach the carburetor to the engine block and fuel tank.||N/A||N/A||Carburetor Body Assembly|
|Final Carburetor Assembly||1||Provides the right mixture of gasoline and air so the engine runs properly.||N/A||N/A||Final Carburetor Assembly|
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