Due to the implementation of BS-VI emission norms, the method of carbureting the engine is totally neglected where the electronic fuel injection system started to rule the method of introducing the fuel into the engine.
In the FI system, separate function will be mapped in order to start the engine at the cold condition. Intake Air Temperature plays a vital role here. If the temperature is too low, then the ECU will inject more amount of fuel in order to prevent the engine from stalling. That's why we can able to see the fuel injected motors idles slightly higher at the starting condition.
But in the carbureted engines, nothing will happen automatically. We have to pull the choke lever manually and have to close it. Inside the carb, there will be separate passages for different conditions. Idling circuit comes handy when the engine idles. We can able to see small holes at the intake side of the carb which is the entry point of the idling circuit.
Main circuit is used when the throttle valve gets opened. Jets are used for atomizing the fuel. Atomization is the process of breaking the fuel into minute particles which results in better combustion process. At the top, a diaphragm setup will be present in which the top of the diaphragm is connected to the main air passage via a separate path.
Air always flows from low to high pressure side. The same principle is used in all the above mentioned paths. At the idling condition, throttle valve will be closed. Due to the downward movement of the piston, a suction pressure will be created and sucks air through the idling passage. Due to the pressure difference, fuel from the float chamber also gets sucked and mix with the intake air, enters the combustion chamber via the pilot jet.
If the throttle valve gets opened, the intake passage for the diaphragm is opened and the pressure difference pulls the diaphragm up, thereby lifting the needle. The needle opens the main jet in the float chamber, air sucks the fuel through the main jet to mix with it due to the pressure difference. Yeah..... Its slight complicated until you understand it.
Whenever the throttle is is closed, the needle comes back to its original position with the help of return spring, thereby closing the main air passage and handover its duty to the idling passage. This simple principle of air pressure plays a vital role in carburettors.
In the FI system, separate function will be mapped in order to start the engine at the cold condition. Intake Air Temperature plays a vital role here. If the temperature is too low, then the ECU will inject more amount of fuel in order to prevent the engine from stalling. That's why we can able to see the fuel injected motors idles slightly higher at the starting condition.
But in the carbureted engines, nothing will happen automatically. We have to pull the choke lever manually and have to close it. Inside the carb, there will be separate passages for different conditions. Idling circuit comes handy when the engine idles. We can able to see small holes at the intake side of the carb which is the entry point of the idling circuit.
Main circuit is used when the throttle valve gets opened. Jets are used for atomizing the fuel. Atomization is the process of breaking the fuel into minute particles which results in better combustion process. At the top, a diaphragm setup will be present in which the top of the diaphragm is connected to the main air passage via a separate path.
Air always flows from low to high pressure side. The same principle is used in all the above mentioned paths. At the idling condition, throttle valve will be closed. Due to the downward movement of the piston, a suction pressure will be created and sucks air through the idling passage. Due to the pressure difference, fuel from the float chamber also gets sucked and mix with the intake air, enters the combustion chamber via the pilot jet.
If the throttle valve gets opened, the intake passage for the diaphragm is opened and the pressure difference pulls the diaphragm up, thereby lifting the needle. The needle opens the main jet in the float chamber, air sucks the fuel through the main jet to mix with it due to the pressure difference. Yeah..... Its slight complicated until you understand it.
Whenever the throttle is is closed, the needle comes back to its original position with the help of return spring, thereby closing the main air passage and handover its duty to the idling passage. This simple principle of air pressure plays a vital role in carburettors.
So why choke valve?
The engine temperature will be lowered, if it isn't started for hours. At that time, it requires extra amount of fuel in order to bring up the operating temperature. For this purpose, choke valve is used. The word choke translates to unable to breathe. Like passages for idling and throttling, a separate passage is also present for the choke.
At the idling condition, the main passage is closed and the air fuel mixture enters the combustion chamber through the idling circuit with the help of pilot jet. Due to the working temperature, the engine starts to stall and turns off sometimes. Here comes the action of choke valve. The valve is connected to the lever at the handlebar. Whenever it gets pulled, the valve in the carb gets opened, thereby letting the extra amount of rich fuel air mixture required to prevent the engine from stalling. With the help of return spring, the valve gets closed and the engine will reach its operating temperature.
Don'ts
The main use of the choke valve is to send the rich air fuel mixture into the combustion chamber. The main air passage will be closed during the whole choking process. But I've seen many who twists the throttle while cranking the engine at the same time when the choke valve is opened. The main point is to cut the air supply. Then what's the use of the choke valve if the throttle is opened... ? So, the throttle must be always at the closed position whenever we use the choke valve.
Image courtesy - YouTube/Athanasio Dolaptsoglou
The engine temperature will be lowered, if it isn't started for hours. At that time, it requires extra amount of fuel in order to bring up the operating temperature. For this purpose, choke valve is used. The word choke translates to unable to breathe. Like passages for idling and throttling, a separate passage is also present for the choke.
At the idling condition, the main passage is closed and the air fuel mixture enters the combustion chamber through the idling circuit with the help of pilot jet. Due to the working temperature, the engine starts to stall and turns off sometimes. Here comes the action of choke valve. The valve is connected to the lever at the handlebar. Whenever it gets pulled, the valve in the carb gets opened, thereby letting the extra amount of rich fuel air mixture required to prevent the engine from stalling. With the help of return spring, the valve gets closed and the engine will reach its operating temperature.
Don'ts
The main use of the choke valve is to send the rich air fuel mixture into the combustion chamber. The main air passage will be closed during the whole choking process. But I've seen many who twists the throttle while cranking the engine at the same time when the choke valve is opened. The main point is to cut the air supply. Then what's the use of the choke valve if the throttle is opened... ? So, the throttle must be always at the closed position whenever we use the choke valve.
Image courtesy - YouTube/Athanasio Dolaptsoglou