Carburetor

11 Oct 2010Carburetor Functions

Carburetor parts and its functions


Carburetor

• What is a carburetor ?

A device for automatically mixing fuel in the proper proportion with air to produce a combustible air fuel mixture.

• Functions of the carburetion system. 1. To mix the fuel with the proper proportion of the air.

2. To vaporize the fuel.

3. To deliver the correct amount of the air fuel mixture to the engine.


Kinds of Air / Fuel Mixture

• Optimum mixture

The chemically correct mixture of air and fuel, its ratio is 15 air to 1 part petrol by mass.

• Rich mixtures

Mixtures containing more than the optimum amount of petrol, produces more power than optimum and weak mixture.

Its maximum mixture has an air/fuel ratio of between 10 to 1 and 13 to 1.

The exhaust products of the rich mixtures normally have an excess of carbon monoxide and have dark cloudy exhaust smoke.

• Weak mixture

Mixtures containing less than the optimum amount of petrol, usually produces less power than the optimum and the rich ones, but give good fuel economy.

Its mixture has a air/fuel ratio between 16 to 1 and 18 to 1.

Burning is generally slow and misfiring, overheating and incomplete combustion.


Typical Motorcycle Air-Fuel Mixture Requirements

• Operating ranges @ different loads and speeds.

1. Idling.

2. Cruising.

3. High Power.


Parts of the Carburetor

Fuel bowl.

Fuel Float

Float Needle

Pilot Jet

Main Jet

Starter Jet / valve

Needle Jet


Fuel bowl , Fuel float & Float needle

• Fuel bowl

1. Stores fuel @ atmospheric vapour pressure.

• Fuel float

1. Fuel float regulates fuel flow into the fuel bowl.

2. Float is usually a hollow floating device inside the fuel bowl attached to a pivoted hinge.

3. As the fuel level in the fuel bowl varies, the vertical level (angular displacement) of this float changes as well, this movement controls the opening and closing of the float needle.

• Fuel needle

1. It usually has a pin connection in one end and a rubber sealing element in the other end.

2. As the fuel float moves the float needle , its rubber seal opens / closes the fuel opening from the fuel tank.

Fuel bowl


Pilot Jet, Main Jet, Needle Jet

• Pilot Jet

1. Part of the carburetor pilot’s circuit 2. Pilot jet controls the maximum amount of fuel that the pilot circuit will supply.

• Main Jet

1. Part of the main fuel supply circuit, the main jets is basically a screw with a very accurate sized hole through it.

2. This hole’s size determines the maximum flow rate of the fuel into the venturi.

3. The carburetor uses the main jet’s full flow capacity, it is the main passage of fule from the fuel bowl in to the carburetor throat.

• Jet needle


Idle Screw & Air / Mixture screw

• Idle Screw

1. Adjust a slight amount of throttle slide displacement @ closed throttle condition. 2. Controls Idle engine speed.

• Air / Mixture Screw

1. Adjust the flow of fuel through the pilot jet at a constant air flow.

2. This screw controls the air/fuel mixture of the pilot circuit from almost no fuel added (lean) to full capacity of the pilot jet, according to the pilot jet size (rich, possibly).


Throttle slide

• Throttle slide

1. A spring loaded gate valve which is directly controlled by the twistable throttle grip. The linear vertical displacement of this valve is equal to the linear displacement of the needle.

2. To regulate engine operating speed a throttle valve is employed to restrict air flow.

3 . This is just mounted above the venturi.

4. When the throttle is fully opened the air flow is restricted very little, when the throttle is closed the flow of air is restricted to the intake passage.

5. This decreases the speed of the engine and power of the engine, at the same time it allows the pressure in the venturi area to increase.

6. The difference between the air pressure in the fuel chamber and the venturi is decreased and therefore the movement of fuel through the nozzle is reduced.


Types of Circuits in a Carburetor

• Various types of circuits in a carburetor

1. Starter Circuit.

2. Pilot Circuit.

3. Main Circuit.

4. Fuel needle jet Circuit.


Starter Circuit

• Starter Circuit (Types – Plunger type & Butterfly valve type)

Plunger type starter circuit

1. Choking the Carburetor actually opens plunger to allow the fuel flow.

2. Fuel is drawn from the bottom of the bowl through the starter jet.

3. Air is drawn from the front side.

4. As the air passes over top of the starter jet, due to pressure difference fuel is drawn from the bowl and mixes with the air which is achieved by the emulsifiers present in the starter jet.

5. Atomized air/fuel mixture is delivered to the engine for easy starting during cold start / first start of the day.


Starter Circuit

Butterfly type starter circuit

1. Choking the Carburetor actually chokes the inlet air flow.

2. Fuel is drawn from pilot circuit.

3. Air is drawn from the front side.

4. As the air passes over butterfly valve plate, due to pressure difference air mixes with fuel and get atomized..

5. Atomized air/fuel mixture is delivered to the engine for easy starting during cold start / first start of the day.


Pilot Circuit

Pilot circuit

1. Pilot circuit is used in the Idle to ¼ throttle opening.

2. Fuel flow is controlled by the pilot jet size, this jet is the first to get plug if the fuel gummy formation happens.

3. Air flow is controlled by the air screw. @ Idling, air flows through the pilot air hole passage .

4. As the air passes over the pilot circuit due to pressure difference air rushes in and mixes with the fuel drawn from the pilot jet and the atomized air/fuel mixture is being delivered to the engine.


Main Circuit

Main circuit

1. Pilot circuit is used in the Idle to ¼ throttle opening.

2. Fuel flow is controlled by the pilot jet size, this jet is the first to get plug if the fuel gummy formation happens.

3. Air flow is controlled by the air screw. @ Idling, air flows through the pilot air hole passage .

4. As the air passes over the pilot circuit due to pressure difference air rushes in and mixes with the fuel drawn from the pilot jet and the atomized air/fuel mixture is being delivered to the engine.

Carburetor

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