Carburettor assignment

The Carburettor is a mechanical device that is designed to mix air and fuel on an internal combustion engine. For an internal combustion engine to work a very precise amout of air/fuel mixture is needed. the carburettor works with simple principles to obtain this mix. It was invented in the late 19th century for this purpose.

How does a carburettor work?

Two stroke: A two stroke Carburettor is much more simple than a four stroke carburettor. It is essentially a tube that mixes and pressurisers the air and fuel., Air enters the carbuerttor through an adjustable plate called a throttle plate, this is controlled by a wire attached to the throttle pedal. Inside the carburettor is a narrowing tube called a venturi, this pressurises the air by forcing it into a smaller space. Inside the venturi is a jet or jets which use the vaccum created to allow fuel to enter the venturi and mix with air to create the right air fuel mix. This mixture then leaves the carburettor and goes to the engine. A choke can be used to increase fuel flow into the carburettor.

Four stroke: A Four stroke carburettor works on the same priciples as a two stroke carburettor, however it is more complicated as it has to operate under different circumstances. The main differences are: More parts, more complication, different operating demands. A four stroke carburattor contains an accelerator pump which assists in clean fuel delivery and smooth acceleration.


Identify the components in a carburettor:

How does an emulsion tube work? and What are the holes in the tube for?:

The emulsion tube meters air and fuel entering the carburettor, the holes regulate the amout of air entering by the change in fuel levels in the tube, under acceleration the holes are uncovered by the fuel going down allowning more oygen into the tube.

How does the manual choke and automatic choke work?

A choke is used to help an engine start when cold. A manual choke works by closing a valve over the air intake of the carburettor, this valve contains a small hole which resctricts tthe amount of air entering the carb, this creates a vaccum so more fuel can be pulled through for startup. An automatic choke works in the same way, but the choke valve is controlled automatically by a temperature sensor.

 Holes on a choke valve

What is the purpose of the float in the Carburettor?

 The float in a carburettor acts to keep the level of fuel in the fuel bowl constant, if the level of fuel drops the float lowers and exposes a pin valve which allows more fuel in.

What is the purpose and how does a mechanical lift pump work?

 The mechanical lift pump is a vaccumm pump that uses pressure to suck fuel from the petrol tank up towards the carburettor. It works by pumping with a diaphram that is run by a camshaft and cam roller.

How does a vacuum fuel pump work on a two stroke engine?
A vaccum fuel pump uses vaccumm created by the movement of the pistons from T.D.C to B.D.C to pump fuel from the fuel tank to the carburettor.


What pressures would you get from these types of pump?
A vaccumm pump depending on its size can be anything from 1 kPa - 50 kPa.

What the difference between petrol and diesel fuel?
Diesel and petrol are both derived from crude oil, using different refining techniques. Diesel fuel has different properties to petrol, the main one being that dielsel is far less volatile than petrol it ignites at a much higher temperature which is required in a Compression ignition engine. Diesel behaves differently to petrol in extreme temperatures with diesel clumping up in the cold.

How does the fuel shut off solenoid work?
A fuel shut off solenoid works by shutting off the supply of fuel to the diesel engine when high temperatures or malfunctions are detected. This is to stop a diesel engine running accidentilly regardless of the throttle.

What is the four stroke cycle?
The four stroke cycle is the cycle of a piston in a four stroke internal combustion engine. The first stroke is intake, the piston travels from T.D.C to B.D.C this creates a vaccum, at the same time the intake valve opens and allows fuel and air mixture to enter the cylinder. The second stroke is compression, the intake valve closes and the piston rises compressing the air fuel mixture into the combustion chamber. The Third stroke is power, the compressed air fuel mix is ignited by a spark plug and expands the air in the chamber forcing the piston down. The last stroke is exhaust, the piston moves up towards T.D.C and the exhaust gas is removed out the open exhaust valve. The strokes are repeated.

What does S.I mean?
S.I means spark ignition. where the fuel in the combustion chamber is ignited by a spark plug.

What does C.I mean?
C.I means Compression ignition. Meaning as the air in the cylinder is compressed by the piston it becomes a very high temperature, fuel is then injected into thishot air and it ignites imediately.

When dismantling a carb what precaution should you take?

Always make sure that the float bowl is facing downwards, if turned upwards too much fuel can enter from the float bowl through the jets. and flood the carburettor. Always remember where the parts go! 

How does a bimetallic spring work in a automatic choke work?

The bimetallic spring is a temperture sensitive metal spring that is part of the automatic choke. The spring is attached to the choke butterfly valve. When the engine is cold the choke is on but as the engine warms up the spring unwinds and slowly opens the air intake butterfly valve.

What is the purpose of the venturi and how does it work?
The venturi is a narrowing in the body of the carburettor, it works by compressing the air that is flowing into the carburettor into a smaller space. Its purpose is to compress the incoming air and create a low pressure area around the fuel nozzle which allows fuel to come out.

An engine lacks power. What is likely to cause this problem?
It is likely a problem with fuel delivery, Not enough fuel going through the carburettor to the intake manifold, possibly a sticky valve or dirty fuel nozzle.

How do you adjust the float in a carburetor?
The float tabs on the inside of the float resivoir tank will need to be adjusted to the right height, so that the float is sitting at the right level. A Carburettor float should sit at 16mm

What is the purpose of a power valve?
The power valve is an extra valve which opens and adds more fuel to the circuit when the carburettor is using more fuel, like acceleration or loading.

What are some of the causes of high fuel consumption?
More fuel can be used when there is a problem with the choke like a sticking butterfly valve. Possibly the carburettor float is set at the wrong height which is allowing too much fuel into the combustion chamber, which in turn overfills the fuel nozzle using more petrol.

How is the the idle and mixture screw adjusted?
The idle mixture screw sets the amount of petrol being used when the car is at idle. The screw is simply turned to adjust the amount of petrol used at idle.

When refitting a three mutiple carburettor what should be done after installing them?
Make sure they are all opening together, and that the choke works on all three.