The Carburettor

Question 1

Purpose of the Carburettor

Answer 1

To mix fuel and air in the correct ratio and deliver it to the cylinders

Question 2

Float Chamber

Answer 2

A fuel reservoir into which fuel is fed from the tank
A vent allows the pressure in the float to remain atmospheric
Fuel flow into the chamber is controlled by a needle valve attached to a float
Normally, the position of the valve allows the fuel flow into the chamber to equal the fuel flow out to ensure an exact level and pressure is maintained at the metering jet

Question 3

Venturi

Answer 3

Air accelerates through the venturi, creating a drop in static pressure
As the pressure in the float is atmospheric, fuel will flow to the area of lower pressure, the venturi

Question 4

Air Bleed

Answer 4

Introduces air directly into the fuel discharge line to atomise the fuel, as it approaches the main discharge nozzle as the fuel is required to be pure vapour

Question 5

Throttle

Answer 5

Controls the throttle butterfly
When the butterfly is open, the airflow through the venturi accelerates, causing lower static pressure
The pressure differential between the float chamber and the venturi will increase and more fuel will flow into the carburettor

Question 6

Accelerator Pump

Answer 6

When the throttle is opened there is a temporarily lean mixture as the airflow increases
The pump sends a surge of fuel to the discharge nozzle to provide a quick source of power when you need it most

Question 7

Idling Jet

Answer 7

Has an inlet near the butterfly valve, where a venturi effect is caused when the valve is almost closed, providing sufficient fuel to keep the engine running at low RPM
The pressure differential between the venturi and float chamber is too low to force fuel through the main metering jet at low RPM

Question 8

Mixture control

Answer 8

Controls the amount of fuel which flows out of the float chamber, by controlling the opening and closing of the metering jet

Question 9

Carburettor Icing

Answer 9

As the fuel evaporates it cools the surrounding surfaces
Any water vapour will condense on to metal
If the temp drops below 0 degrees celcius water forms ice

Question 10

Symptoms of Carburettor Icing

Answer 10

A drop in RPM creates rough running
Low manifold pressure for a variable pitch propellor, but no RPM drop
Backfiring may result, followed by engine failure

Question 11

Conditions conducive to Carburettor Icing

Answer 11

Can occur with temps below 20 degrees celcius and relative humidity greater than 80%
When using low power settings for a long time

Question 12

Types of Carburettor Icing

Answer 12

Impact
Fuel
Throttle

Question 13

Impact Icing

Answer 13

Supercooled water droplets (-40 degrees celcius) hit metal surfaces
Same as airframe icing

Question 14

Fuel Icing

Answer 14

Fuel vaporises in the manifold, absorbing latent heat

Air in manifold may cool below 0 degrees celcius, and water vapour may condense and freeze

Question 15

Throttle Icing

Answer 15

The venturi effect at the throttle butterfly causes a drop in temperature, and may result in icing on the throttle
Increased likelihood at low power settings, when the venturi is almost closed

Question 16

Carburettor Heat

Answer 16

To prevent carburettor ice
Warm air, from a duct next to the exhaust pipe, is less dense creating a richer mixture and a small RPM drop
The engine may run very rough after application as the melted ice enters the cylinders
The air is unfiltered