Carburettors Flashcards
The pressure balance duct ensures …
correct mixture ratio is maintained across a range of airspeeds
If it wasn’t there the pressure increase in the air inlet due to the ram affect would cause less fuel from to spray from the nozzle and a progressively weaker mixture with increasing airspeed
The Diffuser is used to …
It consists of …
maintain the correct mixture ratio across a wide range of engine speeds. As engine speed increases, doubling the mass flow through the throat would cause a fourfold drop in pressure and a correspondingly large addition of fuel, resulting in an excessively rich mixture
finely perforated tube immediately after the main jet
When the engine speed increases, the fuel level in the diffuser falls because of the slight restriction caused by the jet. This uncovers some of the perforations in the diffuser allowing air in from the pressure balance duct to flow in. this reduces the pressure at the nozzle so reducing the fuel flow
The higher the engine speed, the more the fuel level drops in the diffuser and the more air is allowed in to reduce the amount of fuel drawn through the nozzle
What is the slow running jet used for?
take advantage of the strong pressure drop across the butterfly valve a provide a rich mixture at idle
(At very small throttle openings the pressure drop through the venturi is negligible and not enough to draw fuel through the nozzle)
What is the power jet used for?
provides a rich mixture at high power settings to assist in cylinder cooling
At high throttle settings the cam forces down a piston which allows fuel to flow to the power jet
What is the accelerator pump used for?
When the throttle is opened rapidly the air flow to the engine increases almost instantaneously but there is a slight delay before reduced pressure in the throat causes more fuel to flow from the nozzle. This can lead to the engine stopping – known as weak or lean cut. The solution is an accelerator pump
The throttle lever is connected to a small piston type pump in the carb so when the throttle lever is moved rapidly forward the accelerator pump injects an amount of fuel directly in to the venturi
