carburator unit 2
TRANSCRIPT
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Carburetor Theory
Its all due to Air Pressure
Close to sea level pressure is 14.7 psiAir has weight88 lbs in a 12x12x8 ft
room
Vacuum is a pressure less than 14.7 psiOften measured in inches of
mercury 14.7 psi ~ 30 in Hg
As engine runs, intake strokes create vacuum or lower air pressure in
manifolNormal ~10 psi (~20 in Hg)
With throttle plate open, carburetor throat exposed to manifold pressure
Carburetors operate on the venturi effect
The venturi is a narrowing of the bore
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Cont
What causes air flow through carburetor?
Intake stroke of piston creates vacuum Intake valve open, transmits vacuum to
throttle plate
Position of throttle plate determines air flow
Closed- no flow- high manifold vacuum
Openfull flowlow manifold vacuum
Air (at ~ atmospheric pressure) flows from air cleaner side,through venturi, past
throttle plate, through manifold and
intake valve, into cylinder
Model A running at 975 rpm flows about 70 cfm (cubic feet per
minute)
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Cont..
As air flows through venturi, pressure decreases in venturi
Bernoullis Law tells us as Area decreases, velocity increases and
As velocity increases, pressure decreases
Air pressure on fuel in bowl is always ~atmospheric
As pressure difference between 1) fuel in bowl and
2) at tip of nozzle (located in venturi) increases, fuel flow increases from nozzle
Throttle opens, more air flow, greater P, more fuelflow
Throttle closes, less air flow, less P, less fuel flow
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Cont
Important factors
Amount of vacuum created by intake stroke
Less vacuum if
Intake valve guides leak air
Exhaust valve leaks air
Piston rings leak air
Manifold gasket leaks air
Position of throttle plate
Determines air flow through carburetor
Determines difference in pressure on fuel in bowl and at
tip of nozzle in venturi
Greater differencemore fuel flow
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Basic carburetor consists of the following
parts
1-Carburetor body
2-Air horn
3-Throttle valve
4-Ventur
5-Main discharge tube
6-Fuel bowl
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2-Air horn
The air horn is also called the throat or barrel.
The parts which often fasten to the air horn body
are as follows: the choke, the hot idle
compensator, the fast idle linkage rod, the choke
vacuum break, and sometimes the float and
pump mechanisms.
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3-Throttle valve
This disc-shaped valve
controls air flow through
the air horn.
When closed, it restricts
the flow of air and fuel into
the engine, and when
opened, air flow, fuel flow,
and engine power increase.
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Carburetor size is stated in CFM (cubic feet of
air per minute).
This is the amount of air that can flow through
the carburetor at wide, open throttle.
CPM is an indication of the maximum air
flow capacity. Usually, small CPM carburetorsare more fuel-efficient than larger carburetors.
Air velocity, fuel mixing, and atomization are
better with small throttle bores. A larger CPMrating is desirable for high engine power
output
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The seven basic carburetor systems are
the following:
1. Float system
2. Idle system3. Off idle system
4. Acceleration system
5. High-speed system
6. Full-power system