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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