# Small Engine PowerPoint Presentation

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Carburetion. Ch 8. Explain theory of operation of a basic carburetor.. I.D. and understand the function of all carburetor parts. . Understand the effect of atmospheric pressure on the fuel system of a small gas engine.. ID: 460519

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Small EngineCarburetion

Ch 8

Explain theory of operation of a basic carburetor.I.D. and understand the function of all carburetor parts. Understand the effect of atmospheric pressure on the fuel system of a small gas engine.Identify 3 basic types of carburetors.

Chapter Objectives - Upon completion of this chapter you should be able to:

Revised 10/01

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Principles of Carburetion

The amount of air needed for combustion is far greater than the amount of fuel. One pound of air would take up a much greater space than one pound of fuel.Air-fuel by volume:One cubic foot of gas would have to be mixed with 9000 cu ft of air to be at a 15 to 1 weight ratio

Ch 8

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Principles of Carburetion

Primary purpose is to produce a mixture of fuel and air to operate an engine.Engines cannot run on “liquid” gasoline.Must be able to maintain air/fuel mixtureCold or hot startingIdlingPartial throttleAccelerationHigh Speed operation

Ch 8

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Principles of Carburetion

Ch 8

Air-fuel

mixture

1) 14.7-1 Ratio (

15 parts

air

to 1 part

fuel

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Carburetor Pressure Differences

A carburetor is a device that ……Vacuum- Any pressure less than atmospheric pressureAtmospheric Pressure- the pressure produced by the weight of air molecules above the earth. 1. 29.92 Hg

Ch 8

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Carburetor Pressure Differences

Ch 8

Elevation and its effect on Atmospheric Pressure

Hg-Mercury

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Carburetor Pressure Differences

Venturi PrincipleA restriction in a passage, which causes air to move faster. Pressure in the passageway is reduced3. Vacuum created by the decreased pressure which allows fuel to be drawn into the airstream

Ch 8

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Small EngineCarburetion

Ch 8

Venturi Principle

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Types of Carburetors

A. Natural Draft Carburetor 1. Used when there is little space on top of the engine 2. Air flows horizontally into the manifold

Ch 8

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Types of Carburetors

B. Updraft Carburetor

Ch 8

Placed low on the engine and uses gravity fed fuel supply

Air-fuel mixture must be forced upward

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Types of Carburetors

C. Downdraft Carburetor

Ch 8

Operates with lower air velocities and larger passages

Air-fuel must flow downward

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Small EngineCarburetion

Ch 8

Carburetor Draft Types

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Float-type Carburetors

Float SystemFloat-a small sealed vessel made of brass or plasticThe purpose of the float is to maintain a constant level of fuel in the float bowl3. As the fuel is used in the float bowl, a needle valve is unseated to allow more fuel in the bowl. As the float rises, the needle valve seals the line to shut off the fuel.

Ch 8

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Float-type Carburetors

B. Choke System 1. The choke is a round disc mounted at the intake end of the carburetor 2. When closed, the choke cuts off air to allow a richer (higher fuel to air ratio 7-1), air-fuel mixture,….

Ch 8

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Float-type Carburetors

Throttle System 1. The throttle is a round disc mounted beyond the main fuel nozzle 2. The purpose of the throttle is to regulate the amount of air-fuel mixture entering the cylinder(s). 3. Allows operator to vary engine speed

Ch 8

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Float-type Carburetors

Ch 8

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Ch 8

Small Engine

Carburetion

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Float-type Carburetors

Acceleration System 1. On some systems, when the throttle is opened quickly, a large amount of air passes through the carburetor. A pump is used to force fuel into the carburetor to allow proper air/fuel mixture and maintain running engine.

Ch 8

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Float-type Carburetors

Acceleration System 2. Acceleration Well is a reservoir of fuel, during idle fuel rises inside nozzle and flows into acceleration well. When throttle valve is opened quickly fuel rushes through the holes in nozzle. This provides a richer mixture under sudden need for power.

Ch 8

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Float-type Carburetors

Economizer Circuit 1. Designed to retard fuel to the engine during part throttle, by lower pressure in float bowl

Ch 8

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Float-type Carburetors

G. Idling Circuit 1. Throttle valve is closed 2. Fuel transported through an idle circuit pathwayThrottle valve closed-idle system supplies just enough air-fuel to keep engine running

Ch 8

Slide23

Float-type Carburetors

Part-Throttle, Full Throttle Sequence 1. Other circuits outside of idling speed that have secondary discharge holes opened for more air/fuel mixture to open.

Ch 8

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Float-type Carburetors

G. Primer 1. Hand operated plunger 2. Forces addition fuel up the nozzle and/or pumps air pressure onto the float bowl which forces fuel into nozzle

Ch 8

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Ch 8

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Diaphragm-type Carburetors

Does not contain a float systemAtmospheric pressure and the vacuum created in the engine pulsates a flexible diaphragmThe diaphragm draws fuel into the chamber of the carburetor from which it is readily drawn into the venture

Ch 8

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Ch 8

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Negative pressure pulses from crankcase

Fuel pushed to carburetor

Fuel pulled from tank

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Small EngineCarburetion

Ch 8

Carburetion Fuel Systems

Suction Type

B&S = (Vacu-jet)

Pump TypeB&S = (Pulsa-jet)

Gravity Feed TypeB&S = (Flo-jet)

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Small EngineCarburetion

Ch 8

Carburetor Components and Operation

Slide34

Small EngineCarburetion

Ch 8

Old style carburetor

with a fixed Jet

(Jet must be replaced to for altitude adjustment)

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Small EngineCarburetion

Ch 8

When performing an altitude adjustment the jet is removed and replaced with a jet that has a smaller orifice.

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Small EngineCarburetion

Ch 8

Main Jet Location

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Small Engine

Carburetion

Ch 8

Main Jet Location

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Small Engine

Carburetion

Ch 8

Main Jet Location

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Small Engine

Carburetion

Ch 8

Main Jet Location

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A

A

B

C

D

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Ch 8

Small Engine

Carburetion

What other components are related to carburetion???

Slide42

Ch 8

Determine proper Air Fuel Ratio under load

by sparkplug coloration

Small EngineCarburetion

Slide43

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Small EngineCarburetor

Ch 8

Pneumatic Governor

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Small EngineCarburetion

Ch 8

Mechanical Governor

Slide46

Mechanical Governor Operation

Slide47

Mechanical Governor Operation

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Mechanical Governor Operation

Slide49

Small EngineCarburetion

Ch 8

Air Cleaner

Dry Element

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