CarburetionSystems

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Carburetion Systems


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Carburetion

Functions of the carburetion system are:

To mix the fuel with the proper proportion

of air. To vaporize the fuel

To deliver the correct amount of the air-fuel

mixture to the cylinder.

A carburetor may be defined as:

A device for automatically mixing fuel in the

proper proportion with air to produce a

combustible gas mixture.


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Kinds of Air/Fuel Mixture Optimum Mixtures

The chemically correct mixture of air and fuel, its ratio is 15 air

to 1 part petrol by mass.

Rich mixtures

Mixtures containing more than the optimum amount of petrol,

usually produce more power than optimum and weak mixture

Its maximum mixture has an air-fuel ratio of between 12 to 1and 13 to 1.

The exhaust products of these rich mixtures normally have an

excess of carbon monoxide and have dark cloudy exhaust

smoke.

Weak Mixtures

Mixtures containing less than the optimum amount of petrol,

usually produce less power than the optimum and rich ones,

but gives fuel economy.

Its mixture has an air-fuel ratio of between 17 to 1 and 18 to 1.

Burning is generally slow and misfiring, overheating and

incomplete combustion.3


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parts of the carburetor

Main jet

Part of the main fuel supply circuit, the

main jet is basically a screw with a veryaccurately sized hole through it. This hole's

size determines the maximum flow rate of

fuel into the venturi. The carburetor uses

the main jet's full flow capacity. It is themain passage of fuel from the float bowl to

the carb throat.

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Idle mixture screw

Adjusts the flow of fuel through the idle jetat a constant air flow. This screw controls

the air/fuel mixture of the idle circuit from

almost no fuel added(lean) to full capacity

of the idle jet, according to the idle jet size(rich, possibly).

Idle screw

Adjusts a slight amount of throttle slidedisplacement at closed throttle. Controls

idle engine speed.

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Idle jet

Part of the carburetor's Idle circuit, the Idle

jet controls the maximum amount of fuel

that the idle circuit will supply. Sometimes,

after replacing a carburetor with a more

elaborate one, the idle jet might be too rich

to let the engine idle correctly.

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Throttle

A spring loaded gate valve which is directly

controlled by the twistable throttle grip. Thelinear vertical displacement of this valve is

equal to the linear vertical displacement of the

needle.

To regulate engine operating speed a throttlevalve is employed to restrict air flow.

This is mounted just beyond or above the

venturi

When the throttle valve is fully opened the airflow is affected very little.

When as the throttle is closed the flow of air is

restricted to the intake passage

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Throttle This decreases the speed and power of the

engine.

At the same time it allows the pressure in

the venturi area to increase.

The difference between the air pressure in

the fuel chamber and the venturi is

decreased. Therefore the movement of fuel through

the nozzle is reduced.


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Float

Part of a float valve system that regulatesflow into the float bowl, the Float is usually

a hollow plastic floating device inside the

float bowl attached to a pivoting hinge. As

the fuel level in the float bowl varies, thevertical level (angular displacement) of this

float changes as well. This movement

controls the opening and closing of the

float needle.

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Float bowl

Stores fuel at atmospheric vapor pressure.

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Float needle

Part of a float valve system that

regulates flow into the float bowl, theFloat needle is a two force compression

member which is about a centimeter if

not smaller in length. It usually has a pin

connection at one end and a rubber

sealing element at the other end. As the

float bowl moves, the float needle

moves linearly. Its rubber seal blocks anopening through which fuel flows from

the gas tank. Many leaky carburetor

problems are due to a damaged float19


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Venturi

A venturi in the carburetor increases the

velocity of the incoming air.

When the air is forced through a restricted

area, it must accelerate in order tomaintain the volume of flow.

This is a lot like a narrow space in between

to buildings.


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An area in an air flow tube of a carburetor

that restricts the flow of air through the

tube resulting in a high velocity and low

pressure at the restricted area.

Venturies may differ in their generalconfiguration from one carburetor to

another.


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Fuel Filter

It filters the fuel going through the floatchamber.

Choke Plate

It controls the air entering thecarburetor for rich mixture when

starting the cool engine.

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Venturi and Air Flow

Air flow through the venturi area of the

carburetor is due to the movement of

the piston. The pressure in the intake passage is

decreased.

Thus creating a low pressure conditionin the carburetor venturi area.


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Carburetor Types

All carburetors used on small engines

are basically the same.

Carburetors can be classified into three

kinds:

Float Feed

Suction Feed

Diaphram

The difference between these is the

way the fuel is supplied to the fuel

chamber.


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Carburetor Types

Float Feed carburetors locate their fuel

tanks some distance from the

carburetor. The fuel flows either be gravity, or due

to the force of a fuel pump, through fuel

lines to the lower part of the carburetor.

The position of the float controls the fuel

level in the bowl.


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Carburetor Types

Suction Feed Carburetoris very similar

to the float type.

The one exception is there is no float tometer and control the level of fuel in the

fuel chamber.

The difference in pressure between the

tank and the carburetor throat lifts the

fuel up the fuel pipe past the main

needle valve and through the discharge

holes.


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Carburetor Types

The Diaphram carburetorhas a rubber

Diaphram exposed to the cylinder intake

stroke vacuum on one side and toatmospheric pressure on the other.

The Diaphram moves against the inlet

needle allowing it to move from its seat.

A spring returns the needle to its seat

when the vacuum stops.


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CARBURETOR PROBLEMS AND

SOLUTIONS

The following is a chart of potential

carburetor problems and possible

remedies. This chart is intended to be ahelpfule guide and is not complete. Each

incident varies in accordance to

individual use. Regular inspection of

carburetor parts is recommended.

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PROBLEM POSSIBLE SOLUTION

Engine hunts (at idle or high speed)

Carburetor Leaks

Examine idle and main mixture

adjustment screws and O-Rings for

cracks and damage.

Check sealing of welch plugs, caps,

plugs and gaskets

Carburetor out of adjustment

Engine will not start

Engine will not accelerate

Engine hunts (at idle or high speed)Engine will not idle

Engine lacks power at high speed

Engine over speeds

Engine starves for fuel at high speed

(leans out)

Adjust main mixture adjustment screw;

some models require finger tight

adjustment

Adjust control cable or linage, to

assure full choke and carburetor

control

Clean carburetor after removing all

non-metallic parts that are serviceable.Engine will not start

Engine will not accelerate

Engine hunts (at idle or high speed)

Engine will not idle


Idle sped is excessive

Adjust idle mixture screw

Adjust float settings if float type

carburetor


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Carburetor floods

Engine starves for fuel at high

speed (leans out)

Check inlet needle and seat for

condition and proper installation

Check float shaft for wear and float

for leaks or dents

Carburetor out of adjustment

Engine will not idle

Idle speed is excessive

Check for bent choke and throttle

plates

Engine will not startEngine will not idle


Carburetor floods

Idle speed is excessive

Check diaphragm for cracks ordistortion and check nylon check

ball for function if available

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Carburetor Adjustments

Carburetors need adjustment to meet

climatic conditions of both sea level and

temperature variations. Carburetors should be adjusted so it will

not exhaust an unnecessary amount of

unburned hydrocarbons or it should not

be so lean that it will give unsatisfactory

power performance.


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Idle and Main Needle Valve

Adjustments

The main needle valve adjustment is

has many different names.

Power load adjustment, high speedadjustment screw, main jet, etc

The valve is usually turned by hand or

by a screwdriver.

The turning moves the needles tapered

point in and out of a seat in the lower

part of the nozzle, thus varying the

amount of fuel that can be metered.


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Adjustments

Adjustments of the main needle valve

should be done before adjusting the idle

valve. Turn the main needle until the engine

starts to flood.

Then quickly turn the valve in the

opposite direction until the engine starts

to miss from the lack of fuel.


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Adjustments

Somewhere between these points will

be the correct main needle valve

setting. The idle valve should be adjusted next.


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Chokes

Starting an engine when it is cold

requires a richer air-fuel mixture than

when it is warm.

The choke regulates the pressure in the

venturi, thereby increasing the flow of

fuel from the discharge nozzles.


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Carburetion

Additional engine parts closely related

to the carburetor include:

The fuel tank, fuel lines, air cleaner, choke,and speed control devices (governors).


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Governors

The function of the governor on an

engine is to maintain a desired speed

regardless of load.

It is a fixed throttle position, the engine

will speed up or slow down depending

on the load applied.


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Governors

The two most common governor

systems are the air vane type and the

mechanical type.

The air vane governor is operated by

the force of air currents away from the

flywheel fins.

This force and movement of the air on

the air vane opens and closes the

throttle valve.


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Governors

Mechanical governors work in a manner

similar to the air vane type except

centrifugal weights oppose the governor

spring rather than an air vane.


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Air Cleaners

It is important to prevent dirt and dust

from being cared into the engine

through the carburetor.

If the air cleaner is operating improperly

dirt will enter the combustion chamber,

causing excessive wear of the rings,

cylinder walls and other moving parts.


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Air Cleaners

There are several different types of air

cleaners used.

The oil bath type is the oldest type. All air passes through the oil and oil

soaked mesh before entering the

carburetor.

Dust and dirt are removed from the air

and accumulates as sediment in the

bottom of the oil cup.


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Air Cleaners

Oil saturated type of air cleaner,

contains foam or fine wire mesh which

is saturated in oil.

A third type of air cleaner is the dry

element type.

Commonly found on larger engines.


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SG

The SG governor hydraulic speed droop type

governor is designed for use on small diesel,gas, or gasoline engines where isochronous

control is not required. The design of the

speed droop governor is such that the

governor operates at a slower speed asengine load increases. It is through this

characteristic that stability of the system is

achieved and division of load between

paralleled units is made possible.

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CarburetionSystems

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