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Carburetion Systems

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 mixtureto the cylinder.

Carburetor A device for automatically mixing fuel in the proper proportion with air to

produce a combustible gas mixture.

Kinds of Air/Fuel Mixture

Optimum Mixtures

The chemically correct mixture of air and fuel, its ratio is 15 air to1 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 1 and13 to 1.


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The exhaust products of these rich mixtures normally have anexcess 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, butgives 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 andincomplete combustion.

Parts and Functions of the Carburetor

Main jet

Part of the main fuel supply circuit, the main jet is basically a screwwith a very accurately sized hole through it. This hole's size determinesthe maximum flow rate of fuel into the venturi. The carburetor uses themain jet's full flow capacity. It is the main passage of fuel from the floatbowl to the carb throat.

Idle mixture screw

Adjusts the flow of fuel through the idle jet at a constant air flow. Thisscrew controls the air/fuel mixture of the idle circuit from almost nofuel added(lean) to full capacity of the idle jet, according to the idle jetsize (rich, possibly).

Idle screw
http://www.mopedarmy.com/wiki/Main_jethttp://www.mopedarmy.com/wiki/Idle_mixture_screwhttp://www.mopedarmy.com/wiki/Leanhttp://www.mopedarmy.com/wiki/Richhttp://www.mopedarmy.com/wiki/Idle_screwhttp://www.mopedarmy.com/wiki/Main_jethttp://www.mopedarmy.com/wiki/Idle_mixture_screwhttp://www.mopedarmy.com/wiki/Leanhttp://www.mopedarmy.com/wiki/Richhttp://www.mopedarmy.com/wiki/Idle_screw


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Adjusts a slight amount of throttle slide displacement at closedthrottle. Controls idle engine speed.

Idle jet

Part of the carburetor's Idle circuit, the Idle jet controls the maximumamount of fuel that the idle circuit will supply. Sometimes, afterreplacing a carburetor with a more elaborate one, the idle jet might betoo rich to let the engine idle correctly.

Throttle

A spring loaded gate valve which is directly controlled by the twistablethrottle grip. The linear vertical displacement of this valve is equal tothe linear vertical displacement of the needle.

To regulate engine operating speed a throttle valve is employed torestrict air flow.

This is mounted just beyond or above the venturi When the throttle valve is fully opened the air flow is affected very

little.

When as the throttle is closed the flow of air is restricted to the intakepassage

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 theventuri is decreased.

Therefore the movement of fuel through the nozzle is reduced.

Float

Part of a float valve system that regulates flow into the float bowl, theFloat is usually a hollow plastic floating device inside the float bowl
http://www.mopedarmy.com/mediawiki/index.php?title=Idle_jet&action=edithttp://www.mopedarmy.com/wiki/Throttlehttp://www.mopedarmy.com/wiki/Floathttp://www.mopedarmy.com/mediawiki/index.php?title=Idle_jet&action=edithttp://www.mopedarmy.com/wiki/Throttlehttp://www.mopedarmy.com/wiki/Float


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attached to a pivoting hinge. As the fuel level in the float bowl varies,the vertical level (angular displacement) of this float changes as well.This movement controls the opening and closing of the float needle.

Float bowl

Stores fuel at atmospheric vapor pressure.

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 acentimeter if not smaller in length. It usually has a pin connection atone end and a rubber sealing element at the other end. As the floatbowl moves, the float needle moves linearly. Its rubber seal blocks anopening through which fuel flows from the gas tank. Many leakycarburetor problems are due to a damaged float needle.

Venturi

A venturi in the the carburetor increases the velocity of the incomingair.

When the air is forced through a restricted area, it must accelerate inorder to maintain the volume of flow.

This is a lot like a narrow space in between to buildings.
http://www.mopedarmy.com/wiki/Float_bowlhttp://www.mopedarmy.com/wiki/Float_bowlhttp://www.mopedarmy.com/wiki/Float_needlehttp://www.mopedarmy.com/wiki/Float_bowlhttp://www.mopedarmy.com/wiki/Float_needle


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An area in an air flow tube of a carburetor that restricts the flow of airthrough the tube resulting in a high velocity and low pressure at therestricted area.

Venturies may differ in their general configuration from one carburetorto another.

Fuel Filter

It filters the fuel going through the float chamber.

Choke Plate

It controls the air entering the carburetor for rich mixture whenstarting the cool engine.

CARBURETOR PROBLEMS AND SOLUTIONS

The following is a chart of potential carburetor problems and possible remedies. This chartis intended to be a helpful guide and is not complete. Each incident varies in accordance toindividual use. Regular inspection of carburetor parts is recommended.

PROBLEM POSSIBLE SOLUTION

Engine hunts (at idle or high speed)

Carburetor Leaks

Examine idle and main mixture adjustmentscrews and O-Rings for cracks and damage.

Carburetor out of adjustment

Engine will not start

Engine will not accelerate


Engine will not idle

Engine lacks power at high speed

Engine over speeds

Engine starves for fuel at high speed (leansout)

Adjust main mixture adjustment screw;some models require finger tight

adjustment



Engine will not accelerate


Adjust idle mixture screw


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Idle sped is excessive



CarburetorS floods Idle speed is excessive


Check sequence of gaskets and diaphragmsfor the particular carburetor being repaired




Carburetor floods

Idle speed is excessive

Check diaphragm for cracks or distortionand check nylon check ball for function if

available



Carburetor floods

Check float shaft for wear and float for leaksor dents




Carburetor floods


Adjust float settings if float type carburetor




Carburetor leaks

Engine over speeds

Adjust governor linkage

Carburetor leaks

Engine over speeds


Check sealing of Welch plugs, caps, plugsand gaskets

Carburetor floods

Engine starves for fuel at high speed(leans out)

Check inlet needle and seat for conditionand proper installation



Engine over speeds

Clean carburetor after removing all non-

metallic parts that are serviceable.


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Engine over speeds


Engine starves for fuel at high speed(leans out)

Adjust control cable or linage, to assure fullchoke and carburetor control




Check for bent choke and throttle plates

Carburetor Types

Suction Feed Carburetor

It is very similar to the float type. The one exception is there is no float to meter 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 valveand through the discharge holes.

The Diaphram carburetor

It has a rubber Diaphram exposed to the cylinder intake strokevacuum on one side and to atmospheric pressure on the other.

The Diaphram moves against the inlet needle allowing it to movefrom its seat.

A spring returns the needle to its seat when the vacuum stops.

Float Feed carburetors


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Locate their fuel tanks some distance from the carburetor.

The fuel flows either be gravity, or due to the force of a fuelpump, through fuel lines to the lower part of the carburetor.

The position of the float controls the fuel level in the bowl.

Kinds of Carburetor

Compound Barrel Differentially Operated Carburetor

This is a very efficient carburetor which has two barrel, the lowand high speed conflicting venture requirements can be mainlyovercome by having a pair of barrels.

The primary of these two barrels operates alone up to mediumspeed and the secondary barrel is phased in to assist the primary

barrel when the engine load and speed conditions necessitate theadditional supply of air.

Simple Barrel Carburetor

Has only one barrel, the major problem in this kind is that theventure-tube must be small enough to maintain an air speed thatwill support the air/fuel mixture but it restricts the speed inhigher level for it needs more mixture.

When the venture-tube is large it can not carry heavy loads.

Carburetor Adjustments

Carburetors need adjustment to meet climatic conditions of bothsea level and temperature variations.

Carburetors should be adjusted so it will not exhaust anunnecessary amount of unburned hydrocarbons or it should not beso 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 speed adjustment 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 inthe lower part of the nozzle, thus varying the amount of fuel thatcan be metered.


Adjustments of the main needle valve should be done beforeadjusting the idle valve.

Turn the main needle until the engine starts to flood.

Then quickly turn the valve in the opposite direction until theengine starts to miss from the lack of fuel.


Somewhere between these points will be the correct main needlevalve setting.

The idle valve should be adjusted next.

Additional engine parts closely related to the carburetor include:

Fuel Tank


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It is where the fuel is stored

Fuel Lines

Tubes connecting the fuel system

Air Cleaner

It filters the air that enters through the carburetor

It is important to prevent dirt and dust from being cared into theengine through the carburetor.

Governor

It is a speed control device; itsfunction on an engine is tomaintain a desired speed regardless of load.

It is a fixed throttle position, the engine will speed up or slowdown depending on the load applied

Diagrams for Illustration


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REVIEW QUESTIONS


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1. It is a device for automatically mixing fuel in the proper proportion with air toproduce a combustible gas mixture?

a. CARBURETOR c. CARBURETION

b. FUEL MIXER d. NONE OF THE ABOVE

2. It is a part of the main fuel supply circuit and its hole`s size determines themaximum flow rate of fuel into the venturi?

a. IDLE JET c. MAIN JET

b. JET SUPPLY d. POWER VALVE

3. This screw controls the air/fuel mixture of the idle circuit?

a. IDLE SCREW c. MIXTURE SCREW

b. IDLE MIXTURE SCREW d. CONTROL IDLE SCREW

4. It is use to adjust the amount of fuel mixture that flows through the venturiwhen the throttle is closed?

a. ADJUSTER SCREW c. MIXTURE SCREW

b. FUEL MIXTURE SREW d. IDLE SCREW

5. It is a spring loaded gate valve which regulates the engine operating speed,controlling the fuel through the intake manifold?

a. THROTTLE c. SPEED REGULATOR

b. CHOKE PLATE d. GOVERNOR

6. It stores fuel at atmospheric vapor pressure?

a. FUEL TANK c. BOWL TANK

b. FLOAT BOWL d. FUEL PRESSURE

7. It is a valve that controls the air entering the carburetor for rich mixturewhen starting the cool engine.

a. CHOKE PLATE c. AIR CONTROLER

b. AIR FILTER d. BUTTERFLY VALVES

8. Part of the float valve system that regulates the flow of fuel to the float bowlfrom the fuel tank, it close and open the inlet of the fuel?


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a. FLOAT c. FUEL REGULATOR

b. FLOAT NEEDLE d. VALVE OPENER

9. It filters the fuel going through the float chamber?

a. SCREEN FUEL c. CHAMBER CLEANER

b. FLOAT REGULATOR d. FUEL FILTER

10. It is an area in an air flow tube of a carburetor that restricts the flow of airthrough the tube resulting in a high velocity and low pressure at the restrictedarea?

a. VENTURI c. AIR TUBE

b. DISCHARGE NOZZLE d. PRESSURE TUBE

11. Its function on an engine is to maintain a desired speed regardless of load?

a. LOAD REGULATOR c. GOVERNORS

b. SPEEDOMETER d. ENGINE SPEEDER

12. It prevents the dirt and dust which can carry by air to the carburetor?

a. AIR CLEANER c. DIRT-DUST SHIELD

b. DUST PREVENTER d. VACCUM

13. What is the mixture that has the correct ratio of air and fuel for combustion?

a. OPTIMUM MIXTURE c. WEAK MIXTURE

b. RICH MIXTURE d. CORRECT MIXTURE

14. It is a carburetor that is very efficient to use because it can run in goodcondition in any speed at amount of any load because of its two barrel?

a. COMPUND BARREL DIFFERENTIALLY OPERATED CARBURETOR

b. DOUBLE CARBURETOR

c. COMPACT BARREL CARBURETOR

d. UNIQUE CARBURETOR


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15. It is like the float type carburetor but it does not have a float tocontrol the fuel in the fuel bowl instead depend on the difference inpressure between the tank and carburetor throat?

a. FLOAT FEED CABURETOR c. DIAPRAMCARBURETOR

b. SUCTION FEED CABURETOR d. NONE OF THEABOVE

16. An air-fuel mixture that gives fuel economy but causes overheatingand incomplete combustion?

a. OPTIMUM MIXTURE c. WEAK MIXTURE

b. RICH MIXTURE d. CORRECTMIXTURE

17. It is the process of mixing the fuel and air correctly for combustion?

a. CARBURETION c. CARBURETOR

b. FUEL-AIR MIXING d. MIXING PROCESS

18. What is the maximum ratio of air-fuel mixture for rich mixtures?

a. BETWEEN 17-1 AND 18-1 c. BETWEEN 15-1AND 13-1

b. BETWEEN 12-1 AND 13-1 d. BETWEEN 14-1AND 16-1

19. What is the system in the carburetor that supplies air-fuel mixture inthe engine when the butterfly valve or throttle valve is closed?

a. CARBURETION SYSTEM c. AIR-FUEL SYSTEM

b. IDLE SYSTEM d. ENGINESUPPLY SYSTEM

20. What is the size of venturi tube that restricts the speed of theengine?

a. LARGE c. EXTRA LARGE

b. MEDIUM d. SMALL SIZE


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1. Answer: CARBURETOR2. Answer: MAIN JET

3. Answer: IDLE MIXTURE SCREW

4. Answer: IDLE SCREW

5. Answer: THROTTLE

6. Answer: FLOAT BOWL

7. Answer: CHOKE PLAT

8. Answer: FLOAT NEEDLE

9. Answer: FUEL FILTER

10. Answer: VENTURI

11.Answer: GOVERNORS

12.Answer: AIR CLEANER

13.Answer: OPTIMUM MIXTURE

14.Answer: Compound Barrel Differentially Operated Carburetor

15.Answer: SUCTION FEED CABURETOR

16. Answer: WEAK MIXTURE

17. Answer: CARBURETION


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18. Answer: BETWEEN 12-1 AND 13-1

19. Answer: IDLE SYSTEM

20. Answer: SMALL SIZE

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