Page 1

9. CARBURETOR DISASSEMBLY Till now we have learnt to disassemble almost each and every major component of an engine. In previous lessons, we have seen how to inspect the various components of the engine so that we can decide which component needs repair or replacement. All these methods and processes are carried out to overhaul the engine so that it gives optimum performance. However other than those innumerable components, one major item which affects the performance of an engine is the carburetor. Carburetor is responsible for fuel supply to an engine in proper form and quantity. If the carburetor is not working properly, engine performance will be poor and also fuel average will be affected. In this lesson, we will see how to disassemble a carburetor to clean it and restore it to optimum performance.

9.0

Objectives

After going through this lesson, you will be able to: i. Recognize the different parts of a carburetor. ii. Understand the stepwise procedure of removing the various parts of a carburetor.

9.1

Introduction

In a petrol engine, a mixture of air and petrol is inducted into the engine cylinder during suction stroke. This mixture should have air and fuel in correct proportions and the quantity of the mixture should be according to the requirement of the engine. It is the function of the carburetor to prepare this mixture, with the correct ratio of air and petrol and to provide the correct quantity of mixture to the engine cylinders. We have seen the various systems of the carburetor and also studied its working in the lesson on fuel and exhaust systems. In this lesson, we will learn to disassemble the carburetor and understand its various systems and parts, thereby also revising what we have studied in the earlier lessons.

9.2

Dismounting the Carburetor

In the lesson on engine disassembly, the carburetor along with the inlet manifold was removed. The carburetor could be serviced then and fitted back during the engine reassembly. But sometimes a car requires only engine tuning and carburetor servicing at the workshop. In such a case, only the carburetor needs to be removed from the engine.


To remove the carburetor from the engine, the nuts on the air cleaner cap and bracket are opened first and then the breather hose is removed from the cylinder head cover. The positive crankcase ventilation (PCV) hose is removed from the air cleaner and lift off it. Next, we remove the accelerator, the choke cables, the vacuum Fig. 9.1: Dismounted carburetor. pipe at the vacuum advancer, the fuel inlet and the fuel return pipes and the solenoid valve coupler. After having removed all these connections, the bolts holding the carburetor to the manifold are loosened and the carburetor lifted off.

9.3

Components of Carburetor

Let us look at the various parts of the carburetor and some of its systems. The fuel from the tank which is under pressure from the pump enters the float chamber. A constant level of fuel is maintained in the float chamber for proper working of the carburetor by a needle valve. Fig. 9.2: Float chamber. After the level in the float chamber has been attained, the excess fuel comes out and after circulating around the accelerating pump, it leaves back for the fuel tank. This fuel return system ensures that relatively cold fuel is present in the float chamber and vapour locking is prevented. The accelerator cable is connected to the carburetor and it operates the primary throttle valve and controls the quantity of mixture supplied to the engine cylinders. The carburetor also has a choke Fig. 9.3: Choke valve. lever which is operated only when the engine is started from cold. A vacuum unit is provided for increasing the idle speed of the engine by 100 revolutions per minute. It is also known as the idle up actuator. It opens the primary throttle valve a little more than that opened by the accelerator pedal. A higher rpm from Fig. 9.4: Primary and secondary the engine is required for more (a) Sections. (b) Venturies. power to drive the air conditioner. There is another vacuum unit for operating the secondary throttle valve. The diaphragm unit helps to operate the secondary throttle valve after the primary throttle has opened about 40째 and the vacuum in the primary venturi reaches 7 mm of mercury column.


Fig. 9.3 shows the choke valve. When closed, the choke valve restricts the supply of air and a rich mixture is provided which is needed when starting the engine from cold. A choke is present only in the primary circuit because the air-fuel mixture during starting and at low speeds is provided by this circuit. The primary and secondary sections and the primary and secondary venturies can be seen in Fig. 9.4.

Fig. 9.5: Solenoid valve.

Fig. 9.5 shows the solenoid valve. It prevents engine run-on by blocking the slowspeed mixture passage when the ignition key is turned off. The passage opens again when ignition is turned on and the magnetic force pulls the needle valve back. The accelerator pump seen in Fig. 9.6 operates to give a spurt of fuel when the accelerator pedal is pressed suddenly and the primary throttle opens to let in a greater amount of air. This excess fuel prevents stalling of the engine and provides smooth acceleration. The idle mixture screw is shown in Fig. 9.7. It is set manually during the setting of the carburetor. It may be screwed in or out to decrease or increase the quantity of idle mixture supplied at low engine speeds. The idle speed adjusting screw controls the throttle opening to increase or decrease the quantity of air supplied.

Fig. 9.6: Accelerator pump.

Fig. 9.7: Idle mixture screw.

Fig. 9.8: Throttle valves. Two throttle valves – the primary and the secondary can be seen in Fig. 9.8. The primary throttle valve is smaller in diameter and is directly connected to the accelerator linkage. The secondary throttle opens only after the primary throttle has opened around 40° and helps improve the high speed and high load performance.

Self-check Questions 1. Give the function of a carburetor.


2. 3. 4. 5. 6.

Where does fuel first enter the carburetor? Fuel return system prevents ________________ The accelerator cable operates the ____________________ Choke is operated when _________________ Vacuum unit (idle up actuator) is provided for increasing the idle speed of the engine by _____________ when _______________. 7. The diaphragm unit helps to operate the _________________ after the primary throttle has opened about ______ 8. Choke is present in the ______________ only. 9. ____________ circuit provides air-fuel mixture during starting and at low speeds 10. ____________ prevents engine run-on by blocking the slow-speed mixture passage when the ignition key is turned off 11. The idle mixture screw controls the quantity of idle mixture supplied at ____________________. 12. The idle speed adjusting screw controls the throttle opening to increase or decrease the quantity of _______ supplied. 13. The primary throttle valve is ___________ in diameter

9.4

Disassembly of Carburetor

9.4.1 Vacuum Unit The vacuum unit is removed first. The tubes of the vacuum unit and the depression chamber are detached and then the depression chamber is removed. The diaphragm in the depression chamber is checked for proper working. When it is pushed up and the vacuum pipe is closed, it should remain in that position. If the diaphragm comes back, it indicates leakage of air and a defective diaphragm.

Fig. 9.9: Diaphragm checking.

9.4.2 Solenoid Valve The solenoid valve which is held with one screw is removed next. The needle valve or pin on the underside is provided to block the slow-mixture passage when the ignition is turned off.

Fig. 9.10: Float and gasket.

9.4.3 Top Cover of Carburetor The top cover is removed by unscrewing the 7 screws. The top cover has the air horn, over which the air cleaner is fitted. The carburetor gasket and the float are on the underside of the top cover.

Fig. 9.11: Accelerating pump nozzle.


9.4.4 Secondary Emulsion Tube The primary main jet and the secondary main jet supply fuel to the respective nozzles in the primary and secondary venturies. The emulsion tubes consist of an inner tube, which has small air bleed holes, and the outer tube opening into the nozzle. When the level of petrol is low in the tubes, air leaks through the air bleed holes and mixes with the petrol before being discharged in the venturi. Thus, the mixture is prevented from getting rich at high speeds. Fig. 9.11 shows the nozzle from the accelerating pump. It provides spurts of fuel during sudden acceleration so that the engine does not hesitate and the pick-up is smooth. The passage has a steel ball and a weight which function as the valve on the discharge side. 9.4.5 Throttle Body The linkage to the primary throttle is removed next. Then, the two screws are opened and the throttle body is separated from the main body of the carburetor. The throttle body houses the two throttle valves. 9.4.6 Idle Mixture Screw Fig. 9.12 shows the idle mixture screw being removed. The taper at the tip of the screw controls the slow mixture passage. When the screw is screwed in or out, the passage reduces or increases, controlling the quantity of mixture discharged through the idle discharge port.

Fig. 9.12: Removal of idle mixture screw.

9.4.7 Primary Emulsion Tube The primary emulsion tube is removed next. There are small bleed holes in the brass pipe through which air enters and dilutes the petrol that leaves the nozzle. 9.4.8 Jets

Fig. 9.13: Jets (a) Coasting fuel. (b) Secondary main. (c) Primary pilot.

The various jets of different sizes are removed. These are the coasting fuel jet, the primary main jet, the secondary main jet, the primary pilot jet and the secondary pilot jet. Three of these are shown in Fig. 9.13.


9.4.9 Accelerating Pump The accelerating pump is removed next. It is made up of several components – packing, plate with fuel return pipe, spring, diaphragm, and cover with pump lever and linkage. 9.4.10 Deceleration Control System The deceleration control system is removed by unscrewing the three screws. This system helps to overcome the problem of incomplete combustion when the throttle valve is suddenly closed and air required for that engine speed cannot be supplied. It supplies a proper air-fuel mixture to the intake manifold at that time. Fig. 9.14 shows the various components of the deceleration control system. These are the coasting valve, springs, diaphragm, cap and cover. 9.4.11 Float and Needle Valve

Fig. 9.14: Components of deceleration control system.

The float is removed by removing its fulcrum pin. The needle valve is placed below the float. The taper face of the needle should not be worn; otherwise, it leads to the problem of overflow.

Fig. 9.15: (a) Top cover. (b) Main body. (c) Throttle body.

Fig. 9.15 shows the three main sections of the carburetor – the top cover, the main body, and the throttle body. Self-check Questions 14. Air cleaner is fitted over the __________ of the carburetor. 15. The _______________ consists of an inner tube, which has small air bleed holes, and the outer tube opening into the nozzle. 16. Emulsion tube prevents the mixture from _____________________. 17. The ______________________ system helps to overcome the problem of incomplete combustion when the throttle valve is suddenly closed and air required for that engine speed cannot be supplied. 18. Name the 3 main parts of a carburetor.


9.5

Class Assignment

1. Discuss the role of carburetor in a petrol engine.

9.6

Summing Up

In this lesson, we have learnt the procedure of dismantling a carburetor and looked at its various components. The carburetor used in a Maruti 800 car engine has some special systems which were not used in earlier makes of cars. These systems are the fuel return system, which prevents vapour locking; the deceleration control system which helps the engine perform better at low speeds; and, the solenoid switch which prevents engine run on. Besides this is a double barrel carburetor. This carburetor has two venturies and two throttle valves. The primary ventury supplies the air-fuel mixture to the engine at low to medium speeds and both the venturies are used for supplying mixture at medium to high speeds. This not only improves engine performance at high speeds but also the fuel average of the engine. In the next lesson, we will see how to clean the carburetor. We should use carburetor cleaning solvent, or if it is not available then petrol, to clean the various components of the carburetor. All the passages in the carburetor are cleared with the help of compressed air. The carburetor can then be assembled and fitted back on the engine.

9.7

Possible Answers to Self-check Questions

1. Carburetor prepares the mixture of air and petrol in correct ratio and supplies it to the engine cylinder in the required quantity 2. Float chamber 3. vapour lock 4. primary throttle valve 5. starting a cold engine 6. 100 revolutions per minute, air conditioner is put ON 7. secondary throttle valve, 40O 8. primary circuit 9. Primary 10. Solenoid valve 11. low engine speeds 12. air 13. smaller 14. air horn 15. emulsion tube 16. getting rich at high speeds 17. deceleration control 18. top cover, the main body, and the throttle body


9.8

Terminal Questions

1. Give the procedure of carburetor dismounting. 2. List the parts of the carburetor and give the function of each.

9.9

References

1. Maruti 800 Service Manual

9.10 Glossary Air-fuel ratio

The ratio of air-to-gasoline by weight in the fuel mixture drawn into the engine.

Air horn

Collar at the mouth of the carburetor over which air cleaner is placed.

Choke valve

A choke is present in the primary circuit to provide airfuel mixture during starting and at low speeds

Engine stalling

Act of stopping of engine due to load / fuel starvation

Idle mixture screw

Screw used to maintain the idle mixture so that adequate fuel is supplied during idling

Idle speed

Rpm of engines at idling

Idle speed screw

Controls the quantity of mixture discharged through the idle discharge port.

Intake manifold

A casting of passages or set of pipes which connect the cylinders to the inlet source (air cleaner)

Jet

High pressure stream

PCV hose

Positive crankcase ventilation hose

Throttle body

A housing containing a valve to regulate the airflow through the intake manifold.

Throttle body

A housing containing a valve to regulate the airflow through the intake manifold.

Throttle valve

Butterfly valve to regulate air or mixture flow in the intake manifold

Vacuum

A space from which air has been partially or completely removed

Vacuum advance

A device which advances the ignition timing in response to increased engine vacuum.


Vapour lock

Vaporized fuel, usually in the fuel line, that prevents or retards the necessary fuel delivery to the cylinders

Venturi

Area of decreased cross section in the choke tube of the carburetor.

204-L-9-E  

9.2 Dismounting the Carburetor 9.0 Objectives 9.1 Introduction In the lesson on engine disassembly, the carburetor along with the inlet mani...