10. CARBURETOR REASSEMBLY In the last lesson we studied the process of dismantling the carburetor. As indicated earlier, a carburetor is disassembled to clean and repair it to get optimum performance out of it which results in smooth operation of an engine and better fuel average. In this lesson we will see the method of cleaning the different parts of carburetor and then fitting them again to reassemble the carburetor.
10.0 Objectives After going through this lesson, you will be able to: i. Understand the method of cleaning the various components of a carburetor. ii. Understand the stepwise procedure of reassembling a carburetor.
10.1 Introduction In the last lesson, the carburetor was dismantled and its various systems were studied. In this lesson, we will learn about washing the parts of the carburetor with petrol and cleaning the passages and jets with compressed air. It is important that pins or wire are never used to clean the jets or passages, as they spoil the metering characteristics of the carburetor. Also, jets of correct size should be used to replace those that are worn. The carburetor is the heart of the fuel system and optimum performance of the engine depends to a large extent on proper setting of the carburetor. Carburetor setting will be covered under engine tuning in Lesson 6 of course CAF 205.
10.2 Cleaning and Drying of Parts After disassembling the carburetor, its various parts are first washed with proprietary cleaning solvent or petrol and dried with compressed air. Thinner should never be used to clean the carburetor. You should take care not to immerse the fuel Fig. 10.1: (a) Washing of parts using petrol. (b) Blowing compressed air for cleaning and drying. cut solenoid valve, depression chamber and the idle up actuator, also known as the vacuum unit, in petrol. Then compressed air is blown into all air holes, fuel jets and passages to clear them. Wires or pins should not be used.
10.3 Assembly of Carburetor Let us now study the procedure for the assembly of the carburetor and look at the order in which the components are fixed. The following sequence should be followed: • • •
Fix the primary main jet in the float chamber. Screw in the secondary main jet in the float chamber. Place the accelerating pump components in the proper sequence and fix with the screws.
Fig. 10.3 shows a magnified view of the other side, focusing on the fuel return pipe which carries the fuel back to the fuel filler neck in the tank after circulating it around the accelerating pump. • • • • •
Fig. 10.2: Accelerating pump.
Fig. 10.3: Fuel return pipe.
Fit the components of the deceleration control system in proper sequence and fix the screws. Fit the coasting fuel jet. Fit the primary pilot jet by pressing it in place and then fixing the screw over the plate to hold the jet in place. Screw in the secondary pilot jet. Turn over the main body of the carburetor and place the packing onto it before mounting the throttle body.
Make sure that holes in the main body, packing, and throttle body match properly. • •
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Fix the throttle body to the main body with the help of screws. Fix the linkage of the accelerating pump to the throttle shaft of the primary throttle valve. Fix the idle mixture screw.
Fig. 10.4: Mounting throttle body.
Even though the idle mixture screw may be fully screwed in at this time, it will be turned and screwed according to need when the carburetor is being set. The setting of the carburetor is done with the help of the two screws – the idle mixture screw and the idle speed screw.
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Fix the primary emulsion tube assembly in place by tapping it lightly with a plastic hammer. Similarly, fix the secondary emulsion tube in place.
The primary emulsion tube assembly has two main air holes whereas the secondary emulsion tube assembly has one main air hole. Check that both the emulsion tube assemblies have been properly fitted. • •
Place packing on the top surface of the main body of the carburetor. Place the needle valve in position in the float and then anchor the float in the top cover with the help of the anchor pin.
The float moves up or down depending on the level of fuel in the float chamber and operates the needle valve to close or open the fuel inlet passage. •
Measure the float height in order to ensure that the fuel level in the float chamber is properly maintained.
To measure the float height, the cover is inverted; the float is held up hand and lowered slowly. Just when tongue touches the needle valve, the distance between the bottom of the float and the face of the top cover is measured. This measurement should be between 31 and 32 mm. If the float height is out of specification, it should be adjusted by inserting shims between the valve seat and the upper cover.
Fig. 10.5: Measuring float height.
Fig. 10.6: Mounting upper cover.
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Place the steel ball and weight of the discharge valve of the accelerating pump. Place the packing on the top surface of the main body of the carburetor and fix the upper cover with the help of screws. Fix the rod of the depression chamber in the shaft of the secondary throttle valve and fit the chamber bracket to the carburetor top cover with the help of screws.
Fig. 10.7: Fixing depression chamber.
The depression chamber helps open the secondary throttle valve only after the primary throttle has opened by about 40°. Fig. 10.8: Fixing solenoid valve.
The accelerator pedal movement is transmitted only to the primary throttle. •
Fix the link rod between the choke lever and the choke shaft in the upper cover.
When the choke knob is pulled, the choke wire pulls the choke lever which not only rotates the choke shaft and closes the choke but also opens the primary venturi slightly through the choke lever which presses on to the primary throttle shaft lever for the rich mixture to pass. • •
Fit the solenoid in place and hold it to the carburetor body with a screw along with the earth wire of the solenoid. Fit the vacuum unit to the upper cover with the help of screws and fix its rod in the primary throttle shaft lever.
The vacuum unit helps increase the speed of the engine by 100 rpm when the air conditioner is put on. It opens the primary throttle valve slightly more over that opened by the accelerator pedal. The higher rpm is required to get more power from the engine to drive the air conditioner. The vacuum unit works on the vacuum of the inlet manifold which is applied through a switch when the air conditioner is put on.
Fig. 10.9: Fixing vacuum unit.
10.5 Mounting of Carburetor • • • • • • •
Mount the carburetor on the inlet manifold in the car. Fix the fuel inlet pipe and the fuel return pipe at their proper places. Fix the fuel cut solenoid valve coupler and the vacuum tubes of the idle up actuator and the vacuum advancer. Fix the accelerator and choke cables. Install the air cleaner on the air horn. Tighten the nuts on the air cleaner cap and the bracket at the cylinder head. Fix the breather hose at the cylinder head cover and the PCV hose at the air cleaner This completes carburetor assembly and re-mounting.
Self-check Questions 1. How are carburetor parts cleaned? 2. Fuel cut solenoid valve, depression chamber, and the idle up actuator should be thoroughly cleaned with petrol. State True or False. 3. How would you recognize the primary and secondary emulsion tubes?
4. The depression chamber helps open the secondary throttle valve only after _____________________________________째. 5. The accelerator pedal movement is transmitted only to the ____________. 6. The vacuum unit helps increase the speed of the engine by 100 rpm when _____________________ by __________________________________. 7. The higher rpm provided by the vacuum unit is required to get more power from the engine to ______________________________-. 8. The vacuum unit works on the ____________________ which is applied through a switch when ______________________________. 9. _________________ carries the fuel back to the fuel filler neck in the tank. 10. The setting of the carburetor is done with the help of _________________ and ____________________. 11. Carburetor is mounted on the ______________ in the car.
10.6 Class Assignment 1. Discuss how carburetor parts are cleaned and why so?
10.7 Summing Up We know now how a carburetor is assembled and fitted onto the vehicle Proper setting of the carburetor is done in a running engine but that alone is not sufficient to get the best performance from the engine. For optimal working, the engine needs to be tuned. This we will study in lesson 6 of course CAF 205.
10.8 Possible Answers to Self-check Questions 1. By washing with proprietary cleaning solvent or petrol and drying with compressed air 2. False. 3. The primary emulsion tube assembly has two main air holes whereas the secondary emulsion tube assembly has one main air hole 4. the primary throttle has opened by about 40째 5. primary throttle 6. the air conditioner is switched on, opening the primary throttle valve slightly more over that opened by the accelerator pedal 7. drive the air conditioner 8. vacuum of the inlet manifold, the air conditioner is switched on 9. Fuel return pipe 10. idle mixture screw, the idle speed screw 11. inlet manifold
10.9 Terminal Questions
1. Describe the method of measuring the float height. 2. Describe the stepwise method of reassembling a carburetor. 3. Describe the steps in re-mounting a carburetor on the engine.
10.10 References 1. Maruti 800 Service Manual.
10.11 Suggested Further Reading 1. http://Autozone.com
10.12 Glossary Choke
The valve/plate that restricts the amount of air entering an engine on the induction stroke, thereby enriching the air:fuel ratio
Choke knob
A choke is present in the primary circuit to provide airfuel mixture during starting and at low speeds
Decelerating control system
System which provides sufficient mixture in the manifold, for proper continued combustion, when the accelerator pedal is released.
Idle mixture screw
Screw used to maintain the idle mixture so that adequate fuel is supplied during idling
Idle speed screw
Controls the quantity of mixture discharged through the idle discharge port.
Proprietary cleaning solvent
Solvent specified by the manufacturer to clean the carburetor
Solenoid
An electromagnetic switch that uses a movable core
Thinner
A general cleaning agent which should never be used to clean the carburetor
Throttle body
A housing containing a valve to regulate the airflow through the intake manifold.