Page 124 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

4 Malfunction of needle valve. These include loose fitting of the needle valve seat due to wear or sticking
during its up-and-down movement. Due to this, the function of stopping the petrol supply is not properly
done by the needle valve and float Petrol level increases in the chamber and petrol starts leaking.
5 Supply of main jet, since the petrol level in the float chamber is up to the main jet, 3. If the main jet is loose,
petrol starts flowing through it.
6 Exceeding fuel pump pressure. If the diaphragm return stroke in the fuel pump is too hard, the fuel pump
will supply more fuel to the carburetor and the fuel level in the float chamber will be higher than normal,
causing fuel leakage.
b Decrease in petrol supply to engine at slow speed
1 Having the pilot jet choked up.
2 Having the hole choke up where the volume control screw is fitted.
3 The gasket below the carburetor is broken or damaged, causing air to be drawn into the suction from that
point.
4 Keeping the slow speed adjusting screw tight causes the throttle valve to open too wide and draw in too
much air.
5 If the inlet manifold is loosely seated, air is drawn from the side of the inlet manifold.
c supply of lean mixture to engine at high speed
If the engine is supplied with a lean (lean or weak) mixture at high engine speeds, the carburetor mouth
mixture fires. This is called “popping-up” or back firing. Poor mixture supply to the engine at high speed can be
due to the following reasons -
1 If the fuel pump diaphragm is weakened or due to other defects in the fuel pump and if the fuel pipe is
choked up, the petrol supply to the carburettor is not proper.
2 Float level should be slightly adjusted. Therefore, the level of petrol in the float chamber remains low.
3 Choking up of orifice of main jet or emulsion tube.

d Supply of rich mixture at high engine speed
1 High pressure fuel supply from fuel pump.
2 Float level setting too high.
3 Float chamber cover loose.
4 Enlarged main jet orifice,

5 Choke valve partially closed.
6 Inadequate air supply due to air cleaner choke up.
e When the engine speed is increased, the engine does not accelerate quickly
When the engine speed is increased, it decreases without increasing the engine speed first happens This
tendency of the engine is called “Flat Spot” in the carburetor.
This error can occur due to the following reasons.
1 Defective accelerator pump diaphragm assembly.
2 Having non return valve or pump jet choke up.

3 Having pump injector choke up.
4 Improper setting of pump control rod.
f Starting the engine and stopping immediately
1 Containing garbage in petrol or having petrol mixed with water.
2 Malfunction of throttle if there is play in throttle shaft bush. Also throttle not opening properly due to
accelerator linkage not set properly.

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CITS : Automotive - Mechanic Diesel - Lesson 29 & 30 CITS : Automotive - Mechanic Diesel - Lesson 29 & 30