Page 112 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

When the throttle is closed, a lot of vacuum is created below the throttle and inside the manifold. The jet or
discharge port of the idle circuit remains below the throttle valve. As mentioned above, there is a lot of vacuum
below the throttle valve and since this port is below the throttle valve, due to the vacuum on it, petrol keeps getting
drawn from the chamber and goes into the carburetor. And even when the throttle is slightly open, the engine
keeps running at idle speed.
There is an adjusting screw to open the idle port or jet more, so that we can increase or decrease the amount of
petrol, or adjust it at idle speed.
Sometimes a hole is made in the idle circuit which opens above the venture, so that air is drawn in along with the
petrol and is expelled by the jet. This is called air-bleeding of the carburetor
Cruising Circuit

Another disc type valve is fitted inside the carburetor but it is in the lower part of the carburetor. It controls the
amount of mixture of air and petrol going into the engine
1 To increase or decrease the speed of the engine.
2 To maintain the speed when the load on the engine is less or more.
When the mixture of air and petrol goes in more quantity, more power will be generated. Similarly, when less
mixture goes in, less power will be generated.
The throttle valve is connected to the accelerator pedal through a linkage and when the driver presses the pedal,
this valve opens due to which more mixture goes into the cylinder and the engine gains speed. As the pressure
on the pedal is reduced, the throttle valve closes and the engine starts running slowly.
In many engines, automatic governors are fitted between the throttle shaft and the accelerator pedal linkage
which automatically opens the throttle wider when there is excessive load so that the mixture can enter and the
engine speed does not decrease.
High Speed or Full Throttle Circuit

To run the engine at higher speeds or to get maximum power from it, a rich mixture needs to be supplied. For this,
the ratio of air and petrol has to be 12:1 as compared to 16:1, when the engine is running at normal speed. The
following methods are used to supply more quantity of petrol to the engine.

1 Power jet or vacuum set up,
2 Metering rod.
Power Jet: Power jet is operated in two ways. Its mouth opens and closes by a vacuum operated piston. When
the throttle is closed, the vacuum in the manifold increases due to which this piston is pulled down and closes the
valve. Due to this, petrol is not able to go out through the jet. When the throttle is opened, the vacuum inside the
manifold reduces due to which the vacuum pressure on the piston also reduces and it opens the jet with the help
of the spring. When the jet opens, more amount of petrol goes into the carburetor and a rich mixture is formed.

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