Page 102 - CTS - Mechanic Diesel - TP

MECHANIC DIESEL - CITS

EXERCISE 4.5: Tuning carburettor

Objectives
At the end of this exercise you shall be able to
• tune the carburettor adjust the idling speed of the engine
• tuning of carburettor
• ensure the following.

Requirements

Tools/Instruments Materials
• Trainee’s tool kit - 1 No. • Petrol
• Ignition timing light - 1 No. • Waste cotton cloth
Equipments/Machines
• 4 cylinder petrol engine
• Air compressor

Procedure

• Choke valve of the carburettor is at fully closed position.

• The ignition timing is set correctly.
• The air cleaner is in good condition and properly in- stalled.
• The engine tappet clearance is set correctly.
• Start the engine and release the choke and run it for some time till it attains the operating engine temperature.

• Idling speed adjustment
• Adjust the idling speed to the specified r.p.m. by repositioning the volume control and throttle adjusting screw.
• With the engine running at idling speed turn the idle mixture adjusting screw clockwise/anticlockwise and set
it where the highest r.p.m. is achieved with exhaust gas flow in good rhythm. This is the best position.
• If the engine misfires at the idling speed check the pilot jet and adjust the volume control and throttle screw.
• In case the engine’s top speed is insufficient, check the main jet; if found defective, replace the main jet.
• If the carburettor is flooding frequently remove the float and needle valve seat, along with the needle valve
from the upper cover.
• Clean the seat and needle.
• Adjust the float level to the specified measurement by inserting a suitable shim between the valve seat and
upper cover by slightly bending the toggle lever.
• If a flat spot occurs during acceleration of the vehicle, the accelerating pump supplies less quantity fuel
resulting in weak mixture. Hence, stop the engine immediately.
• Remove the pilot and main jets, clear them from obstructions and refit them again on the carburettor. If jets
are found defective, replace them.
• Also adjust the accelerator pump stroke correctly to the specified measurement given by manufacturer of the
carburettor. (Refer to Vehicle Operating Manual for correct measurement of stroke of the accelerating pump.)

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