LV23 petrol fuel systems (2)

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Student Workbook

LV23 Petrol Fuel Systems (2)

kap all phase 2 & 3 6/11/03 11:36 am Page 13

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV23 PETROL FUEL SYSTEMS (2)

Contents

Page Page

…

Hot engine starting compensation

When explanations are given on how to adjust the various parts of the

carburettor, a single type of carburettor will be used There are numerous types of carburettors used by the various manufacturers, so before carrying out maintenance or adjustments the manufacturers’ manuals must be consulted

Carburettor Adjustments

Fig 1

The carburettor, as with other components, needs to be checked and adjusted

at regular intervals Incorrect adjustment can lead to high fuel consumption, incorrect emissions and general incorrect engine operation If the carburettor is incorrectly adjusted for a long period of time then serious engine malfunction may occur, including burnt valves and pistons Conventional carburettors have externally mounted adjustment screws, which allow the adjustment of high and low idle speeds as well as the air/fuel mixture Other adjustments can be

carried out by slightly bending the various levers, or by stripping off

components from the carburettor and following the manufacturers’ adjustment procedures

The following diagrams outline the various adjustments needed A single type

of twin choke carburettor has been used to outline the adjustments The

procedure will change depending on the type of carburettor to be adjusted, i.e constant depression carburettors, constant choke carburettors and variable venturi carburettors, although the types of adjustments will be fairly generic

First throttle valve opening

Fully open the first throttle valve

Visually check the first throttle valve angle

Opening angle 90 degrees

Adjust the first throttle valve opening by bending the throttle lever stopper

Second throttle valve opening

First open the first throttle valve

Fully open the second throttle valve lever

Visually check the throttle valve opening angle

Opening angle 90 degrees

Adjust the second throttle valve opening by bending the throttle lever stopper

Secondary touch angle

Open the first throttle valve until the throttle valve lever part A touches part B

At this time, check the first throttle valve opening angle using the

manufacturers’ recommended gauge

Adjust by bending part A

Adjust part A to leave a kick-up clearance of 0.04 – 0.16 mm

Fast idle

Fully close the choke valve by turning the coil housing

Check the throttle valve clearance between the throttle valve and the bore

Adjust by turning the fast idle adjusting screw leaving a clearance of 0.1 mm

Unloader

Fully close the choke valve by turning the coil housing

Fully open the first throttle valve

At this time, check the choke valve opening angle

Adjust by bending part A to leave an unloader angle of 44 – 46 degrees

Choke breaker

Fully open the choke valve by turning the coil housing

Copyright © Automotive Skills Limited 2003 LV23: Petrol Fuel Systems (2) Issue 1 All Rights Reserved

Push the choke breaker diaphragm, and at this time, check the clearance

between the choke valve and bore

Adjust by bending part A to leave a choke breaker clearance of 2.1 – 2.4 mm

Automatic choke

Set the coil housing scale mark so that it will be aligned with the centre line of

the thermostat case

The choke valve becomes fully closed when the atmospheric temperature reaches 20°C

Depending on the vehicle operating conditions turn the coil housing and adjust

the engine starting mixture

If the mixture is too rich – Turn the coil housing clockwise

If the mixture is too lean – Turn the coil housing counter-clockwise

Idle mixture adjusting screw

Screw in the idle mixture adjusting screw and then unscrew it as follows: Initial setting

Unscrew the adjusting screw about 2¼ turns from the fully closed position

Use care not to screw in the adjusting screw too tightly, as damage will occur

on the screw tip

Accelerating pump

Adjust the pump stroke by bending part A by 3.25 mm

Note: After adjustment is made, be sure to check the linkage to see that it operates smoothly

Float level

Allow the float to hang down under its own weight, then using an appropriate type of measuring equipment, measure the clearance between the float tip and

the air horn

The air horn gasket should be removed before carrying out this measurement Manufacturers’ specifications should be used for float level clearance

Adjustment is made by bending the part labelled A

To adjust the lower position of the float, lift the float and check the clearance between the needle valve plunger and the float lip An appropriate type of

measuring equipment should be used for this

The lower float level position should be adjusted by bending the part labelled B

Idle Mixture Adjustment

Using a CO Meter

When adjusting the mixture using the idle mixture adjusting screw, it is good practice to always use a CO meter If the vehicle is operating correctly, then adjustment of the carburettor may not be needed However the vehicle

emissions should still be checked using the CO meter The following procedure

is specific to one manufacturer, so each individual manufacturers’ specification should be checked before carrying out any idle mixture adjustment

Adjust the idle speed and the idle speed mixture, by using a CO meter to

measure the concentration of CO in the exhaust gases To adjust the CO concentration turn the mixture adjusting screw until the given value is obtained

The idle speed should be approximately 800 rpm

The adjustments should be carried out with no electrical load placed on the

engine

The CO meter to be used should now be calibrated The engine rpm should be raised to around 2000 and held there for approximately 30 to 60 seconds The engine should then be stabilised by allowing it to tick over for approximately 3 minutes The engine should be warmed up before carrying out any of the above operations

The test probe should now be inserted into the exhaust pipe, and a

measurement taken over the next few seconds

The CO concentration should be between 0 and 0.5% for an engine fitted with a catalytic converter, and 1 and 2% for an engine not fitted with a catalytic

converter

If the readings are outside the manufacturers’ specifications then adjustments should be carried out

Four Gas Analyser

As described previously the engine needs to be warmed up before carrying out any adjustment using a CO meter This is more important for an engine

fitted with a catalytic converter, as the catalytic converter needs to reach a temperature of approximately 400°C before it begins to work effectively The above shows the difference in readings achieved from a four-gas analyser when the engine is cold and when the engine has warmed up

The four-gas analyser can be used to test if an engine is either running rich or lean or if it is within its given parameters If an engine is running slightly rich then the following symptoms can occur:

• maximum engine output

• higher emissions

• higher fuel consumption

• lower tendency to knock or pink

When the engine is running too rich then the symptoms will change slightly These are listed below:

• poor fuel mileage

The diagram above left shows the readings achieved with a four gas analyser when the engine is running too rich This was achieved using a petrol injection engine with one of the ignition coils disconnected The same results can be achieved by adjusting the idle mixture screw of a carburettor to make the engine run rich Other items that may cause an engine to run rich include incorrect adjustment of the float level, worn main jet or a blocked air filter

If the air/fuel mixture is lean then a different set of symptoms will occur For a slightly lean engine the following list of symptoms will occur:

• improved fuel economy

• low exhaust emissions

• reduced engine power

Fig 37

• slight tendency to knock or pink

When the engine is running too lean then the symptoms can become more serious, and the types of fault are listed below:

• poor engine power

• misfiring at cruising speed

As with the rich mixture section the diagram above left shows the four-gas analyser readings achieved when the engine is running lean This was

simulated by removing the power to one of the injectors of an electronic fuel injection system Within a carburettor, incorrect adjustment of the idle mixture screw, float chamber or a blocked main jet will cause lean air/fuel mixture to occur

Adjustment without a CO meter

Fig 38

Idle mixture adjustment can be carried out without a CO meter although it is not advisable With rigorous MOT procedures in force the following adjustment method does not guarantee that the emissions will be correct

The following procedure is called the lean drop method and as explained earlier should only be carried out if a CO meter is not available

First set the engine to the maximum idle speed by turning the idle mixture adjusting screw Now adjust the idle speed down to 860 rpm by adjusting the idle speed adjusting screw Continue this procedure until the adjustment of the idle mixture screw does not raise the idle speed above 860 rpm Now reduce the idle speed to 800 rpm by re-adjusting the idle mixture screw These

adjustments should be carried out with no electrical load placed on the engine

Air filter

Within Phase 1 Petrol Fuel Systems LV10 an in-depth explanation was given into the need for an air filter to be fitted to a carburettor engine If small

particles of dirt enter the carburettor then various components within the

carburettor will begin to wear quickly From a diagnosis and adjustment

perspective, if the air filter becomes blocked then the engine will run rich,

leading to the symptoms listed earlier With some manufacturers the air filters are replaced at periodic intervals but they will be inspected at every service interval At given mileages they will also be blown out, which extends the life of the air filter

Carburettor diagnosis

To diagnose a fault correctly the technician must first decide the correct cause

of the problem The fault may not be in the fuel system, but it may be in the ignition system, or within the engine itself For this reason it is important to carry out basic checks on the engine to decide where the fault lies To assist in the diagnosis of a fuel system fault, a diagnosis procedure chart is given below

Analyse the customers fault

Ask the customer to outline the symptoms of the fault and under what conditions the fault occurs This should help the technician to decide on how to proceed.

Test drive the vehicle to

confirm the symptoms

1 Was the vehicle hard to start

2 Was there a flat spot during acceleration

3 Does the vehicle use excessive fuel

Analyse the customers fault

Ask the customer to outline the symptoms of the fault and under what conditions the fault occurs This should help the technician to decide on how to proceed.

Test drive the vehicle to

confirm the symptoms

1 Was the vehicle hard to start

2 Was there a flat spot during acceleration

3 Does the vehicle use excessive fuel

Fuel system inspections

Decide in w hich area the

Final check

Progress check 1

Answer the following questions:

1 List five adjustments carried out on a carburettor based fuel system:

2 List the items checked during the preliminary inspection and adjustment:

3 When adjusting the idle mixture why is it important not to over tighten the

adjusting screw?

4 What type of gas analyser is most effective at assessing all three

pollutants that are emitted from the combustion process?

5 Draw a picture of the fuel filter showing the flow of fuel through it:

Exercise 1

For the five following engine symptoms, list the possible causes and repairs needed to rectify the faults:

Engine will not start

Erratic idle

speed/engine stalling

Symptom Possible cause Repairs needed

Hesitation/misfire

High fuel consumption

Pre-ignition/

detonation

Vapour Lock

When diagnosing faults within the fuel system, you will find that most of them are associated with wear or failure of components In some cases it is the fuel itself that causes the failure Vapour lock is when the fuel inside the fuel line between the tank and the fuel pump begins to boil The boiling of the fuel results in vapour being generated within the fuel line causing only fuel vapour to

be delivered to the carburettor If vaporised fuel is delivered to the carburettor, then the engine will run lean, and as outlined earlier, this will cause problems such as poor engine power, rough idling, misfiring and burnt valves etc

Manufacturers can take different forms of action to try and eliminate or reduce the possibility of vapour lock occurring One important consideration is the positioning of the fuel pipes that run along the underside of the vehicle

They must be placed so that they are not near any heat generating components such as the exhaust system A fuel-return system will reduce the temperature

of the fuel that is delivered to the fuel pump This is achieved by constantly passing fuel between the fuel pump and the fuel tank, as the fuel tank is

subjected to air travelling under the vehicle it is constantly cooled By passing cool fuel around the system vapour lock is eliminated On some systems the fuel is returned from the carburettor and not the fuel pump, but the principle is the same

With the introduction of modern fuel injection systems the need for a fuel return system has been eliminated and most manufacturers have returned to the non-return fuel system To remove vapour lock the fuel has now been pressurised and this increases the temperature that the fuel needs to reach before it boils

Percolation

Percolation is similar in it characteristics to vapour lock, they are both caused

by the fuel boiling As described previously, vapour lock is caused when the fuel boils in the fuel line, whereas percolation is caused when the fuel boils in the float chamber of the carburettor Vapour lock causes the air/fuel mixture to become lean, but percolation causes it to become rich Percolation is caused when the fuel in the float chamber boils and this is usually caused by the

vehicle being driven at high speeds or under high load Sitting in a traffic jam in hot weather will increase the possibility of percolation occurring due to the increase in engine and exhaust temperatures Percolation leads to the vehicle rough idling, due to the rich air/fuel mixture being delivered to the engine If this symptom continues then the engine will stall When the engine has stalled due

to percolation it may be hard to start while the vehicle is hot, so the vehicle will need to be allowed to cool

Each of the manufacturers will have their own adaptations fitted to the

carburettor to reduce or eliminate percolation One type of adaptation is the hot idle compensator which reduces the possibility of the air/fuel mixture becoming over rich An explanation of this system will be given later in the workbook