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Ignition System Vacuum Advance
On standard points systems and some electronic ignition systems there are two types of advance methods used, both of which must function correctly to obtain
maximum performance and fuel economy.
The first method Is Mechanical or Centrifugal, which operates by the use of weights located in the base of the distributor. The weights move outwards advancing
ignition timing as engine RPM increases. This is tested by firstly removing the vacuum advance line to disable the system, then with a timing light connected, run
the engine RPM up checking that the timing advances in accordance with the manufacturer's specification.
The second method is Vacuum Advance, which senses engine load via manifold vacuum. A vacuum diaphragm is mounted onto the distributor and connected to
a rotating internal base plate which advances or retards timing as required to suit varying engine loads. To test this system for correct operation, again with the
timing light connected, raise the engine RPM and check timing advance against manufacturer specifications. In the event that the vacuum advance is not
operating, remove the vacuum line from the distributor advance mechanism. Connect the VS401 (fig.2) and create a 5-10 inHg vacuum, monitoring the timing at
the same time. If a timing advance is noted this confirms that the vacuum diaphragm and mechanical links are in order and that the fault is a vacuum supply. To
confirm this, connect the VS401 to the vacuum supply line and check the gauge reading. No vacuum should be noted at idle but when the engine RPM is
increased a vacuum increase should be observed. If this does not occur, trace the vacuum line back checking for restrictions and breaks.
fig.3
fig.4
fig.2
FUEL SYSTEMS
Testing Mechanical Fuel Pumps
The VS401 vacuum tester can be used to evaluate the condition of a mechanical fuel pump by testing the vacuum that it is able to create. Locate and remove the
suction line from the pump. Connect the VS401 vacuum tester to the suction port of the pump, start and run the engine at idle. The vacuum reading that should be
observed will vary slightly on different makes and models but as a general rule approximately 15 inHg of vacuum should be created. This should also be held for
approximately 1 minute after engine shut down. If this vacuum reading is not achieved or the vacuum drops off immediately with the engine shut down, the fuel
pump requires either overhaul or replacement.
Carburettors
There are many different types of vacuum control systems used on carburettors. Using the VS401 vacuum tester kit allows quick and accurate testing of these
systems. Listed below are just two examples of tests that can be carried out.
Example 1.
Testing a Choke Break Diaphragm. With the engine at normal operating temperature but not running, disconnect the vacuum line to the diaphragm module.
Connect the VS401 vacuum tester (fig.3) and apply approximately 15 inHg of vacuum and wait for 30 seconds. No drop in gauge reading should be observed.
With the vacuum still applied ensure that the choke butterfly is pulled to the fully open position.
Example 2.
Testing Vacuum Operated Carburettor Secondary Barrel. With the engine at normal operating temperature but not running, remove the vacuum line from the
secondary diaphragm module. Connect the VS401 vacuum tester (fig.4), hold the throttle and secondary air valve flaps open. Operate the hand pump whilst
observing free and easy opening of the secondary throttle butterfly.
Testing Fuel Injection Pressure Regulator
Multi-point fuel injection rail pressure must vary to suit changing engine loads and fuel delivery requirements. This is done using a vacuum operated regulator
which is connected to the engine manifold vacuum to sense the varying loads. To test the fuel rail pressure, a gauge is attached to the rail, then engine loads must
be created to vary engine manifold vacuum. Simply remove and block off the vacuum supply line to the pressure regulator, connect and operate the VS401
vacuum pump (fig.5) to simulate vacuum pressures in accordance with the manufacturer's specifications and note variation in fuel pressure reading.
Testing Emission Control Exhaust Gas Reclrculation Valves (EGR)
Start engine and run at idle until normal operating temperature is reached. Remove the vacuum line from the EGR valve and attach the VS401 vacuum tester kit
(fig.6). Operate the hand pump to apply approximately 15 inHg of vacuum. If the EGR Valve is working correctly the engine idle will become rough. If the idle
remains unchanged the valve is possibly seized in the closed position. If the vacuum is not held, the diaphragm in the valve has failed.
fig.5
fig.6
VS401 - 1 - 061006
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