Vacuum Advance System
The vacuum advance system is designed to provide additional advance to increase the timing accuracy given by the mechanical advance system at varying driving speeds. During normal running conditions, the down movement of the piston on the combustion stroke produces a partial vacuum in the inlet manifold. The butterfly in the venturi is open and this partial vacuum draws an increased amount of petrol/air mix through. During certain running conditions, notably cruising, the butterfly is only partially open. The vacuum increases and less petrol/air mix is therefore drawn into the cylinders. The reduced amount of petrol/air mix takes longer to ignite and the spark must therefore be advanced to maintain maximum power. The vacuum advance uses this extra vacuum to do this automatically.
Distributor vacuum advance unit.
Distributor vacuum advance system.
And, because the vacuum both causes the need for the advance and supplies advance, the solution is at all times perfectly matched to the problem. In its simplest form, the vacuum advance system consists of a chamber with an airtight diaphragm sealing the carburettor from the distributor and connected to both by a pipe. The chamber is found attached to the side of the distributor and can easily be recognised by its shape. A spring is fitted inside the chamber so that when no vacuum is present, the diaphragm is held against the pipe leading from the chamber to the distributor.
This effectively seals the distributor from the chamber. When the vacuum is produced in the carburettor inlet manifold, the diaphragm is drawn away from the distributor. The diaphragm is connected to the distributor base plate by a linkage system, and as the diaphragm is drawn against the spring, the linkage moves the base plate. This moves the contact breakers so that they are opened earlier by the cam on the distributor shaft and the spark timing is advanced. The vacuum and diaphragm action have the same effect on electronic ignition systems.
Checking the Vacuum Advance System
The vacuum advance system will only work if the piping from the carburettor to the diaphragm, the diaphragm itself, and the diaphragm chamber on the carburettor side are all airtight. Carry out the following checks; First, remove the distributor cap. Next disconnect the vacuum unit from the carburettor and suck the open end of the pipe. Move the base plate gently forward with a screwdriver. Remove the screwdriver and the base plate should remain in the new position as long as your tongue maintains the suction. Removing your tongue should allow the breaker plate to spring back to its original position.
If the breaker plate does not move, first make sure that there is no obstruction -all wiring should be routed free of the breaker plate. Next, check that the piping is unblocked. To do this, remove the piping from the diaphragm chamber and blow through it. If the piping is clear, unbroken and in good condition, the fault is most likely to be in the diaphragm. It may be a ruptured diaphragm, a jammed spring, or a faulty air seal. Alternatively, but rarely, the linkage from the diaphragm to the breaker plate may be broken or damaged. If any of these is the cause you will have to fit a new unit. The vacuum unit is held onto the distributor by a bracket and by connecting linkage. Undo the bracket and fine tuning nut holding the linkage to the breaker plate. Take care to retain the spring on the linkage. Fitting the new unit is the reverse of removal. Finally, retime the ignition.
Vacuum Modulator System
Manufacturers now have to consider the effect of petrol wastage and pollution, and are fitting emission controls to their cars. One type of emission control device is the vacuum modulator (or idle vacuum retard). This device acts in the same way as the vacuum advance system during normal driving conditions, but during idling it has the alternative effect of retarding the ignition timing. This prevents petrol being wasted by only partial ignition during idling. The unit consists of two diaphragms: one provides timing advance as described above, the other - the retarding diaphragm - is connected to the intake manifold in such a way that it slightly retards the breaker plate. The vacuum modulator is fitted to the same place as the vacuum advance system which it replaces. It can be distinguished by its much larger size. The mechanical advance will still be fitted, and should be examined and repaired as has been described above.
Checking the Vacuum Modulator
Checking the operation of the vacuum modulator requires specialized knowledge. You can make sure that the advance action occurs during higher revving by the method described for vacuum advance systems, but checking retarding during idling and fitting a new unit should be left to your specialist dealer garage.