How to service Air Filters on a Classic Car

How to service Air Filters on a Classic Car

This guide captures ‘bygone’ advice on how to service a variety of air filters on Classic Cars. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.

Why service Air Filters on a Classic Car

Changing the air, oil and petrol filters around the engine is an easy introduction to automotive engineering for the weekend mechanic and a vital part of routine maintenance.

Clean filters can produce bonuses in terms of better performance and higher mileage. Dirty filters can make performance sluggish and allow small particles of metal to find their way into vital areas, in extreme cases-prevent a car from running at all.

When to change Air Filters on a Classic Car

Air and oil filters should be changed about every 10,000 km or 6,000 miles as part of regular servicing. Dependent on use. Petrol filters are usually changed twice a year. Ideally once before winter, if you drive in winter months, and once before the summer fun starts.

What do Air Filters do on a Classic Car

The air filter’s job is to trap dirt particles in the air supply to the carburettor.

In addition the breather pipes which disperse the slight air pressures that build up in the valve rocker box are taken to the air cleaner, where the oil-mist content is filtered out.

This oil mist helps the filter to trap the atmospheric dust, but itself adds to the build-up of dirt on the filter. A worn engine is ikely to pump extra oil mist into the rocker box.

When the filter becomes clogged the balance of air and petrol vapour entering the engine is upset. This is unlikely to actually stop the engine but there is a loss of power and an increase in petrol consumption.

There is also likely to be an accumulation of oily deposits in the breather pipes lowering the engine’s efficiency and probably causing a build-up of sludge in the rocker box.

Hence regular checks of the air filter are essential.

How often should you change Air Filters on a Classic Car

How frequent they should be depends on the conditions in which the car is used and on the age of the engine. A car driven regularly in heavy traffic in an industrial area or on a dusty unsealed road is likely to inhale more dirt than one driven in a country town with sealed roads and light traffic.

An old engine tends to inhale more dirt than a new one. But regular checks at say, 5,000 km intervals will show how much dirt is being trapped and help to establish the correct checking interval.

What do they look like

On a Classic Car the air cleaner is typically a large metal container usually round but sometimes rectangular or tubular. It is either fitted directly on top of the carburettor or connected to it by a large-diameter pipe and there is often an open­ended induction pipe pointing towards the exhaust pipe to breathe in air.

The filter element is inside the metal container and reaching it depends on individual design. A single thumb-screw is sometimes used to hold the top of the unit in place or there may be a number of screws. Sometimes it is easier to disconnect the main pipe to the carburettor and rocker-box connection and lift off the entire unit.

There are many types.

How to service Paper Leaves Air Filters on a Classic Car

The commonest kind of air-cleaner element is the paper type. A concertina-like series of leaves inside a plastic mounting. The leaves allow air but not dirt to pass through. So the outside edges of a used filter will always be dirty to some extent.

How to change Paper Type Air filters on a Classic Car Image 1
The paper-type air filter is housed in a large box on top of the carburettor. To reach it remove the wing nut and box cover

Sometimes the contamination will be of a dry dusty type which can be removed by tapping the filter sharply against a wall. It is more likely though that the dirt will be sticky oily stuff that cannot be removed.

Usually the area nearest the air-induction pipe will accumulate most of the dirt but the air cleaner unit is designed so that the incoming air passes right round the filter element so that contamination on the rest of the element should be fairly even. It shows up vividly against the original colour of the element a lightish brown or off-white.

How to change Paper Type Air filters on a Classic Car Image 2
Dirt on the filter element shows up clearly against the light brown of the paper. A discoloured element should be discarded

If the original colour is obscured by black clinging muck the filter has reached the end of its life and should be replaced.

It may be possible to rotate the element through 180 ° to present a reasonably clean face to the air induction pipe. But this is usually false economy as you are likely to save the price of a filter only at the expense of using more petrol.

How to change Paper Type Air filters on a Classic Car Image 3
Fit a new element in place of the old one. Replace the filter cover and align the air induction pipe according to the season

How to service Wire Mesh Air Filters on a Classic Car

An alternative to the paper element filter is the wire­ mesh type, in which a form of wire wool is used to trap dirt. This type can be cleaned by swilling it in paraffin (kerosene). Note two or three changes of paraffin may be necessary to get rid of all the dirt.

How to service a Wire Mesh Air Filter on a Classic Car Image 1
The ‘pancake’ type of wire-mesh air filter has mesh around its edges. Another kind looks somewhat similar to the paper-type filter

Some older cars have air cleaners of this type which cannot be dismantled. The whole unit has to be taken off the engine and its filter end immersed in paraffin.

How to service a Wire Mesh Air Filter on a Classic Car Image 2
In either type, the filter element slips out when the cover is removed. Wash it thoroughly in either petrol or paraffin. But remember modern day PPE !

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How to service Oil Bath Air Filters on a Classic Car

Although rare these days to come across one some cars have an oil-bath air cleaner.

This type incorporates a wire-mesh filter and a quantity of engine oil (half a litre or a pint) to filter out the dust. In this type of cleaner the wire mesh is cleaned with paraffin (kerosene).

How to service Oil Bath Air Filters on a Classic Car Image 1
Oil bath air filters arena rare to find, but can be found in some older vehicles – like this Land Rover. Diesels also use them

The oil is tipped out, the container cleaned, and new oil put in. Make sure that the sealing ­ring gasket between the lid and the body of the unit is in perfect condition. Any breaks or distortion mean that a new gasket is needed.

How to service Oil Bath Air Filters on a Classic Car Image 2
Remove the large-diameter air hose and then the filter top. If the dirt does not just tap out, rinse the filter in paraffin
How to service Oil Bath Air Filters on a Classic Car Image 3
Wipe out the bowl and refill with oil to the mark. When replacing the lid, make sure the sealing ring is in good condition

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SCOTTYS Top Tips – Eliminate Air Leaks

With all types of air cleaner it is necessary while checking or replacing the filter element to eliminate any air leaks.

Correct carburation settings depend on the efficiency of the air cleaner and can be upset by any air that finds its way into the carburettor other than through the element.

  • The lid of the air-cleaner container must be fastened down tightly with no distortion caused by uneven tightening of screws. As with all other car components groups of nuts or screws should be tightened in rotation, a little at a time, to spread the load evenly.
  • Pipes from the air cleaner to the carburettor and engine must be sound and free from cracks (to which they are vulnerable because of the heat in the engine compartment and inevitable oiliness around the engine).
  • The pipes themselves should be securely tightened at their ends and any supporting brackets also tightened.
  • The position of the air-induction pipe should also be checked if it is of the swivelling type. For near or sub­zero temperatures it should be close to the exhaust pipe, whose heat will help it vaporize the petrol.
  • In warmer weather it should be turned away. Moving the pipe head usually entails loosening a locking-screw, moving the pipe to the correct position, and tightening the screw again.

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