article, together with several pictures from Dave's '73 SB,
appeared in the April 2004 issue of "VW Trends" magazine.
covered in this article include -
component on the VW engine that is often taken for granted
is the air cleaner. Sitting as it does on top of the carburetor,
the big clunky stock air cleaner always seems to be in the
way on tune-up day, and the average VeeDubber is tempted to
change it out for one of those shiny chrome aftermarket models.
the 1972 model year the air cleaner was a large, round, dome-topped
metal black thing with metal clips on it, possibly with several
thin and fat hoses going to it. These round air cleaners,
of which there are several different designs, have oil in
them to clean the air before it goes into the carburetor and
on into the engine. The oil is changed by removing the clips
and the top of the air cleaner.
1973 and later models, the air cleaner is a flat or round
black plastic thing with clips on it and several hoses running
to it. Removing the clips and the top of the air cleaner exposes
the paper filter inside, which serves the same purpose as
the oil in earlier models -- to remove dust particles from
the air before it is drawn into the carburetor for mixture
with the fuel. The cleaning medium, whether it be oil or a
paper filter, should be changed every 18,000 miles.
Oil Bath Air Cleaner
Paper-Element Air Cleaner
the air for mixture with the fuel is the primary purpose
of the air cleaner, but over the years a number of other
important functions have come to be associated with it.
In addition to the main air inlet port(s), there are other
hoses attached to the air cleaner -- simple arrangements
(or none) in early models, more complicated systems of hoses
later on. These hoses are associated with the following
ancillary air cleaner systems.
response to clean air requirements, VW put a rubber hose (about
1/2" ID) from the oil filler pipe to the air cleaner to take
the oil fumes from the crankcase to the carburetor for burning.
The VW engine pulls fresh air into the crankcase from a spiral
groove in the crankcase forward of (front is front) the engine
crankcase pulley, thus providing positive crankcase ventilation.
air, plus the "blow by" from the cylinders, is pulled into
the air cleaner (and on to the carburetor to be re-burned)
by way of the breather on top of the oil filler. Draw of air
into the upper chamber of both the oil-filled air cleaner
and the later paper types creates a slight vacuum inside the
air cleaner. This vacuum was enhanced in 1973 and later models
by a vacuum line running to the air cleaner from a port in
the intake manifold right below the carburetor.
negative pressure in the air cleaner, plus the positive pressure
from the spiral groove pulling air into the crankcase, results
in the oil staying put in the crankcase. Fumes are burned
as they are sucked into the carburetor by way of the breather
and the air filter.
little or no pressure drop at the top of the breather (e.g.,
because of substitution of the stock air cleaner with an aftermarket
one), any extra pressure in the crankcase causes the oil mist
to push out past the spiral groove and through the slots behind
(actually, in front of) the pulley. This oil mist, of course,
settles out inside the engine compartment and can make a real
-- for the crankcase ventilation system to work properly,
it is essential that 1) a vacuum is established in the air
cleaner, and 2) the hose between the oil filler and the air
cleaner be in place so that crankcase gases are sucked up
into the air cleaner and thence into the carburetor and intake
manifold for burning.
later models there is a rubber hose from the right wheel well
to the air cleaner. This hose provides suction on the evaporative
emissions control system so that gases are directed to the
carburetor for burning rather than escaping into the atmosphere.
fuel, expelled from a full tank by expansion, is stored in
the expansion chamber under the cowl panel in the luggage
compartment. From there it escapes into a ventilation line
that carries it to an activated charcoal canister located
in the right rear wheel well. The activated charcoal absorbs
the vapor, preventing it from entering the atmosphere.
the engine is started, air from the engine's cooling fan,
through a hose on the upper right side of the fan housing,
is directed into the canister, blowing trapped fumes from
the activated charcoal into the air cleaner. The filtered
vapors are drawn through the carburetor and into the cylinders
of the engine where they are burned.
- Steel ventilation line from fuel tank.
- Plastic tube from steel line to canister.
- Exit hose to air cleaner.
- Air hose from engine fan.
Photo courtesy of Bentley.
for the evaporative emission control system to work properly,
there is a nozzle on the upper right side of the fan house,
through which air is blown to the into the activated charcoal
canister, and another on the air cleaner into which air is
returned to the carburetor.
the 1973 and later air cleaners, there are two nozzles on
the right end of the aircleaner. The furthest one forward
has a baffle behind it inside the air cleaner; it is to this
nozzle that the hose from the oil filler is directed to draw
a vacuum on the crankcase. The other nozzle, furthest to the
rear, is the one to which the hose from the activated charcoal
canister is attached.
The Bentley Manual says "the activated charcoal filtering canister must be replaced after each 48,000 miles or two years, whichever comes first." The canister replacement procedure is easy -
- Label and then disconnect the vacuum hoses. Remove the
mounting screws and separate the canister from the bracket.
Installation is the reverse, making sure the routing of all of
the hoses is correct.
note of caution: If you have disabled the
evaporative emission control system (as many people have),
the outlet nozzle on the fan housing and the inlet nozzle
on the air cleaner must be plugged. Otherwise valuable cooling
air is lost, and the vacuum inside the air cleaner cannot
some point, VW added an intake preheating system for faster
cold-weather warm-up. The system initially consisted of a
pull wire running from the thermostat to a movable flap inside
the air cleaner intake. In 1973 and later models, this temperature-controlling
flap is controlled by a vacuum system.
intake air pre-heating system added yet another hose, at 2"
the largest of the several hoses to the air cleaner. This
hose, which is the same size as the fresh air hoses to the
fan shroud, collects warm air from the #2 cylinder by way
of an L-shaped tube underneath the engine tin. This warm air
is then routed up to the air cleaner.
Wire” Pre-Heat Control
carburetor pre-heat system on older model cars works like
cooling control flaps inside the fan shroud are linked by
a flat rod in the front (front is front of car) and outside
of the fan shroud. The cooling flaps are worked by the brass
bellows thermostat which is mounted under and between the
right cylinders, inside the ducting for the used cooling air.
The thermostat has a vertical rod extending from the top,
running up between the two cylinders.
rod connects to the cooling flap linkage, and also works the
air intake pre-heat system by way of a pull wire (a black
‘sheathed’ single strand of piano wire) which starts on the
back (back of car) of the fan shroud just under the generator/
alternator, and loops to the left and up behind the carburetor
to the flap in the rectangular air cleaner intake pipe. Under
this intake pipe is the 2" paper tube which pulls in warm
air from the #2 cylinder by way of an “L-shaped” tube that
protrudes up through the tinware.
is an air-preheating regulator valve inside the air cleaner
intake which normally closes off the pre-heater inlet. A lever
on the side of the inlet connects the pull wire with the cooling
flaps inside the fan shroud, and thus to the thermostat pushrod.
If the pull wire is missing, you'll be sucking cold air into
the carburetor all the time, instead of after the engine has
removing the air cleaner you can leave the pull wire attached
and just rest the air cleaner at the back of the engine pulley
(the cable is just long enough), or you can disconnect the
wire to remove the cleaner completely. The regulating flap
in the air cleaner is adjusted by sliding the sheath back
or forwards through the clamp, so that the flap is open to
the warm air pipe when the engine is cold.
Someone wrote - I have been reading your looking for knowledge on my 1967 Beetle. I have the original oil bath air cleaner with a neck on each side of it with a flap on each side. I noticed the other day that they are clipped up to stay open. There is a weight that hangs down on the outside of the neck that seems to hold it closed. I was playing around with the engine idle and noticed that when I rev up the engine the weight moves. I live in southern California in the city. Should these flaps be clipped open
all the time or should they be allowed to move with the engine revs? Also what do they do?
Rob responded - The flap on the left side should be loose but weighted so it's normally closed, but will open when the engine is running hard and pulling in a lot of air. The flap on the right side of the car should have a pull-wire (like a mower throttle cable) running to a crank on the flap unit inside the fan shroud on the right side of the engine (I bet it's not there!).
With this system, when the engine is cold both air inlet flaps are shut and the carburetor air is pulled up from the right side cylinder head area through the paper tube, so it gets warm air as soon as the engine starts to heat up. As the engine gets fully warmed, the thermostat opens the cooling flaps inside the fan shroud to increase the cooling airflow, and this action also opens the right side air inlet flap so now the engine is running on cool inlet air (the underside of the carburetor is now being heated by the heat riser under the manifold which uses hot exhaust gas). Now the engine is running on the right side air inlet until you floor the throttle and then the weighted flap on the left side opens too.
That's used on the 67 and 68 years only. For 69-71 they dropped the left side air inlet and increased the size of the right side inlet, so it became all-or-nothing for warm/cold air through the single inlet with the pull-wire flap mechanism (and from about 72 on they used a vacuum/temp switch device).
If the pull wire is missing, and the cooling flaps inside the shroud will probably be missing too (should get them replaced as soon as you can - they are quite important for fast and EVEN warmups), then the flap in the air inlet should be fixed open. It will mean the engine is a little rough when cold, but the engine only needs warm air for the initial warmup period, then it should be running on cold air with the heat riser under the manifold providing the necessary heat to stop icing etc.
1973 VW changed to a vacuum-controlled intake pre-heating
system for faster cold weather warm-up. A small vacuum line
from a port in the intake manifold directly below the carburetor
supplies vacuum via the tube clipped to the air cleaner to
the temperature sensor in the bottom of the air cleaner. From
there vacuum is carried by another small tube to the operating
mechanism for the warm air intake, mounted on the air intake
the inlet air cold and the engine running, the air cleaner
sensor feeds vacuum to the mechanism on top of the air inlet,
which opens the regulating valve allowing warm air from the
cylinder heads, by way of the paper intake, into the air cleaner
and carburetor. As the air inside the air cleaner warms up,
the sensor shuts down the vacuum and the air inlet gradually
switches to the cold air inlet as the vacuum bleeds out of
the inlet flap mechanism.
either the pull wire or vacuum system, ff there is a large
hole in the tinware behind the engine (next to the right cabin
heater paper tube), it means the “L shaped” metal tube is
missing too. This means that the pre-heat system has been
disabled (which many people do). We do not recommend this!
But -- if the system is disabled in your car, you must plug
the large hole in the tinware where the warm air tube connected
to the L-shaped warm air adapter pipe. Otherwise you will
have hot, dirty gases sucked into the engine compartment,
contribution to overheating and smelly air in the cabin.
is very common practice to remove the thermostat. Again, we
do not recommend this. If the thermostat is removed, the condition
will be the same as if the pre-heater tube were missing --
the engine would be very slow to warm up and the engine may
Aftermarket Air Cleaners
frequently-asked question - I'm thinking about changing out
the big hurky air cleaner on my Bug for a smaller chrome one.
regarding aftermarket air cleaners generally runs against
them, in favor of the stock VW air cleaners. Although the
chrome filters look nice on a tricked-out engine, our advice
is to retain the oil-bath or paper-filter air cleaner since
it's more reliable and provides several functions which the
aftermarket filters do not. Here are some specifics:
air cleaners are usually an open design, so there is little
or no pressure drop inside the air cleaner to help the
crankcase breather work the way it should. In extreme
cases, oily mist in the engine compartment may result
from “blow-by” through the crankcase breather.
chrome air cleaners usually don't allow for an intake
air pre-heat system. This will make the engine take longer
to warm up on cold mornings.
carburetor likes warm air in the winter; without it, ice
may form at the base of the carburetor. The air intake
pre-heat system reduces the possibility of icing up the
is possible that the small, chrome cleaner may not provide
enough air through-put, especially as the filter becomes
contaminated. The engine may become starved for air at
high RPM's because the filter element is too restrictive.
stock cleaners (both oil bath and paper type) have a "ram
tube" (sometimes also called a "velocity stack") about
three inches high inside the air cleaner, which sits above
the carburetor inlet. This helps smooth the airflow as
it enters the carburetor throat, so you get a more consistent
mixture as the fuel flows out the delivery tube into that
squat chrome aftermarket cleaners invariably lack this
internal feature, so the airflow into the top of the carburetor
is more turbulent, and a small amount of engine performance
is therefore lost.
you doubt the worth of this feature, have a look at almost
any modern engine. Carbureted engines have one ram tube
for each carburetor, and fuel injected cars usually have
one per cylinder, either curved or straight, as part of
the induction manifold.
will fall through the vents in the decklid, rusting that
chrome in no time at all, unless you have one of those
plastic "rain guards" installed in the underside of the
must of course give a line or two for an opposing opinion
that we have received - "Taking off the original aircleaner
won't harm the engine at all. Go ahead and change it, it will
do the engine a lot of good, saves weight, looks better. "Everyone"
has one." (And "everyone" is surely right! :-).
on our experience, the "tricked-out" chrome aftermarket air
cleaners have several disadvantages:
do not provide pre-heated air to the carburetor for smoother
starting on cold mornings,
do not provide sufficient air to the carburetor,
do not provide vacuum for positive crankcase ventilation,
do not smooth the airflow into the carburetor and thus
ensure a consistent fuel/air mixture,
may cause icing of the carburetor.
terse bottom line from a source we trust (Bob Hoover) -- "The
aftermarket chromie filter assemblies are bad news."
- You describe a pull wire on older VW's, older than my '72.
Since VW had moved on to a vacuum based system by '72, I wouldn't
have the pull wire. Considering the construction of my aftermarket
aircleaner doesn't have a means of attaching any vacuum, I'd
like to employ the pull wire instead. Do you know the part
number, or where I would start looking for this wire? I've
tried searching the 'net, but I've been unable to locate one.
responded - It's very unlikely you'll find a replacement
pull-wire for the the assembly you won't to install on your
72 Westy, but you can easily manufacture that part from the
stock throttle cable used on lawn mowers etc - a stiff inner
wire with coil/wound outer.
really tricky part would be getting the control flaps with
the crank projecting out of the shroud under the generator
on the upright Bug engines - these were used on only the '68-'71
models and a lot of the control flaps have been ditched by
"knowledgable" experts over the years so are hard to find...
Obsolete Parts, and wreckers. I don't know if your '72
has the 1600cc upright engine or the 1700cc pancake Type 4
engine (in some countries the 1600cc upright engine was offered
as a cheap option right through the '70s), so I'm not even
sure if the control flaps are the same as for the upright
pull wire looped up behind the carburetor from the control
flap crank, up to the carburetor inlet. This method provides
only warm air (from the paper tube mounted running up from
the right cylinder head) when the engine is cold, and cold
inlet air once the engine has warmed up. The later vacuum-operated
method was better in that it provided mixed warm/cold air
via the temperaure sensor set in the air cleaner which controlled
the vacuum airflow to the flap mechanism in the air inlet.
Having said that, the pull-wire system does help with cold-climate
engine running - a faster warm-up and less stutters whilst
it is warming up.
a pinch, you could also use a manual system to operate the
inlet flap. Any arrangment which would hold it closed for
warm-air intake in colder months will work fine. The engine
will be running on warm air all the time but that seems to
work okay too (I've used this on Bug engines with the pullwire
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