The MV air filter thread is very interesting and touches on some other
aspects:
It cannot assumed that MVs are the epitome of engineering design and
manufacturing excellence, the recent thread on useless parts is an
illustration and the air filter certainly isn't exempt. Those with UK
vehicles may have observed the small black "Mod Record" plates, with a
grid of numbers starting at 1, attached here and there; this is a
modification record and each number struck through is in effect a
factory recall, we wouldn't be best pleased if our private cars had such
a catalogue of disasters.
Alan Bowes has identified the principle of oil bath filters and
correctly proportioned (read big) these are excellent devices with an
inherent feature that has not yet been identified.
Some of you have vehicles with a RR B range engine and the coffin like
Vokes filter, two being fitted on the larger motors. These carry nearly
2 litres of oil each and require a 360 degree change in the high
velocity air flow before entering the massive amount of oiled mesh some
12" deep and 12" x 18" in footprint. The cleanliness of the inlet tract
is a good indicator of the efficiency and these devices were the first
post war designs drawing on much hindsight.
A well proportioned oil bath cleaner can hold far more crud than a paper
type and still work effectively, the early WWII Morris Commercial
vehicles (e.g. PU, CS8.) used in the desert had a tubular concertina
like felt filter fitted in the cab with a hand mechanism to "pump" the
device regularly (several times a day) in an effort to dislodge the
dust.
The B range is a sealed and submersible unit with watertight electrics
and ignition, all breathers are vented to the carb intake. The other
feature of the air cleaners is to seal the whole system. When stopped,
the oil uptake into the mesh runs back forming a liquid seal to the
intake system whilst during running the slight negative pressure ensures
continuous purging of the distributor and ignition circuit. To my cost
a low filter level caused serious condensation in the electrics and
hours of remedial work.
Be very wary of replacing the air filter system, the carburettor only
works by the pressure differential between its float chamber
(atmosphere) and its venturii, if this is changed by the usually less
restrictive modern filters the mixture will alter greatly at the most
dangerous point i.e., high/full throttle. Over weak mixture at large
throttle openings is seriously damaging, with luck only plugs and valves
burn but there is a real possibility of melting the piston crowns and
sending them off down the exhaust pipe.
As a _rough rule of thumb_ carbs with a strangler or choke flap usually
have the float chamber vented to atmosphere and will suffer from any
change to the designed air filter restriction, those with a separate
cold start system, like a little secondary internal carb in it own
right, normally have the float vented into the carb mouth and are
impervious to intake restriction as the pressure is identical on both
the carb mouth and float chamber.
Very best wishes to all for 1998,
Richard
(Southampton UK)
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