Zundapp 281 - Page 41

4. BING Carburettors
Operation
The carburettor is designed to supply the engine with the correctly proportioned and
thoroughly mixed fuel/air mixture to match any load.
The mixture is prepared in one idling and one main jet system. The idling jet system
supplies the engine at the lower speed stages. It consists of the idling fuel jet, the
idling air jet and the air adjuster screw. If the air inlet is partially throttled with the
adjuster screw, the idling mixture becomes richer, if more air is admitted the mixture
becomes leaner. Carburetors on small engines have no separate idling jet system;
instead
the
needle
jet
system
provides
the
mixtures
at
all
stages.
As
engine
speed
increases,
the
main
jet
system
comes
into
action,
consisting
of
main
jet,
mixing
chamber
insert
or
fixed
atomiser
and
needle
jet.
The
exchangeable
main
jet
sits
in
the nozzle
block,
screwed
from
below
—
or
from
the
side
on
inclined-jet
carburettor
designs
—
into
the
carburettor
housing.
On
carburettors
with
fixed
atomisers,
the
main
jet
is
screwed
to
the lower
end
of
the
needle
jet.
As
the main
jet
system
comes
into
action,
the
fuel
flows
via
the
main
jet
to
the
needle
jet.
The
outlet
bore
of
the
needle
jet
is
located
in
the
mixing
chamber
where
the
preliminary
mixing
of
fuel
and
air
takes
place.
Here,
little
fuel,
air
bubbles
form
which
are then
mixed
with
the
main
air
flow
stream
and
drawn
into
the
engine
combustion
chamber.
A
tapered
needle,
fixed
to
the
throttle
slide,
controls
the
available
needle
jet
cross
section.
Operation
of
the
throttle
slide
drives
the
needle
further
into
the
jet,
thus
narrowing
the
free
cross
section
between
needle
jet
bore
and
needle,
or,
vice
versa,
draws
the
needle
further
out
and
thus
increases
the
cross
section.
The needle shank has several grooves, so that the needle can be re-set in relation
to
the
throttle
slide.
If
the
setting
is
altered
so
as
to
drive
the
needle
deeper
into
the bore, the engine is supplied with a leaner mixture. If the needle is set higher on
the throttle slide, the available flow cross section increases and, consequently, the
mixture becomes richer. The needle setting will only influence fuel consumption un-
der throttle control, With the throttle fully open, fuel. consumption is exclusively
governed by the main jet.
Fitting Carburettor.
Carburettors must be mounted with special care. They must be positioned accurately
vertical
and
fitted
flush
and
precisely
on
their
connecting
socket.
The
slots
of
the
clamping
connecting
fitting
must
on
no
account
admit
secondary
intake
air,
other-
wise a steady, quiet idling speed cannot be set. Where connecting flanges are pro-
vided, use only gaskets in perfect condition and tighten nuts evenly. Fix Bowden cable
sleeves
without
any
sharp
bend
or
kinks.
Check
that
throttle
slide
opens
and
cioses
all the way as you operate either lever or twist grip. On carburettors with starter
jets,
the
Bowden
cable
hooked
to
the
starter
piston
should
have
a
little
play
to
ensure that the piston provides a reliable seal.
4)