Zundapp 281 - Page 43

Carburettor Setting
Manufacturers of carburettors work out the most suitable design and select the jet
sizes in close collaboration with the engine makers. They also specify the setting
which tests have shown to give best performance and maximum economy. It is, there-
fore, inadvisable to depart from their specifications
Idling
Idling speed should always be set or regulated while the engine is warm. With
the adjuster screw, close the throttle slide until the engine just ticks over slowly and
steadily. On carburettors without separate idling speed system, the needle jet as-
sembly supplies the amount of fuel required for idling speed. Carburettors with
separate idling speed system work as follows:
The dir adjuster screw controls the ratio of the fuel/air mixture supplied by the idling
speed system. By turning the adjuster clockwise, the mixture becomes richer, by
turning anti-clockwise, it becomes leaner. The setting is correct when the engine ticks
over quietly and evenly. As soon as it does, do not alter the air adjuster setting again,
since this also governs the lower and medium speed range, so that any further ad-
justment might increase fuel consumption.
When the throttle is slowly opened out, the engine should steadily rev up. It should
neither splutter nor should the speed drop back again at any throttle setting. If the
engine splutters or speed rises in surges, or if black fumes escape from the exhaust
silencer, the mixture has been made too rich. If the engine coughs or cuts out for
brief moments, if a blue flame blows back from the carburettor, or if the engine is
difficult to start, the mixture has been set too lean.
Driving Speed Range
In order to determine the most suitable main jet for a carburettor, first take the
machine on a straight, level road and find out its maximum speed by the speedometer
or with a stop watch. Generally, the main jet which will produce the highest speed on
a level road will be the most suitable for any machine. But If on long continuous
runs at full throttle the engine starts knocking due to overheating, select the next
larger jet size.
In the medium speed range, carry out fine settings between two needle jet sizes by
means of the needle. By setting the needle higher, the mixture will become richer and
vice versa.
Note that needle position only affects the mixture ratio in the lower and medium
speed ranges, not for driving at full throttle. If the carburettor is correctly set, the
insötator
of
the
spark
plug
will
show
an
even
brown
combustion
coating.
A
spark
plug
coated
with
black
coking
residues
or
wet
to
the
touch
is
a
sure
sign
that
the
mixture is too rich, a white insulator shows that it is too lean.
Always remember that only correct carburettor setting guarantees maximum running
economy.
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