The carburetor's job is to produce the correct fuel/air mixture for the
engine. The air sucked in by the engine is mixed with fuel in the carburettor
via a nozzle. The correct amount is determined by an adjustable nozzle
needle. If the carburettor is also to be used to make the power or speed
of the engine adjustable, it needs an air throttle slide. In order to
obtain the correct mixture at every throttle position, the fuel flow must
be adjusted at the same time as the throttle. With the Webra type TN carburetor,
the air intake cross section is adjusted by a rotatable throttle valve.
At the same time, the fuel flow from the nozzle assembly is changed by
a superimposed axial displacement using the idle nozzle needle so that
the correct mixture is created in every position of the throttle valve.
Before starting up a new engine, the carburettor is roughly adjusted.
The main jet needle must be opened by approx. three turns. The idle needle
is adjusted so that when the throttle slide is almost closed, just a little
air can still be blown through the throttle slide. The engine, installed
ready for operation, is started in idle position.
After a short warm-up time, the correct idle setting is sought. By slowly
turning on the idle needle, the mixture becomes richer and the engine
runs irregularly. From this position, the needle is slowly closed again,
with the motor running faster and more steadily until it finally stops.
Starting from this too lean idle setting, the needle is turned up again
by approx. 1/2 turn and then left like that for the time being. The adjustment
of the main jet needle is carried out with the throttle valve fully open.
To do this, the main jet needle is first turned on until the engine runs
at four strokes. By slowly closing the needle, the engine speed increases
and the engine switches to two-stroke operation. In new engines, the carburetor
is left in this "rich" setting. Since all carburetors work according
to the same principle, this type of setting applies to all adjustable
carburetors. Basically, we can note that with the TN carburetor types,
the idle needle has its range of action up to about 3/4 throttle position,
but with the PROMIX and ULTRAMIX carburetor types the effective range
of the idle needle is available up to the full throttle range. In the
DYNAMIX carburettor, the idle needle is replaced by a slotted fuel slide
which regulates the idle range to the full throttle range. Its position
opposite the throttle slide is adjusted in the same way as the idle needle
on the other carburettors, but with the screw turning in the opposite
direction (tighten = rich setting).
Arrangement of the fuel tank
Regardless of whether the engine is installed in an aircraft, ship or
car model, it is advisable to install the tank in such a way that the
fuel level when the tank is full corresponds to the height of the carburettor's
nozzle assembly. This installation position ensures that the fuel does
not run over and unintentionally fill the engine when the model is parked.
Furthermore, it is advantageous to move the tank as close as possible
to the engine in order to ensure the best possible fuel supply in different
flight positions or acceleration conditions.
The right choice of fuel hoses with the appropriate cross-section should
also be made. Vibrations that are transmitted from the cell to the tank
must be avoided at all costs. They cause the fuel to foam, making it impossible
to safely adjust the carburettor. This can be remedied by a vibration-damped
installation of the tank, e.g. in foam rubber. Pressure tank High-performance
engines usually have a carburettor with a large intake cross-section.
They can no longer suck in the required fuel themselves. To compensate
for this disadvantage, the tank is supplied with excess pressure from
the exhaust. This type of fuel supply of course only works if the tank
system has no other openings or leaks.
The causes of inadequate fuel supply with large carburettor cross-sections
or with tanks installed further away can also be eliminated by installing
a pump. However, no pressure tank may be used here. When installing a
pump, Webra recommends using a "Promix" - "Dynamix"
- "Ultramix" - "3N-60H" - or "MC" carburetor
type as the carburetor.
To avoid engine problems caused by unclean fuel, use a fuel filter. This
is housed as a tank pendulum or in the supply line to the carburettor.
However, installing filters is only successful if you do not refuel through
them, as dirt particles remain in front of the filter and later get into
the nozzle assembly.