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DA100L showing the mounted Carbie
The majority of petrol two stroke motors used in R/C models utilise some form of pump carburettor which are often sourced from specialist manufactures such as Walbro, Tillotson etc. These carburettors typically operate by applying crank case pressure pulses to one side of a diaphragm which is vented to atmospheric pressure on the other side.
Some installations require an external flexible tube to connect a
crankcase vacuum port to the carbie whilst others have a vacuum passage
integrated into the carbie body and its mounting spacer which aligns
with an appropriate port on the motor. The carbie pump diaphragm is
vented to atmospheric pressure through a hole or vent fitting on the
diaphragm cover plate.
During testing of our new tow plane we uncovered an interesting
condition that was directly related to the operation of the diaphragm
fuel pump on the DA100L motor. The tug is a semi scale Piper Pawnee
with a fully cowled engine installation as per pic 2.
Pic 2 - Pawnee landing
The vent hole on a DL100
The vent hole for the carbie diaphragm can be seen facing forward
directly into the air flow. During ground runs the motor performed
faultlessly at high power however we were unable to obtain a stable idle
which was initially put down to a tight new motor. In the air we
experienced a significant number of engine failures when descending at
low throttle and the motor was clearly loading up in an over rich
condition. We experimented with the high and low mixture settings
however whilst there was a small improvement the over rich condition
when descending at low throttle continued to plague subsequent flights.
The solution turned out to be quite simple after a quick call to the
Australian distributor confirmed our suspicion that problem was caused
by positive air pressure on the carbie diaphragm due to the vent hole
facing the oncoming airstream. This occurs because build up of air
pressure over the vent hole upsets the position of the pump diaphragm
causing it to provide an over rich mixture to the motor.
Pic 3 - tube connected to carbie
The modification is relatively straight forward and consists of
soldering a small piece of copper or brass tubing over the diaphragm
vent hole and attaching a piece of fuel tube. In our case the other end
of this tube was connected to a fitting provided on the air cleaner
mount as per pic3. However, you can simply run the tube back into the
fuselage or any other point in the cowling where there is no positive
pressure build up caused by the outside air flow. Before attempting to
solder the fitting you must remove the plate covering the diaphragm to
prevent the heat from damaging it. Be careful here as the diaphragm is
quite delicate and any tear or leak in the seal will cause the carbie to
malfunction. Once this modification was finished the following flights
went off without a hitch and dead stick landings are now in the pas and
the idle and mid range throttle transitions are no longer a problem.
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