O.S. Engines 8AA Carb - 29383000 - Manual

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 8AA CARBURETTOR
The O.S. Type 8 A A carburettor has been designed to provide a correctly balanced mixture of fuel and air at
all throttle settings. It ensures steady revolutions and a smooth response even to abrupt operation of the
throttle.
Idle Mixture
Control Screw
Throttle Lever
Rotor Guide Screw
INSTALLATION NOTES
Note: This carburettor is not fitted with a throttle stop
screw. Instead, idling speed is adjusted by means of
the throttle trim lever on the transmitter. This
enables the full r.p.m. range, from idling to full
power, to be controlled by the throttle stick, and
then allows the engine to be stopped, from the trans-
mitter, by closing the throttle completely with the
trim lever. Set up the throttle linkage as follows:
1) With the transmitter throttle trim lever fully retarded,
adjust the throttle servo linkage so that the throttle
rotor is (a) fully open when the transmitter throttle
stick is fully advanced and (b) fully closed (i.e. engine
stopped) when the stick is fully retarded.
2) The idling speed is then set by advancing the throttle
trim lever to the point where the engine runs, steadily
and reliably, at the desired idling speed.
Be sure to use a muffler-pressurized fuel feed and, before
attempting to start the engine, check that the carburettor
controls are set as indicated below.
ADJUSTING THE CARBURETTOR
1) Open the Needle-Valve 1~1-1/2 turns (or 1-1/2~2 turns if a
tuned silencer not is used) from the fully closed position.
2) Set the throttle very slightly open from the idle position
(see Fig. 1) and start the engine in the usual way. It is
preferable to have the throttle only slightly open, to avoid
unnecessarily high revolutions when the engine starts.
3) Now open the throttle fully and gradually close the
Needle-Valve until the engine is running at its maximum
speed.*
* Warnings:
(a) Do not close the Needle-Valve to too "lean" a setting,
as this will cause overheating which may result in
internal damage and reduced engine life. Set the
Needle-Valve approximately 1/8 to 1/4 turn open (i.e.
"rich") from the peak r.p.m. setting, even though this
will slightly reduce r.p.m. on the ground.
(b)Make sure that the engine is fully "run-in" before
operating it continuously at full power.
(See engine instruction leaflet.)
4) Having set the Needle-Valve as described above, close the
throttle. The engine should idle steadily and continuous-
ly. (If it stops immediately, first try setting the idling
speed a little higher by means of the throttle trim
lever.)
(a) If, however, the engine idles unevenly and smokes a
good deal when the throttle is re-opened, before pick-
ing up speed, it is probable that the idling mixture is
too rich. In this case it will be necessary to turn the
Mixture Control Screw clockwise about 30 to 45
degrees.
(b)Alternatively, if the Mixture Control Screw is set too
lean, the engine may stop when the throttle is closed,
or it may idle for a few moments, then speed up very
slightly before coming to a stop. Re-opening the
throttle under these conditions will usually result in
the engine cutting out abruptly, without smoking.
Corrective action is to turn the Mixture Control
Screw about 30 to 45 degrees counter-clockwise.
Mixture Control Screw adjustment is not unduly critical,
but do not turn it more than 30—45 degrees at a time.
Carry out readjustments progressively and patiently,
remembering the symptoms of rich and lean running,
quoted above, until the engine responds quickly and
positively to the throttle.
2~3mm
Fig.(l)
Needle Valve
Fuel Inlet