O.S. Engines 3H Carb - 22981000 - Manual (Page 3)

ADJUSTMENT (FIXED-WING AIRCRAFT)
1. Start the engine and, with the throttle fully open, gradual-
ly close the Needle-Valve until it is running at its maxi-
mum speed.
Caution: Do not close the Needle-Valve to too "lean" a
setting as this will cause the engine to overheat
and slow up. Set the Needle-Valve very slightly
to the "rich" side of the peak r.p.m. setting.
Make sure that the engine is fully "broken-in"
(about 1 hour of total running time in short runs)
before operating it continuously at full throttle.
2. Having set the Needle-Valve as detailed above, close the
throttle. The engine should idle continuously and steadily
without further adjustment.
(a) If, however, the engine begins to idle unevenly, open
the throttle. If the engine then hesitates before pick-
ing up to full speed, it is probable that the idling
mixture is too rich. Check this by closing the throttle
again and letting the engine idle for a little longer
before again opening up. If the engine now puffs out
a good deal of smoke and hesitates or even stops, it
will be necessary to close the Mixture Control Screw.
Do this by turning it clockwise: about 10~20 degrees
should be sufficient.
(b) If instead of being set too rich, the Mixture Control
Screw is set too lean, the engine will stop when the
throttle is closed, or will lose speed while idling and
then cut-out abruptly (without smoking) when the
throttle is opened again. In this case, turn the Mixture
Control Screw counter-clockwise about 90 degrees,
then turn it clockwise slowly to find the best setting.
Mixture Control Screw adjustment is not critical and
by remembering the symptoms of rich and lean run-
ning quoted above, it is a very simple matter to
establish the best setting.
Note 1 If the engine stops when the transmitter throttle
stick is fully pulled back, advance the throttle trim
lever so that the idling speed is slightly raised. Do
not change the Needle-Valve setting.
Note 2 As previously stated, the Mixture Control Screw is
pre-set before the engine leaves the factory. If the
original setting is lost, it may be reestablished as
follows: 1. Close the throttle completely. 2. Screw
in the Mixture Control Screw carefully until it
stops. 3. Unscrew the Mixture Control Screw
exactly 2 1/2 turns.
CARBURETTOR CLEANLINESS
The correct functioning of the carburettor depends on its
small fuel orifices remaining clear. The minute particles of
foreign matter that are present in any fuel, can easily partial-
ly obstruct these orifices and upset mixture strength so that
engine performance becomes erratic and unreliable.
O.S. 'Super-Filters' (large and small) are available, as optional
extras, to deal with this problem. One of these filters, fitted
to the outlet tube inside your refueling container, will pre-
vent the entry of foreign material into the fuel tank. It is also
recommended that a good in-line filter be installed between
the tank and carburettor.
Do not forget to clean the filters regularly to remove dirt and
lint that accumulate on the filter screens. Also, clean the
carburettor itself occasionally.
If, despite the use of f i l t e r s , foreign matter should reach the
carburettor, causing it to malfunction, it should be carefully
dismantled and cleaned as follows:
1. Remove complete carburettor from engine. Clean off any
external dirt.
2. Remove throttle stop screw.
3. Withdraw rotor from carburettor body, taking care not to
lose the rotor spring placed behind rotor.