O.S. Engines 2F Carb - 22681010 - Manual

O.S. 2F, 2FB
INSTRUCTIONS FOR O.S. TYPE 2F & 2FB AUTOMATIC CARBURETTOR
This new carburettor incorporates an automatic mixture control device which ensures that the engine receives a correctly
balanced mixture of fuel and air at all throttle settings. The device progressively reduces the effective size of the fuel jet orifice
as the throttle is closed, thereby preventing the engine from running too rich at low speeds. This also means that an airbleed is
no longer required and, with its elimination, maximum suction is maintained at the fuel jet at all times. This is a most im-
portant factor where manoeuvres have to be executed at low engine speeds and through wide variations of fuel level within the
fuel tank.
Under average operating conditions, the carburettor will normally
function satisfactorily as factory set. Simply start the engine in the
normal way and adjust the needle-valve for maximum r.p.m. On
closing the throttle, the engine should idle at between 2,500 and
3,000 r.p.m. and also run steadily at all intermediate speeds.
However, different fuels and/or climatic conditions, may require
minor readjustments for optimum results.
Throttle Stop Screw
Needle Valve.
Mixture Control Screw
Note: In the event of the factory carburettor settings having been
accidentally disturbed — or otherwise interfered with — pro-
visional settings may be re-established as follows:
1. Unscrew the Throttle Stop Screw just sufficiently to allow the
throttle to close completely.
2. With the Throttle Rotor set as above, carefully screw in the Mix-
ture Control Screw until it stops. Now unscrew the Mixture
Control Screw 1 1/2 turns from this point.
3. If necessary, fine-tune the carburettor in accordance with the
procedure explained below.
ADJUSTING THE CARBURETTOR
Three adjustable controls are provided on this carburettor:
1. The Needle-Valve (located on left-hand side of carburettor).
2. The Mixture Control Screw (located on right-hand side).
3. The Throttle Stop Screw (angled at rear of body).
1. The Needle-Valve is used in the same way as on all model engines,
i.e., for adjusting the high-speed mixture strength. Start the engine
and, with the throttle fully open, gradually close the Needle-Valve
until it is running at its maximum speed. Caution: Do not close
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. The Mixture Control Screw is for adjusting fuel mixture strength
at part-throttle and idling speeds. Having set the Needle-Valve
as detailed above, close the throttle. The engine should idle con-
tinuously and steadily without further adjustment.
(a) If, however, the engine begins to idle unevenly, open the throttle.
If the engine then hesitates before picking 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 neces-
sary 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 30 degrees,
so that the mixture becomes slightly over-rich, then turn it
clockwise very gradually to achieve the desired setting.
Mixture Control Screw adjustment is not critical and by re-
membering the symptoms of rich and lean running quoted
above, it is a very simple matter to establish the best setting.