O.S. Engines 2B Carb - 22681005 - Manual

O.S.2B.2CA.2CB.2H
INSTRUCTIONS FOR O.S. TYPE 2B, 2CA, 2CB & 2H 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 main-
tained at the fuel jet at all times. This is a most important 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 readustments for optimum
results.
Throttle Rotor Set-Screw
Needle Valve
Mixture Control Screw
Photo shows carburettor
type 2B. 2CA or 2CB.
If mixture control screw is moved unnecessarily, observe the
re-setting as follows:
(1) Unscrew the throttle rotor set screw and set the rotor at a
position where the rotor hole is completely closed from the
carburettor body hole.
(2) With the rotor set as above, slowly screw-in the mixture
control screw until it reaches the maximum point. Avoid
trying to force it in abruptly. Otherwise, the tapered tip
of the mixture control screw ruins the fuel jet hole. Un-
screw the mixture control screw one turn from the maxi-
mum point.
(3) Re-set the rotor so that the rotor hole opens approximately
0.5 mm (0.02 in.) from the fully closed position. Screw-in
the throttle rotor set screw and fix it with lock-nut.
(4) Do the final setting of the mixture control screw in accord-
ance with the carburettor adjustment stated 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 Rotor Set-Screw (angled at rear of body).
I. 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.
Lock Nut
Phot shows carburettor type 2H.
Needle Valve
Throttle Rotor Set-Screw
Mixture Control
Screw