O.S. Engines 7B Carb - 26781008 - Manual

INSTRUCTIONS FOR O.S. TYPE 7B. 5 B . 4 B K - 4 D & 4E 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 2,000 and 2,500 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.
MAX-60FSR
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 Valve (recessed screw on right-hand
side).
(3) The Throttle Rotor Set-Screw (angled at rear of body).
I. T h e 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.
II. 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 continuously 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 necessary to close the Mixture Control Screw. Do this by
inserting a small screwdriver into the recessed screw on the
right-hand side and turning it clockwise. About a quarter turn
should be sufficient.