O.S. engine FS-120S-E - Adjusting the Carburettor; Realignment of Mixture Control Valve

ADJUSTING THE CARBURETTOR
The carburettor of your FS-120S-E has been factory set for
the approximate best result, but the settings may, in some
cases, vary slightly in accordance with fuel and climatic
conditions.
If the desired throttle response is not obtained after the
completion of the running-in period, the carburettor should
be re-adjusted as follows. Please refer to Photos 2 and 3.
Three adjustable controls are provided on this carburettor.
@ The Needle-Valve: For adjusting the mixture strength
when the throttle is fully open.
@® The Mixture Control Screw: For adjusting the mixture
strength at part-throttle and idling speeds, to obtain
steady idling and smooth acceleration to medium speeds.
è The Throttle Stop Screw: For establishing the minimum
idling speed.
Mixture control screw
Needle valve
Rotor guide screw
f- Photo 2
E
1. Set the throttle lever 1/4 open from the fully closed
position (see Fig. 3) 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.
2. Now open the throttle fully and gradually close the
needie-valve until the engine is running at its optimum
rp.m. in the air. The needle-valve setting at this time
will be 1/8 to 1/4 open from the maximum r.p.m.
setting.
Note: Make sure that the engine is fully “‘run-in’’ before operating it
continuously at full power. (See RUNNING IN section.)
3. Having set the needle-valve as described above, close the
throttle and run the engine at idling speed for approxi-
mately 5 seconds, then steadily move the throttle to the
fully open position by means of the throttle servo.
(a) lf, at this point, the engine hesitates before picking
up speed, with a low-pitched exhaust note and an
excess of exhaust smoke, 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 45
degrees.
(b) If, on the other hand, the engine hesitates or even
appears to cease firing for an instant, before in-
creasing speed, it is probable that the idling mixture is
too lean. In this case, it will be necessary to turn the
mixture control screw counter-clockwise about 90
degrees, then turn it clockwise 45 degrees.
Turn the mixture control screw 30 to 45 degrees at a time
in the early stages. When the response improves, turn the
mixture control screw 10 to 15 degrees at a time. Carry
out adjustments progressively and patiently, remembering
the symptoms of rich and lean running, quoted above, until
the engine responds quickly and positively to the throttle.
4. The throttle stop screw is for fixing the minimum idling
speed. If, after carrying out mixture adjustments, the
idling speed is found to be too high, the throttle stop
screw should be turned counter-clockwise until the
desired idling speed is obtained.
Note: Once the correct carburettor settings have been established, it
should be unnecessary to alter them. Such slight needle-valve
readjustments as may be required to compensate for varia-
tions in atmospheric conditions, will not normally affect the
other two controls. Slight readjustments may be necessary
for optimum performance if different types of fuel, glow-
plugs or propellers are used. In this case, begin by flying the
model with the needle-valve approximately 1/8 to 1/4 turn
open from the peak r.p.m. as a safety measure, then readjust
as necessary.
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just
possible that the Mixture Control Valve may be inadver-
tently screwed in or out too far and thereby moved beyond
its effective adjustment range.
lts basic setting can be reestablished as follows:
The basic (factory) setting is as shown in the main sketch,
i.e. with the shoulder portion ‘A’ exactly at a tangent to the
throttle rotor hole.
To return the Mixture Control
Valve to its original position,
first screw in the Mixture
Control Valve, while looking
into the rotor hole. Then
gradually unscrew the Mix-
ture Control Valve until ‘A’ is
precisely tangential to the
rotor hole (i.e. so that ‘A’ and
‘B' are superimposed) as in
Throttle
‘A
rotor hole an)
the main sketch. B Fig. 4
CARBURETTOR CLEANLINESS
The correct functioning of
the carburettor depends on
its small fuel orifices remain-
ing clear. The minute particles
of foreign matter that are pre-
sent in any fuel can easily
partially obstruct these
orifices and upset mixture
strength so that engine per-
formance becomes erratic and
unreliable. =
It is recommended that fuel is passed through a filter when
the tank is filled and that a good in-line filter is installed
between the fuel tank and carburettor and, furthermore,
that this filter is frequently cleaned to remove dirt and lint
that accumulate on the filter screen. Finally, occasionally
remove the needle-valve holder from the carburettor, using
the wrench supplied, and extract any remaining foreign
matter that may have lodged in the location shown in
Photo 3.
Throttle
stop screw
Dirt and fibrous
matter mostly
accumulate here.
Photo 3
VALVE CLEARANCE ADJUSTMENT
Valve clearances are correctly set before any O.S. four-
stroke engine leaves the factory and, in normal use, will
seldom require adjustment. However, if, after a considera-
ble amount of running time has accumulated, a loss of
power is detected, or if the engine has to be disassembled or
repaired as a result of a crash, valve clearances should be
checked and readjusted as necessary. For checking and
adjusting the valve clearances, a valve adjusting kit, contain-
ing two feeler gauges and two wrenches, is available as an
optional accessory.
Note: Valve clearances on this engine. must be checked and reset
only WHEN THE ENGINE IS COLD.
1. Remove the rocker cover by unscrewing two Allen
screws from the rocker box on top of the cylinder head.
2. Turn propeller until compression is felt, then turn it one
quarter turn and stop. Both valves should now be closed.
3. The required valve clearance is between 0.04 mm and
0.10 mm (.0015 to .004 in.) measured between the valve
stem and rocker-arm. Use the 0.04 mm (thinner one)
and 0.10 mm (thicker one) feeler gauges to check
clearance. The 0.04 mm feeler should pass through the
gap: the 0.10 mm gauge should not.
4. If the gap is found to be less than 0.04 mm or more than
0.10 mm, carefully slacken the locknut on the rocker-
arm with the 5 mm wrench supplied, turn adjusting
screw with 1.5 mm Allen key to open or close gap, then
retighten locknut.
Finally, re-check gap and readjust if necessary.
IMPORTANT: Incorrect valve clearances may cause difficult
starting (due to valve not closing properly) or loss
of power (due to valve not opening sufficiently).