O.S. Engines 4BK Carb - Manual

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 4BK,4D,5B CARBURETTORS
This 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. It ensures steady revolutions and a smooth response to any change of the
aircraft's attitude.
Throttle Stop Screw
Mixture Control Valve
Photo (1)
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 Valve (Mixture control screw): For adjust-
ing the mixture strength at part-throttle and idling speeds, to
obtain steady idling and smooth acceleration to medium speeds.
The mixture control valve has been factory set for the approxi-
mate best result. First, run the engine as received, and re-adjust
the mixture control valve only when necessary.
• The Throttle Stop Screw: For setting the position where the
carburettor rotor is closed.
PROVISIONAL SETTING
Open the needle-valve according to the engine instruction leaflet.
ADJUSTING THE CARBURETTOR
1) Set the throttle very slightly open from the idle position (See
Fig. 1) 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 Needle-Valve
until the engine is running at its maximum speed.*
*Warnings:
(a) Do not close the Needle-Valve to too "lean" a setting, as this
will cause overheating which may result in internal damage
and reduced engine life. Set the Needle-Valve approximately
1/4 to 1/2 turn open (i.e. "rich") from the peak r.p.m. setting,
even though this will slightly reduce r.p.m. on the ground.
(b) Make sure that the engine is fully "run-in" before operating it
continuously at full power. (See engine instruction leaflet.)
3) Having set the Needle-Valve as described above, close the
throttle. The engine should idle steadily and continuously. (If
it stops immediately, first try turning the Throttle Stop Screw
clockwise a few degrees to raise the idling speed a little.)
(a) If, however, the engine idles unevenly and smokes a good
deal when the throttle is re-opened, before picking up speed,
it is probable that the idling mixture is too rich. In this case
it will be necessary to turn the Mixture Control Valve clock-
wise 30 to 60 degrees.
(b) Alternatively, if the Mixture Control Valve is set too lean, the
engine may stop when the throttle is closed, or it may idle for
a few moments, then speed up very slightly before coming to
a stop. Re-opening the throttle under these conditions will
usually result in the engine cutting out abruptly, without
smoking. Corrective action is to turn the Mixture Control
Valve about 30 to 60 degrees counter-clockwise.
Mixture Control Valve adjustment is not unduly critical, but do
not turn it more that 30 — 60 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.
Do not alter the Needle Valve setting while adjusting the Mixture
Control Valve.
Warning: For safety reasons, it is advisable to stop the engine
before carrying out adjustments to the Mixture Control
Valve or Throttle Stop Screw.
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
variations in atmospheric conditions will not normally affect
the other two controls. Slight readjustments may be neces-
sary for optimum performance if different types of fuel,
glowplugs or propellers are used.
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just possible
that the Mixture Control Valve may be inadvertently screwed in or
out too far and thereby moved beyond its effective adjustment
range.
Its 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.
Needle Valve
1.5~2.0 mm
Fig 1