O.S. Engines 4BK User 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.

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 adjusting 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 approximate 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.

Throttle Stop Screw

Needle Valve

Mixture Control Valve

Photo (1)

1.5~2.0 mm

Fig

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

1/2

turn

open

(i.e.

"rich")

from

the

peak

r.p.m.

even though this will slightly reduce r.p.m. on the ground.

(b)

Make

sure

that the

engine

fully

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

Note: Once the correct carburettor settings have been established,

before carrying out adjustments to the Mixture Control Valve or Throttle Stop Screw.

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.

"run-in"

before operating

setting,

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.

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 Mixture Control Valve until 'A' is precisely tangential to the rotor hole (i.e. so that 'A' and 'B' are superimposed) as in the main sketch.

CARBURETTOR CLEANLINESS

The correct functioning of the carburettor depends on its small fuel orifices remaining clear. The minute particles of foreign matter that are present in any fuel can easily partially obstruct these orifices and upset mixture strength so that engine performance 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 accumulates on the filter screen. Finally, occasionally remove the needle-valve holder from the carburettor as shown in Photo (2) and extract any remaining foreign matter that may have lodged in the location shown in Photo (3).

Remove this with an 8mm spanner

Dirt and fibrous matter mostly accumulate here

Photo (2)

PARTS LIST

Description

Throttle Lever Set-screw Carburettor Rotor Mixture Control Valve "0" Ring Rotor Spring Throttle Stop Screw Assembly Throttle Stop Screw Throttle Stop Screw Holder Nipple No.1 Carburettor Body Carburettor Rubber Gasket Carburettor Fixing Screw Needle Valve Assembly Needle Valve Holder Assembly Ratchet Spring Needle "0" Ring Set-screw

The specifications are subject to alteration for improvement without notice.

Code No.

4BK 24981405 26381501 24981219 26781309 24881824 26781506 26781619 26681305 26681803 22681953 24981122 22115002 25081700

27881900 27381940 26711305 24981959 24981837 26381501

4D 24981405 26381501 25381203 26781309 24881824 26781506 26781619 26681305 26681803 22681953 25381117 22115002 25081700

27881900

27381940 26711305 24981959 24981837 26381501

Photo (3)

5B 26781400 26381501 25181206 26781309 24881824 26781506 26781619 26681305 26681803 22681953 25181111 22115002 25081700

27881900 27381940 26711305 24981959 24981837

26381501

6-15 3-chome Imagawa Higashisumiyoshi-ku Osaka 546, Japan. TEL. Osaka (06) 702-0225

88804