O.S. Engines 8AA User Manual

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 8AA CARBURETTOR

The O.S. Type 8AA carburettor has been designed to provide a correctly balanced mixture of fuel and air at all throttle settings. It ensures steady revolutions and a smooth response even to abrupt operation of the throttle.

INSTALLATION NOTES Note: This carburettor is not fitted with a throttle stop

screw. Instead, idling speed is adjusted by means of the throttle trim lever on the transmitter. This enables the full r.p.m. range, from idling to full power, to be controlled by the throttle stick, and then allows the engine to be stopped, from the transmitter, by closing the throttle completely with the trim lever. Set up the throttle linkage as follows:

1) With the transmitter throttle trim lever fully retarded, adjust the throttle servo linkage so that the throttle rotor is (a) fully open when the transmitter throttle stick is fully advanced and (b) fully closed (i.e. engine stopped) when the stick is fully retarded.

2) The idling speed is then set by advancing the throttle trim lever to the point where the engine runs, steadily and reliably, at the desired idling speed.

Be sure to use a muffler-pressurized fuel feed and, before attempting to start the engine, check that the carburettor controls are set as indicated below.

ADJUSTING THE CARBURETTOR

1) Open the Needle-Valve 1~1-1/2 turns (or 1-1/2~2 turns if a tuned silencer not is used) from the fully closed position.

2) 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.

3) 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 1/8 to 1/4 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

fully

"run-in"

before operating it continuously at full power. (See engine instruction leaflet.)

4) Having set the Needle-Valve as described above, close the throttle. The engine should idle steadily and continuously. (If it stops immediately, first try setting the idling

speed a little higher by means of the throttle trim lever.)

(a) If, however, the engine idles unevenly and smokes a

good deal when the throttle is re-opened, before pick-

ing up speed, 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 30 to 45 degrees.

(b)Alternatively, if the Mixture Control Screw 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

Screw about 30 to 45 degrees counter-clockwise.

Mixture Control Screw adjustment is not unduly critical, but do not turn it more than 30—45 degrees at a time. Carry out readjustments progressively and patiently, remembering the symptoms of rich and lean running, quoted above, until the engine responds quickly and positively to the throttle.

Fuel Inlet

Needle Valve

Rotor Guide Screw

2~3mm

Idle Mixture Control Screw

Throttle Lever

Fig.(l)

PARTS LIST

8.&3-K Code No.

28281900 28281970 26381501 24981837 27381940 26711305 22681953 28281600 24925202 29381100 29315000 28281500 28281210 28281200

29381300

27881820 28281400

The specifications are subject to alteration for improvement without notice.

Description

Needle Valve Assembly Needle Set-screw "0" Ring Needle Valve Holder Ratchet Spring Fuel Inlet

Rotor Guide Screw Carburettor Fixing Screw Carburettor Body Carburettor Rubber Gasket Rotor Spring Venturi Insert Carburettor Rotor Mixture Control Screw "0" Ring Throttle Lever Assembly

5) If, after carrying out mixture adjustments, the idling speed is found to be too high, the throttle trim lever on the transmitter may be retarded 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 variations in atmospheric conditions will not normally affect the other two controls. Slight readjustments may be necessary for optimum performance if different types of fuel, glowplugs or propellers are used.

REALIGNMENT OF MIXTURE CONTROL SCREW

The Mixture Control Screw is factory set at the approximate best position. If, however, it has been tampered with, or moved accidentally, the factory setting can be reestablished as follows:

1) Set the throttle rotor in the fully closed position;

2) Carefully screw in the Mixture Control Screw until it stops;

3) Unscrew the Mixture Control Screw 2 1/2 turns.

CARBURETTOR CLEANLINESS

Minute particles of foreign matter (present, to some extent,

in all fuels) will cause problems if not removed before they reach the carburettor jet. Use high quality fuel filters and make sure that the fuel tank and your refuelling container are clean. The use of an O.S. Super-Filter Type L, with its special double-filter system, is recommended. Installed inside the refuelling can at its outlet, it will prevent the passage of foreign matter into the fuel tank, and a good inline filter, between tank and carburettor, will give added protection.

Remember to clean the filters regularly. It is also advisable to check the carburettor, itself, at least once every 30 flights. Remove the needle-valve holder, using the openended wrench supplied, and extract any remaining dirt or fibres that may have lodged in this part of the carburettor body. If allowed to build up, such material can restrict fuel flow and cause the engine to stop; or to run lean, with the attendant risk of engine damage due to overheating.

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

FAX.

(06)

704-2722