O.S. Engines 86 Carb - 27881000 - Manual

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 86 CARBURETTOR
The O.S. Type 86 carburettor has been designed exclusively for use with the O.S. PD-02 fuel pump unit. These two
components, with appropriate plumbing, form the O.S. PA-102 Pump System. The O.S. Pump Systems offer the twin
advantages of increased power and the virtual elimination of variation in fuel "head" at the carburettor.
The Type 86 carburettor has an extra large throat, which improves breathing and increases power. With normal suction fuel
feed, or low-pressure (exhaust gas) assisted fuel feed, such a large choke area would be impracticable, but the special pump
unit ensures that fuel is delivered to the carburettor at the pressure required to maintain optimum mixture strength, irrespec-
tive of fuel tank location or the aircraft's attitude.
These qualities are especially valuable for international class pattern aerobatic flying and in models (including scale aircraft)
where it may be difficult to locate the fuel tank close to the engine.
The Type 86 carburettor is not suitable for engines not equipped with a pump: for these the O.S. Type 7L carburettor is
available.
Mixture Control Valve Stopper
1.5~2.0mm
Fig 1
Photo-1
Three adjustable controls are provided on the Type 86
carburettor. (See photo below.)
• 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.
• The Throttle Stop Screw: For establishing the minimum
idling speed.
PROVISIONAL SETTING
Before starting the engine, open the Needle-Valve three full
turns from the fully closed position.
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.*
* Warning:
(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 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 continuous-
ly. (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 mix-
ture is too rich. In this case it will be necessary to
turn the Mixture Control Screw clockwise about 5
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 5 degrees counter-clockwise.
Mixture Control Screw adjustment is not unduly critical,
but do not turn it more than 5 degrees at a time. Carry
Plug Screw
Throttle Lever
Nipple
Nipple
Mixture Control Valve
Idle Mixture
Control Screw
Needle Valve Throttle Stop Screw