O.S. Engines 3H Carb - 22981000 - Manual

INSTRUCTIONS FOR O.S. TYPE 3H AUTOMATIC CARBURETTOR
This new 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. The device progressively reduces the effective size
of the fuel jet orifice as the throttle is closed, thereby preventing the engine from running too rich at low speeds. This
also means that an airbleed is no longer required and, with its elimination, maximum suction is maintained at the fuel
jet at all times. This is a most important factor where manoeuvres have to be executed at low engine speeds and
through wide variations of fuel level within the fuel tank.
Three adjustable controls are provided on this carburettor
• The Needle-Valve: For adjusting the mixture strength
when the throttle is f u l l y open.
• The Mixture Control Screw: For manually adjusting the
automatic mixture control valve in order to obtain a
stable idle and a smooth transition through the medium
speed range.
• The Throttle Stop Screw: For setting the position where
the carburettor rotor is closed.
Note: The Mixture Control Screw has been factory set for
the approximate best result. However, different fuels
and/or climatic conditions may necessitate minor
compensatory adjustment. Therefore, first run the
engine as received and re-set the Mixture Control
Screw only if necessary.
PROVISIONAL SETTINGS
Before starting the engine, check that the controls are set as
follows:
1) In the case of a fixed-wing aircraft, use a muffler pres-
surized fuel feed, and open the Needle-Valve 1 1/2 to
2 turns from the fully closed position. For helicopter
use, open the Needle-Valve 2 1/2 turns (or 2 turns if a
muffler-pressurized fuel feed is used) from the fully
closed position.
2) Make sure that the Throttle Stop Screw is set, in conjunc-
tion with the throttle lever linkage, so that (a) the throttle
rotor is fully open when the transmitter throttle stick is
fully advanced and (b) that the throttle rotor is fully
closed when both the throttle'stick and throttle trim lever
are fully pulled back.
ADJUSTMENT (HELICOPTERS)
In helicopter f l i g h t , t h r o t t l e response at medium revolutions
(i.e. hovering speeds) is most important, since this is a speed
range widely used in helicopter f l i g h t . The optimum mixture
at mid range will be obtained with a balanced adjustment of
both the Needle-Valve and the Mixture Control Screw.
A — Low Speed Performance — Mixture Control Screw
1) Start the engine with the transmitter throttle stick in the
closed position and the throttle trim lever set at the mid
position.
2) Adjust the throttle trim so that the highest possible idling
r.p.m. are obtained without actually engaging the clutch.
3) Now increase engine r.p.m. by means of the throttle stick
until the helicopter just 'floats' above the ground, allow
the engine to warm up for about 10 seconds, then gradual-
ly reduce r.p.m. and allow the engine to idle for about 5
seconds.
4) Repeat, several times, this procedure of increasing r.p.m.
so that the model 'floats' after idling for 5 seconds on the
ground. While doing this, carefully observe the exhaust
smoke and the behaviour of the engine. If, as the throttle
is opened, the engine tends to hesitate and to run with a
low-pitched exhaust note and an excess of exhaust smoke,
the mixture is too rich.
Throttle Rotor Set-Screw
Needle Valve
Mixture Control
Screw