O.S. Engines 40C User Manual

INSTRUCTIONS FOR O.S. TYPE 40C AUTOMATIC CARBURETTOR

This carburettor, as fitted to the O.S. MAX-46FX-H RING helicopter engine, 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 for reliable helicopter ascent and
descent.

Two adjustable controls are provided on this carburettor.

• The Needle Valve:
When set to produce maximum power at full throttle, the
Needle Valve establishes the basic fuel/air mixture
strength. This is then maintained by the carburettor's
automatic mixture control system to cover the engine's
requirements at reduced throttle settings.

•The

Needle Valve

Rotor Guide Screw

Mixture Control Valve
(Mixture Control Screw)

Throttle Lever

Photo 1

Mixture
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.

A PROVISIONAL SETTINGS (see ADJUSTMENT CHART)

• Open the Needle Valve 2 turns from the fully closed
position. (Be sure to use a muffler-pressurized fuel feed.)

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:
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.
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.

RE-ESTABLISHMENT OF MIXTURE CONTROL VALVE­SETTING
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 factory setting can be reestablished as follows:

1. Disconnect the throttle linkage by removing the con­trol rod from the throttle lever.

2. Fully close the throttle.

3. Turn the Mixture Control Valve clockwise gently until it

stops, while keeping the throttle fully closed. Do not
over-tighten the valve.

4. Turn the Mixture Control Valve 2-1/2 turns counter­clockwise from the above position. This is the basic

(factory) setting.

ADJUSTMENT PROCEDURE
The following adjustments are approximately correct when
using a fuel containing 25% lubricant and 10—30% nitro­methane.
Bear in mind that fuels containing relatively large per­centages of power-boosting nitromethane operate at richer
mixture settings than are needed for mild fuels and will,
therefore, require the Needle Valve to be readjusted
accordingly. The type and percentage of lubricant used is
also a factor here,as noted later in these instructions.

B As a safety measure, first check the transmitter controls,

including the throttle stick and trim lever positions, and
hold the main rotor securely before starting the engine.

C Warm the engine by allowing it to idle for about 30

seconds. If the engine stops, advance the throttle trim
lever slightly to increase the idling rpm. Then open the

throttle sufficiently to 'float' the model above the ground.

Control Valve

(Mixture

Control

Screw):

D If, at this time, the engine is slow to pick up and pro-

duces an excess of exhaust smoke, the mixture is too

rich. Correct this condition by turning the Mixture
Control Screw clockwise. If the mixture is extremely
rich, engine rpm will become unstable: opening the
throttle will produce a great deal of smoke and rpm may
drop suddenly or the engine may stop. This condition
may also be initiated by an excessively prolonged warm­ing-up period.

E If, on the other hand, the mixture is too lean, this will

be indicated by a marked lack of exhaust smoke and a
tendency for the engine to cut out when the throttle is
opened. In this case, turn the Mixture Control Screw
counter-clockwise to enrich the mixture.

F Turn the Mixture Control Screw 30 degrees at a time.

G Having provisionally set the idle mixture,

the next step is

to adjust the mixture for hovering flight.

H Hover the model and actuate the throttle to observe res-

ponse over the medium speed range. If the engine smokes
excessively and throttle response is poor, the mixture is
too rich; in which case, land the model and turn the
Needle Valve clockwise. Do not close the Needle Valve
too much, keeping it a little on the rich side at this stage.

I If, on the other hand, hovering is not stable and response

to the throttle is over-sensitive, or if the engine tends to

overheat,

this

indicates

that

the

mixture

is

too

leanand

should be corrected by turning the Needle Valve counter­clockwise.

J When satisfactory hovering flight has been achieved, land

the model again and re-check the engine's idling qualities.

K After about 10 seconds of idling, open the throttle to

'float' the model. If the transition is smooth, the idle

mixture is O.K. If the symptoms of either rich or lean

running are observed, readjust the Mixture Control

Screw accordingly.

L Now adjust the Needle Valve to achieve the best per-

formance when the model is flying at full throttle. If, at
full throttle, acceleration is poor, the exhaust unduly
smoky and the model fails to reach expected straight

line speed, the mixture is too rich and the Needle Valve

setling will need to be reduced.
If, however, after smoothly accelerating to satisfactory
high-speed straight and level flight, power is lost when
the model is pulled up into a climb, the mixture is too
lean. In this case, land the model immediately and re­adjust the Needle Valve to a richer setting.

M Now re-check hovering performance and, if

necessary,

fine-tune the mixture for hovering flight.

N For helicopters, good throttle response at medium

revolutions (e.g. hovering speeds) is most important,

since this is a power range widely used in helicopter
flight. The optimum fuel/air mixture strength at medium
speeds is dependent on obtaining balanced adjustment of
both the Needle Valve and the Mixture Control Valve.

If both controls are already at their optimum setting,

some modification to these settings may be necessary
to achieve satisfactory mid-range throttle response, but

such readjustments should only be made within the

range where idling reliability and high-speed perform­ance are not unduly compromised.
Readjustments should therefore be carried out as follows:

O If

the mid-range throttle response is not rapid and posi­tive (indicating a rich mid-range mixture), turn the
Needle Valve 2 or 3 clicks clockwise, or turn the Mix­ture Control Screw 10-20 degrees clockwise.

P If, on the other hand, the response to mid-range throttle

movement is too sensitive (indicating a lean mid-range
mixture), turn the Needle Valve 2 or 3 clicks counter­clockwise or turn the Mixture Control Screw 10—20
degrees counter-clockwise.

Q Throttle response at hovering speeds is also affected by

the relationship of the main rotor pitch angle to throttle
opening. If the optimum mid-range throttle response
cannot be obtained by the carburettor adjustments
described above, try adjusting the helicopter's pitch
control characteristics.