O.S. Engines 70SZ-H - Manual (Page 11)

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Now re-check hovering performance It may be
found that (with the High-Speed Needle Valve
now set for optimum full-throttle performance)
hovering will reveal symptoms of slightly lean
running at medium speeds. DO NOT touch the
High-Speed Needle Valve.
Now adjust the High-Speed Needle Valve to
achieve the best performance when the model is
flying at full throttle. lf, 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 High-Speed Needle
Valve setting will need to be reduced.
lf, however, after smoothly acceIerating to
satisfactory high-speed straight and level flight,
power is lost when the model is puIled up into a
climb, the mixture is too lean. ln this case, land the
model immediately and readjust thw High-Speed
Needle Valve to a richer setting. 3.
4. Having now found the optimum setting for the
High-Speed Needle Valve, re-check the engine's
idle performance, fine tuning the Idle Mixture
Control Screw, if necessary, to perfect the idle.
5.
It is at this point that the Medium Speed Needle
Valve, previously kept one turn open, may be
brought into operation. Open the Medium Speed
Needle Valve 45 degrees, or 4-5 clicks, then fine
tune until the optimum hovering performance is
obtained. If on the other hand, hovering reveals
rich running, close the Medium Speed Needle
Valve gradually until the optimum hovering
performance is obtained.
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CARBURETOR CLEANLINESS
The correct functioning of the carburetor 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 carburetor 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 carburetor as shown in Photo and extract
any remaining foreign matter that may have lodged in
the location shown in Photo, right.
Remove this with
an 8mm spanner
Dirt and fbrous matter
mostly accumulate here
SUBSEQUENT READJUSTMENTS
Once the engine has been run-in and the carburetor
controls properly set up, it should be unnecessary to
alter the mixture settings, except to make minor
changes to the Main (High-Speed) Needle Valve
occasionally, to take account of differences in climatic
condisitions.
However, as previously mentioned, the use of a
different fuel, particularly one containing more, or
less, nitromethane and / or a different type or
proportion of lubricating oil, is likely to call for some
modification to the High-Speed Needle-Valve
adjustment.
As a safety measure, it is advisable to increase the
High-Speed Needle Valve setting by an extra half-turn
counter-clockwise, prior to establishing the new
setting. The same applies if the silencer type is
changed. A different silencer may alter the exhaust
pressure applied to the fuel feed and call for a revised
Needle-Valve setting.
The use of a different glowplug, or changes to the
main rotor and its pitch angles may also require
compensating carburetor readjustments.