O.S. Engines 91HZ-R - Manual (Page 18)

34
CARBURETOR CLEANLINESS
Remove this with
an 8mm wrench
Dirt and fibrous 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. 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 foreign
matter 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.
35
ADJUSTING CHART
High Speed Needle Valve - two and half turns open from fully closed position. Medium Speed Needle Valve - one turn open from fully closed position.
Start the engine
Warm up the engine
Observe the mixture condition while "floating" the model
Turn the Idle Mixture Control
Screw
counter-
clockwise 5 degrees
Idling OK
Turn the Idle Mixture Control Screw clockwise 5 degrees
Lean mixture
Rich mixture
Observe the hovering mixture condition
Open the High Speed Needle Valve (Turn counter-clockwise)
Hovering OK
Lean mixture
Rich mixture
Close the High Speed Needle Valve (Turn clockwise)
Land the model
Idle for approx: 10 seconds, then reopen throttle to observe transition ("Float" the model)
Idling OK
Lean mixture
Rich mixture
High Speed Flight
Lean mixture
Rich mixture
High Speed Flight OK
Fine tuning of hovering mixture
Open the Medium Speed Needle Valve
Hovering OK
Adjustment completed
Open the High Speed Needle Valve (Turn counter-clockwise)
Close the High Speed Needle Valve (Turn clockwise)
Fine tuning of idling mixture
Lean mixture
Rich mixture
Idling OK
Lean mixture
Rich mixture
Close the Medium Speed Needle Valve Turn the Idle Mixture Control Screw clockwise 5 degrees Turn the Idle Mixture Control Screw clockwise 5 degrees
Turn the Idle Mixture Control
Screw
counter-
clockwise 5 degrees Turn the Idle Mixture Control
Screw
counter-
clockwise 5 degrees