O.S. engine MAX-91SZ-Hring Owner's Instruction Manual page 13

2. Having provisionally set the idle mixture, the next
step is to adjust the mixture for hovering flight.
Hover the model and actuate the throttle to
observe response over the medium speed range.
lf the engine smokes excessiveIy and throttle
response is poor, the mixture is too rich ; in which
case, land the model and turn the High-Speed
Needle Valve clockwise. Do not close the High-
Speed NeedIe Valve more than the recommended
initial adjustment (3 to 4 clicks) at a time, keeping
it a little on the rich side at this stage. DO NOT
touch the Medium Speed Needle Valve.
lf, on the other hand, hovering is not stable and
response to the throttIe is over-sensitive, or if the
engine tends to overheat, this indicates that the
mixture is too lean and should be corrected by
turning the High-Speed Needle Valve counter-
clockwise.
When satisfactory hovering flight has been achieved,
land the modeI again and re-check the engine's idle
qualities.
Now adjust the High-Speed Needle Valve to
3.
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.
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.
Now re-check hovering performance It may be
5.
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.
After about 10 seconds of idling, open the throttle to
'float' the model. lf the transition is smooth, the idle
mixture is O.K. If the symptoms of either rich or Iean
running are observed, readjust the Idle Mixture
Control Screw accordingly.
Adjustment range of Medium
Speed Needle Valve
Adjustment range of Idle Mixture
Control Screw
2/3
Fully Closed 1/3
Throttle Opening
NOTE: With this carburetor, neither the Idle
Mixture Control Screw adjustment, nor the
Medium Speed Needle Valve adjustment will have
any effect on mixture strength when the throttle is
fully (or almost fully) open.
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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 30 degrees, or 3-4 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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Needle Valve too
far open
High Speed Needle
Valve Setting varies
according to the curve.
Needle Valve too
far closed
Fully Open