O.S. engine MAX-91FX Owner's Instruction Manual page 21

Optimum needle setting(1)
Slowly advance the throttle to its fully open position,
then gradually close the needle-valve until the
exhaust note begins to change. (4-cycle to 2-cycle)
At this point, disconnect the battery from the
glowplug, taking care that the battery leads or
glowplug clip do not come into contact with the
rotating propeller. If the engine stops when the
battery is disconnected, close the needle-valve about
30˚ and restart.
Optimum needle setting(2)
As the needle-valve is closed slowly and gradually,
the engine r.p.m. will increase and a continuous high-
pitched exhaust note, only, will be heard. Close the
needle-valve 10-15˚ and wait for the change of r.p.m.
After the engine r.p.m. increases turn the needle-
valve another 10- 15˚ and wait for the next change of
r.p.m. As the speed of the engine does not instantly
change with needle-valve readjustment, small move-
ments, with pauses between, are necessary to arrive
at the optimum setting.
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