O.S. engine MAX-15 Owner's Instruction Manual page 20

14 Needle-valve adjustment(Summary)
7
Practical best(optimum) needle-valve setting
Clear, high-pitched two-stroke
exhaust note
5
Intermittent, high-pitched
two-stroke note superimposed
on low "four-stroke" sound.
4
Disconnect battery leads from
glowplug at about this point.
3
Exhaust note starts to change.
Note: Although this is a two-stroke engine it fires like
a four-stroke at these rich needle-valve settings-i.e.
ignition of the fuel charge takes place at every fourth
stroke of the piston instead of at every second
stroke.
NOTE: The above sketch is for reference purposes only.
Actual needle positions may differ from those shown.
7
Maximum rpm setting("Lean").
1
20~30˚
"Rich" needle-valve
setting when starting
the engine.
2
The engine may stop if the battery leads
are disconnected from the glowplug while
the engine is running rich.
19
On starting from cold, with the needle-valve
set at the rich starting position:
a good deal of white smoke is emitted, accompanied by
a relatively low-pitched "four-stroke" exhaust note.
As the needle-valve is closed and the r.p.m. increases:
an intermittent high-pitched two-stroke note will be superimposed
on the low-pitched "four-stroke" note.
Exhaust smoke will be less dense and grey in colour.
Further needle-valve closure:
exhaust note is now a steady high-pitched sound,rising
higher in pitch as needle-valve is closed and increase.
The grey smoke will be lighter.
(However, make sure that engine is fully run in.)
Finally:
maximum rpm is reached and will fall off (or engine will
stop) if needle -valve is closed any further.
Exhaust gas will be very light.
Now, re-open needle-valve 20-30˚
This will produce the practical best (i.e. optimum)
rpm setting (lower than maximum rpm).
A light grey exhaust emission may be observed.
Take note of this position of the needle-valve.