26
17
Needle-valve adjustment (Summary)
Practical best (optimum) needle-valve setting
Exhaust note starts to change.
Disconnect battery leads from
glowplug at about this point.
Clear, high-pitched two-stroke
exhaust note
Maximum rpm setting("Lean").
"Rich" needle-valve setting
when starting the engine.
The engine may stop if the battery leads
are disconnected from the glowplug
while the engine is running rich.
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.
Engine
stops
R
ev
ol
u
tio
n
s
s
ta
rt
t
to
de
cr
e
a
s
e
.
to
d
e
Intermittent, high-pitched two-
stroke note superimposed on low
"four-stroke" sound.
NOTE:
The above sketch is for reference purposes only.
Actual needle positions may differ from those shown.
1
7
6
5
4
3
2
20-30
o
27
Take note of this position of the needle-valve.
This will produce the practical best (i.e. optimum)
rpm setting (lower than maximum rpm).
A light grey exhaust emission may be observed.
Now, re-open needle-valve 20-30
o
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.
Finally:
exhaust note is now a steady high-pitched sound,
rising higher in pitch as needle-valve is closed and
rpm increases.The grey smoke will be lighter.
(However, make sure that engine is fully run in.)
Further needle-valve closure:
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 color.
As needle-valve is closed and rpm increases
a good deal of white smoke is emitted, accompanied
by a relatively low-pitched "four-stroke" exhaust note.
Once the optimum needle-valve setting has
been
established
(see
"Needle-valve
adjustment-Summary") the procedure for
starting is simplified as follows:
1) Open the needle-valve one half-turn (180˚)
from the optimum setting.
2) Open the throttle fully, place your finger over
the carburettor intake and rotate the
propeller through two revolutions to prime
the engine.
3) Set the throttle one-third open from the fully
closed position, energize the glowplug and
apply the starter. When the engine starts,
re-open the throttle and re-adjust the
needle-valve to the optimum setting.
Subsequent starting procedure
17
On starting from cold, with the needle-valve set at the
rich starting position:
