Evolution Engines 20GX 20cc - Manual (Page 13)

13
Low-Speed Needle Adjustment
If you have properly set the idle needle, the low-speed setting should be within 1/16th of a
turn from perfect. Remember we are talking about minute amounts of fuel going through the
carburetor, any adjustments you now make to the idle needle should be very, very small.
Take into consideration the length in service (e.g. break-in time) and the size of propeller you
are using. In the beginning, use a smaller propeller early in the break-in process. You will not be
able to achieve an idle much below 2000 rpm. The larger the propeller, the lower your idle rpm
will be, and the more broken in your engine is will allow a lower idle rpm (15 x 6 should be about
1800–2000 rpm).
Your engine will idle at low-speed needle settings from far to0 rich to far too lean without
any damage, so the quality of the idle is not a good indicator of the proper low-speed needle
settings. The transition from idle to full throttle will be used to determine the position of the
low-speed needle.
1. With the engine started and warmed up,
open the throttle fully then reduce to idle
and note the rpm. Listen to the engine.
2. Let the engine idle for ten seconds and then
rapidly advance the throttle to full open. One
of three things will occur:
a. The engine responds instantaneously.
Your low speed needle is set perfectly.
Now use your throttle trim to achieve the
lowest reliable idle.
b. The engine slowly accelerates to full
throttle. This indicates the low speed
needle is set too rich and that fuel is
building up in the crankcase. Shut the
engine off and lean (clockwise turn) the
low speed idle screw by an amount equal
to the thickness of the screwdriver blade
you are using to make the adjustment.
Restart the engine and repeat steps 1 and 2.
c. The engine stutters or stops on its way to
full throttle. This indicates the low speed
needle is set too lean. With the engine
off, richen (counterclockwise turn) the
low speed idle screw by an amount equal
to the thickness of the screwdriver blade
you are using to make the adjustment.
Restart the engine and repeat steps 1 and 2.
3. Once you have achieved the instantaneous
throttle transition, your low-speed needle is
set perfectly. Now go back and recheck your
high-speed needle valve setting. There is
some interaction between the two needles
so you might need to do this process (HS
and LS needle setting) a couple of times.
Patience here will reward you with an easy to
use engine. Don’t try to do this too quickly.
It is possible that with a heavier-loaded
propeller that you may need to richen
the needle in order to attain a smooth,
instantaneous response again, but using the
above procedure should lead you to the proper
setting everytime.