to adjust needle valves on aircraft engines
|The number of turns
out on a main needle is an arbitrary starting point. No set
number of turns will be correct as the setting will vary with
fuel, air density, elevation, humidity an temperature. You have
to learn to tune your engine.
|A good starting
point is 3 to 5 turns out on the high speed and and the low
speed should be backed out to a point where it is not affecting
the high speed mixture.
|A model engine makes
sounds that will tell you how it's performing. You'll have to
listen very carefully for them, recognize their message, and
make adjustments to the fuel control needle valves accordingly.
The mixture of fuel and air is controlled by the amount of fuel
metered by the needle valve.
SLOPPY RICH MIXTURE
running is characterized by a very slow, irregular, sputtering
exhaust sound. The exhaust gas will be very smoky and contain
many droplets of oil. NEVER RUN AN ABC ENGINE AT THIS SETTING.
The cylinder is not able to heat properly and the fit will
be ruined in a short time.
RICH MIXTURE running
is characterized by a slower, sometimes irregular, sputtering
exhaust sound. The exhaust gas will be smoky and probably
contain small droplets of oil. NEVER RUN AN ABC ENGINE AT
THIS SETTING. The cylinder is not able to heat properly and
the fit will be ruined in a short time.
FOUR CYCLING or
SLIGHTLY RICH running is a rich type setting, but it is fast
enough to pull the airplane. This is the setting you normally
look for before launching the airplane because the engine
will run leaner when airborne.
PEAKED OR TWO CYCLE.
As the main needle is closed (clockwise), it reduces the amount
of fuel mixed with the air drawn into the engine. At a specific
point, which varies with each engine, air temperature, altitude
and relative humidity, the exhaust note will change quickly
into a smooth, powerful note. If the needle is closed further,
the note will stay smooth, but will weaken. The peak occurs
just at the break point from a rich setting and further leaning
will ruin the engine. A lean setting raises the engine heat
above the safe point, reduces lubrication, and destroys glow
plugs due to high combustion temperature. This is very harmful
to the engine and your investment. Learn to tune the engine
before flying. Remember, a little rich is always preferred
for long motor life.
Here are the frequently asked questions...
|How many turns outs should my needle be set.
||Turns out are an arbitrary starting point and should
not be considered a correct setting. If we knew exactly
how many turns out a needle should be set you wouldn't
need a needle to adjust. You have to follow procedures
below to obtain the correct settings.
|How do I set the high speed adjustment?
First be sure the idle needle is not interfering with
the high speed adjustment as described in the next step.
If you have a new engine read How
to break in an ABC engine or How
to break in a Ringed engine
After the engine starts open the throttle to the full
Unscrew the main needle...
if the engine slows,
it is on the rich side of the setting, slowly turn the
needle in until the desired mixture is achieved as described
if it speeds up, it
is on the lean side of the setting, continue unscrewing
the needle until the engine begins to slow. Now you
are on the rich side of the setting, slowly turn the
needle in until the desired mixture is achieved as described
|Can't get low speed adjustment right.
For carbs with a low speed needle in the center of
the throttle barrel like on all the MECOA's & GMS's:
|The high speed must be adjusted correctly before the
idle mixture can be set.
Unscrew the idle needle until it stops or the head
of the needle is flush with out side of the receptacle
it screws into. At this point we know the idle mixture
will be very rich and not interfering with the high
speed needle. Start the engine and open the throttle
to full speed. Adjust the high speed needle to the correct
setting. Close the throttle to the lowest possible speed
without the engine stopping. Now screw the idle needle
in, the engine should speed up. Again close the throttle
to the lowest possible speed without the engine stopping.
Screw the idle needle in again, the engine should speed
up. Keep repeating this procedure until a low idle is
Using this method, you will always obtain the correct
idle setting because you will be starting from rich
setting moving towards a leaner correct setting.
|Engine leans out after running for a while.
||This is usually caused by a pin hole in the pick up
tube inside the fuel tank. Check for air bubbles in the
fuel line with the tank nearing empty while the engine
is running. If you see bubbles when the fuel level is
low and not when the tank is full there is a pin hole
that is being exposed when the fuel drops below it exposing
the hole to air. Replace the fuel line.
|Engine leans out, then richens. Mixture never stays
This is typically caused by debris
in the carb plugging and unplugging the needle orifice.
This causes irregular mixture setting at high RPM's.
Disassemble the carb and clean in the main needle passage
and spray bar.
Another cause of this is the pressure tap in the muffler.
Some oils in fuels can cause the tap to be plugged up
not allowing pressure to the tank and in turn depriving
the engine of fuel, then the oil will blow through the
line providing pressure to the tank providing more fuel
to the engine. Hence, pressure, no pressure, as you
fight to find the correct adjustment.
|Engine leans out when it gets hot.
||This is sometimes caused by loose head screws. Tighten
all the head screws in a cross torquing pattern.
|Fine tuning the idle mixture.
||Start the engine and open the carburetor to the full
open position, then adjust for peak R.P.M. with the main
needle as previously described.
Close the carburetor barrel slowly until the lowest
possible speed is reached without the engine stopping.
Go to full throttle after about 10 seconds of idling.
If the engine gains speed slowly, the idle mixture is
too rich. If the engine stops, the idle mixture is too
lean. Turn the idle needle clockwise if mixture is too
rich and counterclockwise if too lean.
The engine will accelerate from idle to full throttle
smoothly and instantaneously when properly adjusted.
The engine may not idle well at a low setting or accelerate
as quickly until it is has some running time on it.
above information is provided as a guide. Since MECOA/K&B has
no way of determining the ability of the individual using and understanding
this information, we assume absolutely NO RESPONSIBILITY for any
damage to person or property from the use of this information.
Model Engine Corporation of America, All rights reserved.
and K&B are Registered Trademarks of Model Engine Corp. of
Registered U.S. Patent Office
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without written permission from
MECOA -- P.O. Box 5 -- Sierra Madre, CA 91025 U.S.A.