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How
to adjust the needles on your Marine Engine
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To obtain proper
adjustments on a marine engine it must have the proper load
on the prop.
Trying to tune
your engine outside the water will not put any load on the
prop and requires less fuel to operate your engine. A common
problem is someone will think the have the correct setting
outside the water (please read con
rod breakage) then when the boat is thrown into the water
it will run for about 15 feet and die. This is because the
engine now has the correct load on the prop and not enough
fuel to overcome the load.
On the other extreme,
you can't get the proper setting by holding the boat in the
water. This put too much load on the prop because the boat
is not running through the water. This will cause too rich
a setting to overcome the higher load.
The only way to
get the correct settings is to allow the boat to run through
the water and make minor adjustments by bringing the boat
back to shore.
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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 or if you have a mixture disk place
it in the neutral position so as not to affect the high speed
mixture.
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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 push the boat. This is the setting you normally
look for before launching the boat then further adjustments
can be made to fine tune the engine.
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.
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Here are
some 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 launch the boat into the water and
open the throttle to the full open position.
If
the boat only travels 10 or 20 feet and dies the engine
is running too lean, unscrew the main needle one full
turn and repeat this procedure, if it dies again, unscrew
another full turn, and repeat. You should start to see
an improvement. If not screw the needle back in 2 1/2
turns and repeat. Now you should be able to determine
which way gives you the improvement.
If
boat runs through the water slowly and sputtering, it
is probably on the rich side of the setting, Bring the
boat back to shore and turn the needle in 1/4 of a turn.
Repeat this until the desired mixture is achieved as
described above.
See
also
How
to adjust a K&B carb with the brass mixture disk
or
How
to adjust an early style square K&B carb
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| Can't get
low speed adjustment right.
This related
to carbs with a low speed needle in the center of the
throttle barrel like on all the MECOA's & earlier
K&B carbs.
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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.
After the engine starts launch
the boat into the water and open the throttle to the
full open position. 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 archived. Unfortunately correct
setting can only be achieved while the boat is running
through the water.
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.
See
also
How
to adjust a K&B carb with the brass mixture disk
or
How
to adjust an early style square K&B carb
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| 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 correct. |
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.
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| 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 above.
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.
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DON'T FORGET ALL
THESE ADJUSTMENTS
MUST BE MADE WHILE THE BOAT IS RUNNING THROUGH THE WATER.
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The
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.
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©
Model Engine Corporation of America, All rights reserved.
MECOA
and K&B are Registered Trademarks of Model Engine Corp. of
America
Registered U.S. Patent Office
Linking
to this page is permissible, However no part may be reproduced
without written permission from
MECOA K&B Mfg -- 16015 Adelante St -- Irwindale, CA 91702
U.S.A.
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