1⁄3- inch space between the body and screw
head. If turned in any farther, the regulator
diaphragm will be unseated and put out of
operation. The main purpose of the adjustment is for tailoring the mixture mid-range.
If the engine runs rich through the mid-range, back the adjustment out of a half
turn and, if lean, turn it in a half turn. Other
than this, the adjustment should be left
alone with all mixture adjustments made at
I should point out that after 20 years,
there is a good chance that the regulator
diaphragm may have dried out and be ineffective. If the adjustment screw does not
have any effect, this is the reason. This
being the situation, just turn the adjustment screw all the way in and use muffler
pressure in conjunction with the pump. If
the pump will not pump fuel, the butterfly
valves have frozen or corroded away. In this
case, return the pump to Gary Conley, who
now produces the Perry pumps, carburetors
and related accessories for servicing; con-leyprecision.com; (630) 858-3160.
4-STROKES & MUFFLERS
U Could you please settle a debate that has
been going on many years in our Lake Country
RC Flying Club (Ardmore, OK)? The debate is
whether to use muffler pressure on 4-strokes.
One side states that if the fuel tank is in the
proper position to the carb, pressure is not
needed, so your needle setting is not as critical
and exhaust gasses are not being put back into
your fuel tank. We have found that rarely does
the engine lean toward the end of a tank because
the needle setting is much less sensitive. Instead
of a two-click range from lean to rich, it can be
a five- to 10-click range. Power and run time
seem the same as running pressure.
The other side believes that pressure is the
only way to go. That is what the manufacturer
wants us to do, otherwise the pressure fitting
would not be there and the exhaust gasses being
pumped back into the fuel tank will not harm
I realize that 4-strokes have better ability to
draw fuel from the tank than 2-strokes. I run
pressure on my 2-strokes.
Do exhaust gasses in the fuel harm the
engine? Why does the needle valve become less
sensitive without pressure? Is there an advantage to either side?
Thanks from all the fliers in Oklahoma.
Paul, gene rally spe aking, the use of muffler
pressure, whether with a 2-stroke or 4-
stroke engine, is an advantage. Contrary to
what you say, the 4-strokes have considerably less fuel draw ability than an
equivalent displacement size 2-stroke —
hence the use of considerably smaller
throat diameter carburetors. You must bear
in mind that at 10,000rpm, a 4-stroke is
only seeing 5,000 intake pulses verses
10,000 for a 2-stroke. Although a 4-stroke
has longer intake duration, this does not
make up for the lower number of pulses.
The use of muffler pressure makes the fuel
tank position less critical, lessens the
THE USE OF
THE FUEL TANK
THE CHANCE OF
LEANING OUT MORE
IN MORE VIOLENT
chance of the engine leaning out more in
more violent maneuvers, etc. The various
manufacturers also design the carburetor to
be used in conjunction with muffler pressure, i.e., larger throat diameter carburetors,
different needle valve taper, etc. Without
muffler pressure, the needle valve does
become less sensitive due to metering vacuum rather than pressure, but at the
expense of reduced fuel draw ability.
It is true that the exhaust gases are contaminating the fuel, but this does not seem
to be a significant problem unless something is wearing in the engine and metal
particles exiting the exhaust are going back
into the fuel, which is definitely not good.
If an engine runs fine without muffler pressure, and you are satisfied with its
performance, then there would be no reason to use it.
Our last letter is the kind I like to receive
in that the sender, Paul Geders, had a problem that he solved himself. Thanks for
sharing your finding with our readers, Paul.
Sometimes it’s the little things that are overlooked that cause the biggest problems.
I have been enjoying this hobby going on 54
years now and ran into an engine quitting problem that gave me fits for quite a while. Engine
is an O.S. . 61 SF ABC with a model 7L carburetor mounted inverted in a Goldberg Matrix
Extreme 3DNd. It uses Cool Power 15% fuel,
K&B-1L plug and a Master Airscrew 12x6 propeller. Fuel tank is level with carburetor.
Symptoms: start engine, needles nicely. Take
off, fly around for about 2 minutes and the
engine would quit abruptly. Restart and the
engine runs normally; take off and the same
thing happens. Take the airplane home, pull the
tank and internal plumbing of tank out and recheck. Everything checks OK.
Back to field: start engine and it doesn’t needle right. Pull the carburetor off and do a
tear-down at the field. When I removed the nee-dle-valve body from the carburetor body and
inspected the cavity closely, I found a sliver of a
clear material. I removed the material, reinstalled the carburetor and fired it up. Needled
well and took off. Two minutes later, the engine
quits again. This time, I noticed that the engine
was very hot, indicating the engine was getting
lean long before it quit.
Frustrated, I take it home and pull the carburetor apart again to find another sliver of
clear material in the same location as before.
Where the heck is this stuff coming from? A
thorough inspection of the fuel lines and I discover the culprit. Seems that there was a slight
burr on the fuel inlet nipple that was cutting
into the inner wall of the red silicone tubing and
creating a sliver of silicone each time the fuel
line was reinstalled. Removed the burr, replaced
the fuel line and all is well.
Lesson learned is to inspect all fuel connections, especially to ensure that all fuel tubing is
chamfered so that when installing the fuel line
(which most of us do at about a 40-degree
angle) on the tubing we don’t inadvertently cut
off a sliver of material. Check for burrs on the
fuel inlet nipple, too. Even a fuel filter wouldn’t
have caught this because it was right at the carburetor inlet connection.
Problem solved and all is well!
PAUL F. GEDERS, ST LOUIS, MO
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