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(1) Warm up the engine until all readings
are normal
(2) Adjust the engine rpm to the recommended
idle rpm
(3) Slowly pull the mixture control back
to idle cut-off
(4) Just before the engine quits, the engine
rpm should rise about 50 rpm if the mixture is properly
adjusted. If the rpm drops off without any
increase in rpm, the idle mixture is set too lean. If
the rpm increases more than 50 rpm, the idle mixture
is set too rich.
NOTE: Some amateur-builders, after properly
setting the idle mixture/rpm to the
manufacturer’s specification, increase the
engine idle rpm by 100 rpm for the first 10
+ hours of flight testing. This is to ensure
that the engine will not quit when the throttle
is pulled back too rapidly, or when power
is reduced on the final approach to landing.
b. Magneto Check:
(1) The magneto checks should be smooth
and the difference between both magnetos rpm drops
should average about 50 rpm. The builder also
should perform a ‘‘HOT MAG’’ check, to ensure
against the engine, on its own, deciding when and
where to start. To perform a hot mag check, run
up the aircraft until the engine is warm. At idle rpm
turn the magneto switch off; the engine should stop
running. If the engine continues to run, one or both
of the magnetos is hot (not grounded).
(2) The usual causes for a hot magneto are
a broken ‘‘P’’ lead coming out of the magneto or
a bad magneto switch. THIS IS AN IMMEDIATE
23
5/24/95 AC 90-89A
THREAT TO THE PERSONAL SAFETY OF ANYONE
NEAR THE AIRPLANE AND MUST BE
REPAIRED AT ONCE.
c. Cold Cylinder Check:
(1) If the engine is running rough and the
builder determines it may be an ignition problem,
perform the following check:
(i) Run the engine on the bad magneto
for about 30 seconds at 1200 rpm. Without
switching the mag switch back to ‘‘both,’’ shut off
the engine.
(ii) One of the test crew should
quickly use a grease pencil to mark an area of the
exhaust stacks approximately an inch from the flange
that attaches the stacks to the cylinders.
(iii) Check the marks on the stacks. If
one or more of the exhaust stacks with a grease mark
has NOT been burned to a grayish-white color and
the mark on the stack still retains most of the original
color of the grease pencil, the ‘‘cold cylinder’’ has
been identified.
(2) Probable causes of the cold cylinder
problem are defective spark plugs, ignition leads, or
a cracked distributor in one of the magnetos. To
detect if the spark plugs are bad, switch both plugs
to another cylinder. If the grease pencil proves the
problem moved to the new cylinder, the spark plugs
are bad. If the problem remains with the original
cylinder, the ignition lead or magneto is bad.
d. Carburetor Heat:
(1) It is strongly recommended that all
amateur-builders install a carburetor heat system that
complies with the engine manufacturer’s recommendation.
If no recommendation is available, the
FAA suggests a carburetor heat system for a sealevel
engine and a conventional venturi should be
designed so that it will provide a 90 degrees F
increase in the venturi at 75 percent power. For altitude
engines using a conventional venturi carburetor,
120 degrees F increase in venturi temperature at 75
percent power will prevent or eliminate icing.
Remember: Too little carburetor heat will have no
effect on carburetor icing, and too much carburetor
heat will cause a overly rich mixture which will
reduce power and may shut down the engine.
(2) During the engine tests, make numerous
checks of the carburetor heat system. To avoid
overly rich mixtures from oversized carburetor heat
ducts, ensure that the carburetor heat duct is the same
size as the inlet of the carburetor.
(3) Be certain there is a positive reduction
in rpm each time ‘‘carb heat’’ is applied. If there
is no reduction, or the rpm drop is less than expected,
check the carb heat control in the cockpit and on
the carb heat air box for full travel. Also check for
air leaks in the ‘‘SCAT TUBE’’ that connects the
heat muff to the carburetor air box.
e. Fuel Flow and Unusable Fuel Check: This
is a field test to ensure the aircraft engine will get
enough fuel to run properly, even if the aircraft is
in a steep climb or stall attitude.
(1) First, place the aircraft’s nose at an
angle 5 degrees above the highest anticipated climb
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Amateur-built Aircraft & Ultralight Flight Testing Handb(16)
