May-Aug
2001
Here are a few scenes from the last
episode…
I have decided to abandon the Revflow (POSA)
carburetor for the following reasons,
1) Even
when full throttle mixture is within specification "stock" needles
seem to cause the engine to run lean in the 2700 - 3000 RPM range. CHTs climb
rapidly in this RPM range suggesting detonation.
2) When
full throttle mixture is adjusted per instructions idle mixture is excessively
rich. This has caused the engine to stall on a number of occasions (all on the
ground so far) during acceleration.
3) Friction
on the slide requires higher operating force especially at low throttle
settings. High temperatures increase this to uncomfortable levels.
4) Mixture
is sensitive to relatively small changes in fuel pressure. Vent position, fuel
level in tank, atmospheric changes and flight maneuvering all affect engine
performance significantly.
5) The
absence of an accelerator pump causes the engine to stumble slightly on rapid
acceleration such as during a balked approach.
6) From
my reading it seems that most of the actively flying KRs have either changed
away from the POSA or never used one in the first place.
After nearly 9 months of testing I simply
have too many issues with its operation.
The Tillotson carb, after some initial
success, also proved unsuited to my aircraft.
The biggest challenge proved to be getting
enough fuel into the carb at high power settings and low tank levels. Even
after drilling out the needle valve seat as large as possible the engine would
still run lean at these settings. It was not possible to find a larger needle
valve and seat even after a great deal of investigation.
Had I wished to install a fuel pump I
believe this carb could have been been made to work, however this would have
required considerable additional engineering. I would not have felt comfortable
unless I had a mechanical pump with an electric back-up similar to the
Cherokee.
I decided to try out the new Aero Carb form
the Sonex folks. Since this is essentially a POSA on steroids I considered this
an act of near-desperation. But the literature seemed to indicate that many of
the concerns I had with the earlier Revflow had been addressed.
The Sonex folks were pleasant, answered my
questions and offered to refund my money (less the cost of the bell which is
drilled out for each application). This seemed fair so I ordered the 32mm
spigot mount version.
http://www.aeroconversions.com/
Installation was straightforward (it would
have been very easy had I not modified the intake manifold for the Tillotson).
One of the nice features of this carb is that the throttle and mixture cable
ends are held in place on the body of the carb. No weird brackets to fabricate.
I again tested the fuel flow since the Aero
Carb accepts a 1/8" NPT fitting (different from the Revflow). I had also recently replaced a leaky fuel
shut off. Despite the fact that a
1/8" fitting is not standard for 3/8" fuel line (this really should
be 1/4" NPT) I did find a 90deg fitting with a male 1/8" NPT on one
end and a 37deg male flare end on the other. This allowed the use of 3/8"
line/hose right to the carb fitting which yields a worst case fuel flow of 20
GPH! With the 1/4" hose fittings required by the Revflow the best I could
get was around 15 GPH. In addition to the increased flow I feel the larger hose
is less susceptible to vapour lock. With a low head pressure gravity system
like the KR this makes me feel more secure.
The lion’s share of engine failures are fuel related in homebuilts and
factory-builts alike.
The carb required only one adjustment of the
needle to run well. Within 30 minutes
the top-end EGT was set (150 degrees below peak) and the idle was smooth.
Several tests were run with consistent EGT readings. These first runs were done at about ˝ tank as recommended by Rex
Taylor. Further tests were run at low
fuel (2 gal) and full tank levels. Full
power EGTs varied no more than 50 degrees. CHTs stayed well within limits during these
ground runs. The idle was also
consistent and smooth.
After about 10 hours of flight the engine
consistently turns the following numbers:
EGT 1100
(after leaning -150 below peak)
CHT 380
Oil Temp 190
Oil Pressure 36PSI
Fuel Cons 3.1 GPH
Oil Cons < 1 QT in 10 hours
The plugs are clean and I have not seen CHTs
in excess of 400 deg even during a 3000’ climb on a 95deg F day.
The only concern so far is a miss at idle
after the engine reaches operating temperature. This occurs at irregular intervals (between 5 and 20 seconds) but
is not serious enough to stall the engine.
Additionally it does not occur at RPMs above 1400, or when the engine is
cool. I have increased the size of the
blast tube to the magneto without effect. Additionally I have installed a
baffle under the oil cooler to direct hot air away from the carb. This seems to delay the onset of the problem
somewhat however it has not completely cured it.
