At the last annual, we replaced the 3" scat tube in our 74 Cherokee 140
Cruiser that runs from the nose cowling to the carb air box. The old one
was getting tattered and had several small pinholes in it. It ran fine
all summer and fall. About 2 months ago, I went out to fly the beast. It
fired up, then it immediately shut down, and then refused to restart.
Upon pulling the plugs, I found all 8 to be filled with water. Once
cleaned out, I started the thing up again, this time with the carb heat
lever half way between "off" and "on". It started fine, but when I moved
the lever slowly to "off", it began running rough as it once again began
ingesting water. A few seconds later and it cleared up.
I inspected the induction system and the carb heat box and control to
find everything working normally. We already replaced the air box door
when the seal became worn and the grommets/bushings. When this first
started happening, I thought that maybe the retaining screws may have
come loose and the door was not operating properly because of it. But
everything looked and operated as it should. We had just had a downpour
the previous evening and I figured water found its way in somehow.
I have had the thing out several times since then, each time starting it
up with the carb heat in the half-way position. On 2 subsequent
occasions (both with temps hovering around 30 degrees - no preheat), the
carb heat lever would not go completely "off" or "on". And, after
warming up a bit with the carb heat on (as far "on" as it would go), it
would run rough as it ingested water. Once I taxied back from the runup
area because I was uncomfortable with the situation and, upon shutdown,
saw a little shower of water running from the carb heat box. And again,
after the thing stopped dripping, it started up and ran fine with full
control over the carb heat.
A couple of days ago, we had a real scare. The carb heat control was
stuck in the "off" position. Ground temp was about 30 degrees so I did
not call for a preheat. Once warmed up for 5 minutes on the ramp, it
would move, but not to full "on" (carb heat position). After letting it
run with heat (and intermittently bogging down because of water), it
appeared to go full "on". But then, it appeared it would not go full
"off". We just thought it was our imagination. It would run up to full
power, so we did one circuit in the pattern. The thing appeared to run
normally. After 10 minutes of flying, we tried to go full on and off
(above an airport). The RPM dropped about 100 or so and returned once
put back in "off". The lever did finally return to full off. 45 minutes
later and near our home airport, I tried the heat full on again and
WHOA! It nearly died at 800 feet AGL with FEW opportunities to land.
Full power made it bog down worse. Heat on or off made no difference. I
immediately pulled the mixture back to lean, figuring we put a slug of
water into the carb throat and it turned to ice and it was now starving
for air. It came right back to smooth power upon leaning. Going rich
made it bog again. I applied full carb heat in the lean position and
eventually it cleared up and would take the mixture being set at full
rich. That was a LOOOOONG 1 minute.
Now, I have experienced carb icing on 2 occasions in the last 10 years
and it NEVER ran this badly when I applied carb heat. We were definitely
going to go down if it were not for me leaning it, A LOT.
I opened up the induction system once again and found the linkage and
door operating properly. I did see a trickle of water in the carb heat
tube. We have been having some freaky warm spells in the Chicago area
with heavy rain. These problems always occur after a rain event,
although I sometimes don't fly it until days later. So, it appears that
the new tubing is holding water while the old tattered one would just
let it leak out. Before this BIG event in the air, we had a big
snowstorm with high wind that blew out my right side cowl plug. I
noticed the engine was packed with snow that night. We did not get out
to fly it until a week later when this big event occurred. It appears we
may have had a bunch of snow accumulate in the carb heat tube during the
storm that subsequently melted and lay in hiding until it was sucked
into the air box. I cannot figure out why it did not get sucked in on
the first carb heat application but instead waited until the second one.
While inspecting the system, I noticed that the new tubing does not run
quite straight but has a small dip in it. I shortened it up to run
straight figuring the small bend may be what is holding water.
One Cherokee owner at the local airport says he routinely puts a small
hole in the bottom of both tubes to let water out. Anyone else heard of
this or is doing it? I am reluctant to bung up a new tube from the air
filter and let even this small amount of unfiltered air into the engine.
Is the small amount of unfiltered air a good trade vs. these freaky
water events? We have not had these problems in the 10+ years we have
owned the beast. Is there another possible explanation?
Thanks for any relevant feedback,
Mike
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