S_P_2015_04_

(Joyce) #1
Andrew and
his Zenair

all the normal things, balanced the prop; balanced the carbs pneu-
matically, changed the carb springs, checked the engine mounts,
mixture control, etc. All of this several times over. I also joined a
really good US CH701 forum, where it appeared quite a few other
CH701 912ULS owners were having similar problems. Many peo-
ple at Serpentine’s Sport Aircraft Builder’s Club offered advice and
support, but we just didn’t seem to be getting to the root cause.
In February, after many people had offered advice, one person
queried why I had not installed carby drip trays. He went to on say
that some Rotax owners used the carby overflow tubes to lean the
mixture even further at altitude and that they were highly sensitive
to pressure changes.
So I splashed out $200 and bought a pair of carby drip trays
from Rotax. I also homed in on the choke operating system. What
I did was to simplify the 2 into 1 cabling, running both choke ca-
bles all the way back to the instrument panel. I replaced the stand-
ard choke knob with a simple crimp in front of the panel. What
also alarmed me was how much static friction there was on the
cable run for one of the carbs. The cable running through the 90
degree metal cable tube was very stiff, much more so than the
other side. Also, the lug crimped onto the end of the choke cable
did not always run smoothly in the two parallel operating arms. Fur-
ther investigation revealed these arms were not quite parallel. This
caused the lug to sometimes jam and the choke on one carb to
stick on intermittently.
Fixing both of these problems (drip tray and choke) has trans-
formed my aircraft. Whether the alloy pipes were forcing air into the
overflow tubes on descent and altering the mixture remains to be
seen. Maybe it was just the sticking choke? Or maybe it was just a
combination the two. A typical case of James Reason’s ‘Swiss cheese’
aviation model, which seems so relevant. Whatever the reason, I now
have a fully functioning, vibration free engine in the 3,000-4,000rpm
range. I can’t tell you what a relief it is and I only wish I had found the
problem much sooner. But better late than never.

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