Hi Rob,
As other coopers have told you there is no way to shut off the engine in a
Coupe or for that matter a Cub, T-craft, Aeronca or any other airplane
with
the A or C series Continental engine except by turning off the ignition or
fuel feed. The mixture system on the Stromberg carb does not work until
there
is a significant flow of air through the engine. There are some other
problems to consider however. Within the last 100 hours the float on my
carb
sank with about 75% fuel in it. You might think that would keep the
engine
from running but it actually ran rather well. The only differences were
that
I didn't need to use much primer to start it and there wasn't much
hesitation
when I opened the throttle at idle. My coupe has full EGT and CHT
metering so
there were other ways to identify the problem. A sunk float is a
possibillity
in your carb since you don't say how much change there was in the way
your
engine runs. An easy way to know whether the float has sunk is to see how
fast the fuel comes out of the carb with the engine off. A sunk float
will
produce a steady flow.
Interestingly the correct answer to the question of how much fuel is
coming from the carburator is a slow drip. If no fuel drips out of the
carb
then you may have a rubber tipped needle valve which should have been
replaced
long ago. The steel needle valve which replaced the rubber tipped needle
is
nearly impossible to fit such that it does not leak a little. I own two
spare
needles and three spare seats used in attempt to get my carb to stop
leaking.
The net result is that I turn off the header tank and let the engine stop
when
it runs out of gas. Look in your log books to see if your needle valve
was
ever changed because the rubber tipped needle in not compatible with 100LL
or
autogas which both contain toluene.
The black soot on your plugs is quite normal and is controlled by
what
the engine was doing just before you shut it off. If it had been idling
for a
while then there is likely to be a considerable coating of soot while if
the
engine was recently running at high power then the plugs will be brown in
color. The black soot burns off quite nicely at normal engine
temperatures.
You also did not say whether your airplane is running 100 LL or car gas.
That
nice white color seen inside exhaust pipes of high compression engines is
lead
oxide which also used to be used as the pigment of white paint many years
ago.
The lead oxide looked nice and it also killed the organisms that attacked
the
wood but the EPA doesn't encourage its' use any more.
One easy test you can perform to establish the proper mixture is to
lean
the engine at idle and watch the rpm. An aircraft engine should run about
50
rpm on the rich side of ideal. That means that as you lean the engine at
idle
there should be a 50 rpm rise before the engine dies. With a Marvel carb
this
test is easy to perform because the mixture control is active at idle.
With
the Stromberg carb the test is performed by shutting off the header tank
and
waiting for the carb to run out of fuel. I cannot really tell you how my
engine performs on this test because my tach is not steady enough to read
well.
I think that your mechanic is telling you the truth, go fly your
airplane
and don't worry about it. Good luck with it. Bob Condon
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