Guys,
For the past week I have made some kind of adjustment, change
or
check in the engine compartment trying to find any source of a problem
from my engine out 3 weeks ago. I have a small stumble that I can now
get to happen when running on the ground. I did not have any hint of
this problem before I started my annual inspection. So I have been
concentrating on the things that I touched during the inspection.
To recap a little the ignition has been completely checked out
as
well as the engine itself. This left fuel delivery and carb. The fuel
lines where
checked and flow was verified to be more than ample.
I have about 4 hours of ground run and fast taxi test over the
past week. I thought that I might have it licked yesterday so I
tried a flight test. I tried to duplicate the flight take off from when
the engine stopped. The temp and density altitude were almost the
same. The run up and take off and the climb all went without the
slightest
hick up. I climbed to 8500 ft while circling the airport. (@ 800'/min) I
circled for awhile and reduced throttle to 2750 rpm. About 20 min had
gone
by and not so much as a hint of a problem. I leveled out to the west an
noticed that my indicated airspeed and ground speed were almost
identical.
This made me curious and distract to check true airspeed on the gps
E6B calculator. I found that I was operating at a density altitude of
10500 and true airspeed was 139 mph at 2750 rpm. by now I was 10 to 12
miles to the west and I decided to go back to full throttle. I was leaned
out during this time also.
When I went to full throttle it started to sputter and spit and
just plain act up. I went full rich (while turning back) That did not
clear it. I reduced throttle and that did not return it to smooth.
These
were not sudden or panicked throttle changes. I moved the throttle
slowly
and methodically to try and figure it out and analyze what was going on
with the instruments. The only instrument that gave me any clue was the
mixture gauge which went to full lean with every sputter. I also tried
switching back an forth the 2 ignition systems. That made no difference
either. The last resort was to try carb heat. That did have an effect on
it and the engine recovered in a matter of 5 to 10 seconds. The mixture
gauge showed that it enriched the fuel to a normal level. I did not try
full throttle the rest of the flight and I did try going back and forth
with the carb heat. After the roughness stopped the carb heat just
dropped the rpm but showed no other effect.
As a side note the air temp at altitude was 59 deg. and I was
at
least 1000' above the haze layer(no visible moisture)
This morning I took the fuel system apart again and flushed it
out and check flow. I ground ran it for about 30 min again. I can get it
to do the stumble on the ground now but the engine has to be completely
up to normal operating temps. I.E. the oil has to be around 220 deg. and
oil pressure is 35 to 38 psi. cylinder heads are all around 350 deg.
The
exhaust temps are the only other clue that I am finding something that
I
never had before. All my time before annual inspection at cruise I
would
set the mixture for the highest exhaust temp to be 1330 deg and they
would all fall into a 20 to 30 deg range. Now at 1330 and all temps in
the normal range the exhaust span while not get closer than 120 deg and
I
have to set it for more like 1300 even to stay out of the stumble
problem.
The way that I can get it to start the stumble problem on the
ground is to lean it out. If I leave it full rich it does it only
occasionally and with not set pattern or time between. It is also very
brief like that too.
Any thoughts or other information needed.
Joe Horton, Coopersburg, Pa.
joe.kr2s.buil...@juno.com
