0-300D Continental roughness Fixed!

Since the last time I had posted the question about the roughness between 1000
and 1700 RPM my wife had taken the plane on a cross country flight and claimed
things seemed worse. Sure enough the roughness seemed worse plus the idle
(below 1000 RPM) was no longer smooth. This time I brought my non-contact
infrared thermometer to the field and checked each exhaust port as it idled.
The whole right side was just over 400°F while the left two cylinders in the
front were mid 200s. The #2 (left rear) was around 120. I sprayed carb clean
around all the manifold joints (usually perks RPM when it finds a leak) with no
results. I began to have doubts about the differential compression test so I
did it again on the #2 cyl. Still 80/72. I thought maybe the pressure may have
been sealing the ring so I did a cranking compression test and it showed as
good as the other cylinders (125 lbs). Since I had just blasted the plugs I
figured it was a billion to one that I got 2 bad plugs on the same cyl and a
million to one on 2 bad wires. I had no choice but to proceed as though there
was an air leak. I pulled off the left intake "tree" and inspected it for
cracks along with the 3 hoses and the gasket on the oil pan joint. All good. I
sprayed cleaner around the intake runner to head joint to see if anything would
run down the inside of the runner and saw nothing. I was ready to button it up
and look elsewhere when I decided to try spraying cleaner up into the intake
runner (with the plastic pipe that comes on the can) and look for leaks on the
outside. Gusher! Unbolted the runner and saw the gasket being sucked in on the
back side of the runner in about the only spot I couldn't reach with the tube
from the outside.
Here is what threw me; it seems like at idle the whole #2 cylinder would shut
down rather than stumble. I guess "smoothness" of idle is very subjective. That
engine idles better on 5 cylinders than my 0-360 Lycoming does on 4. I also
misread the manual where it said that best idle mixture should be about ¾ to
one turn out from base on the mixture screw (I thought I saw one and a half
out). The final setting is now about half a turn out, nearly one third of where
it was. It was only a coincidence that it happened after I installed new rings
on cylinders 5 and 6.
Thanks to all for the suggestions. I should have been more thorough in the
first place.

Jim

On 27 Jun 2004 00:38:48 GMT, (JFLEISC) wrote:


Thanks to all for the suggestions. I should have been more thorough in the
first place.

Jim


Glad you found it!

z

(JFLEISC) wrote in message ...
Since the last time I had posted the question about the roughness between 1000
and 1700 RPM my wife had taken the plane on a cross country flight and claimed
things seemed worse. Sure enough the roughness seemed worse plus the idle
(below 1000 RPM) was no longer smooth.

There's another little-known factor that sometimes affects these
small Continentals. On the O-200, at least, and perhaps the O-300
(been too long, don't remember) the carb is mounted on a "spider," an
intake manifold that is held to the case by two studs and nuts. The
Marvel Schebler/Facet Aerospace/Precision Aeromotive carb doesn't like
being shaken side-to-side as caused by rotational vibration from power
pulses, and can slop fuel from the bowl into the vent, which is in the
carb throat, flooding the engine somewhat and causing rough running at
certain RPM ranges.
So Continental calls for "Lock-O-Seal" washers, (rubber molded
inside steel) to be installed above and below the spider on both
studs. The nuts are to be tightened finger-tight only, with a wrench
used only to take the nut to the next flat for cotter pin
installation. They want that carb to flex side-to-side, with the
rubber intake runner connections keeping things centered while
allowing the flex.
We had an O-200 that drove us crazy for three weeks until one of us
finally read the assembly manual and the lights came on.

Dan

the O-300
(been too long, don't remember) the carb is mounted on a "spider," an
intake manifold that is held to the case by two studs and nuts

I saw nothing like that on my 0-300 but I wouldn't be surprised at all about
the possibility of 'sloshing'.

Jim