Help; Rough running Continental O-300D

I hate to keep bugging this group with my bizarre Cessna questions but it
repeatedly proves to be a good source of information.
History; Since I got my '61 C-172 with an O-300D several years ago it always
ran smooth but had high oil consumption. Before the second annual several of
the cylinders started to show bad compression. Rings were all fouled up and
frozen. Replaced rings and it continued to run great with no oil use for about
3 to 4 hours then went back to using oil (been through 2 annuals now,
compression still good). 8 months later right mag goes bad (cutting in and out
on several cylinders). I replaced the mag and do a timing check. Find both mags
were over advanced about 12° (seems since it ran smooth my AI never bothered to
check timing. I should have done it myself but I thought that was what I was
paying for.) This, of course, was why it used oil. Reset timing, and another
compression check revealed 2 nearly dead cylinders. Broken rings on one
cylinder, frozen on another. Other 4 cylinders were 80/70 or better. Installed
new rings on those 2 cylinders and power great with virtually no oil use.
Strange problem; Idle at 500 RPM is smooth as silk and so is operation over
1700 RPM. Tons of power and 100° cooler head temps to boot. Operation between
1000 RPM and 1700 RPM seems very rough (never did this before).
I doubt it's an air leak since it idles so smooth. Idle mixture screw is out
less than 1 and a half turns so I don't think it's compensating for something
by over richening. Mag check at any speed shows even RPM loss not over 100 RPM
at worst. I could swear it feels like a cross fire (it's that bad) but why
wouldn't it do it at higher power settings? I also find that hard to imagine
with shielded wires (meaning they would short to ground, right?) I thought the
tertiary idle delivery circuit (MA-3SPA carburetor) may have picked up debris
but I had the carb apart to the last nut and bolt and everything and all
passages are clean as a whistle. Seems like this never happened until the
timing was backed to the standard 28° left and 26° right. Anyone seen this? Any
ideas? What is staring me right in the face that I missed?

Jim "boy I like my Lycoming" F

JFLEISC wrote:
/snip/ Any
ideas? What is staring me right in the face that I missed?

Jim "boy I like my Lycoming" F


Jim,

it's a long shot, but are the plug wires routed correctly? As you know,
on that engine, one mag runs all the tops, and the other all the
bottoms. Maybe someone installed them in the more conventional,
staggared arrangement? Just SWAG...

Happy Flying!
Scott Skylane
N 92054

it's a long shot, but are the plug wires routed correctly? As you know,
on that engine, one mag runs all the tops, and the other all the
bottoms. Maybe someone installed them in the more conventional,
staggared arrangement? Just SWAG...

Happy Flying!
Scott Skylane
N 92054

I should have mentioned that I did confirm this. I should note that everything
"wire wise" is the same. I replaced only the right mag but not the wires. That
bad mag, of course, would hardly run correctly for a minute at a time.

Jim

I had the close to the same thing happen on a 172 with O300A. It turned out it
was one spark plug lead that was breaking down.

Idle and mag check was good. Static RPM check was was okay. However when
throttle up engine ran rough.

Replaced bad spark plug lead and everything waqs fine.

On another note, that engine runs 7:1 compression ratio and doesn't generate a
lot of heat (six cylinders - 145 hp). I found that if I did not lead
aggressively during taxi and ground use, the plugs would foul easily. Once I
started leaning aggressively, I never had anymore problems. It should be noted
that when I sold it it had 2600 hr on the bottom end and 1000 on the top.
Temps and compressions were great, the current owner, now flys theplane
regularly.

Just my take - check the leads and lean aggressivrely.

Larry

"Larryskydives" wrote in message
...
I had the close to the same thing happen on a 172 with O300A. It turned
out it
was one spark plug lead that was breaking down.

Idle and mag check was good. Static RPM check was was okay. However when
throttle up engine ran rough.

Replaced bad spark plug lead and everything waqs fine.

I have the same problem, and the bad lead runs from a Slick mag to the #2
cylinder bottom plug. Did you replace a Slick lead, or was your mag
another make, like a Bendix? I'd like to know where to find a replacement
lead and how to get it swaged into the distributor cap on the mag end.
Thanks.

I had Bendix Mags. The shop I use, took care of it but they were able to buy
one lead.

Idle and mag check was good. Static RPM check was was okay. However when
throttle up engine ran rough.

Problem is, when I throttle up it runs fine.

Jim

On 23 Jun 2004 00:26:39 GMT, (JFLEISC) wrote:

Idle and mag check was good. Static RPM check was was okay. However when
throttle up engine ran rough.

Problem is, when I throttle up it runs fine.

Jim

One possibility is that if you throttle up quickly your accelerator
pump could enrichen the mixture a little too muck since the carb is
also adding more feul and air. What happens if you throttle up at a
little more than a snails pace? If it works better going slowly this
may be the problem.

I know I haven't explained it well cuz I'm tired, but let us know how
a very slow advance works out. In my plane I don't have this happen
if I go slowly, otherwise it sounds as you describe.

HTH
z

One possibility is that if you throttle up quickly your accelerator
pump could enrichen the mixture a little too muck since the carb is
also adding more feul and air. What happens if you throttle up at a
little more than a snails pace? If it works better going slowly this
may be the problem.

I know I haven't explained it well cuz I'm tired, but let us know how
a very slow advance works out. In my plane I don't have this happen
if I go slowly, otherwise it sounds as you describe.


No rate of advance necessary. Just hold it steady anywhere from 1000 to 1700
RPM and it is very rough for as long as you stay there.

Jim

JFLEISC wrote:
: No rate of advance necessary. Just hold it steady anywhere from 1000 to 1700
: RPM and it is very rough for as long as you stay there.

Hmmm, do you have a carb with a 1-piece venturi? On some applications these
made the engines run so poorly that the 2-pc was allowed to be reinstalled.

Does leaning, even leaning to the point of the engine dying, change the
roughness? Perhaps the carburettor is incorrectly jetted.

Did you look at the distributor blocks in the mags? Or the coils? Perhaps
the increased temperature of running at 1000-1700 with less cooling airflow
is making something conduct that shouldn't?
--
Aaron Coolidge