This is why we do runups

The particular O-320 in our 172N is the sweetest running aircraft engine I
have ever known. We maintain it rigorously and proactively and lean
aggressively for all ground operations. I have not had to clear a mag once
in the 880 hours since it was installed.

I flew the plane the day before yesterday, 15 hours out of annual with new
plugs installed. I remember being impressed again with how smooth it was.
Two of our more experienced pilots flew it after me and the last confirmed
that it was smooth when shut down.

The next morning, I started it up and taxied to the run up area. When I did
the mag check, the left was rough. The right was so rough that the airframe
was shaking visibly. There was no question about flying the plane.

When the plugs were removed, all were found fouled and one was completely
bridged. The engine ran acceptably on runup but was not as smooth as
before. It gradually returned to normal on a 4 hour flight. We're still
trying to figure out what happened. (Wild ass guesses appreciated)

The lesson here is that fouling isn't always something that gradually gets
worse. It can build up quietly and then show itself suddenly when the
electricity goes through the crud instead of the mixture. This could have
happened on a lunch time stop over, running perfectly at shutdown and then
sick after start up and a 5 minute taxi. A rough, weak engine will probably
get you down safely if it goes bad in flight but may put you in the trees on
a tight takeoff.

Don't skip that runup just because it was running fine 15 minutes ago!

--
Roger Long

"Roger Long" om wrote in
message .. .
| I tried all the tricks I learned with our previous, old, tired, engine and
| before we learned to lean on the ground and in descent. I did all you
| suggest until the CHT's got too high. Nada.
|

The only other things I can think of are dirty fuel, leaking valves, or
perhaps the wrong type of spark plugs was installed.

I think the most significant point is that the engine got steadily better
over an hour of subsequent cruise flight. Four hours later, it's back to
normal. If there had been an underlying cause, it should have gotten worse.

The weather here in the east has been terrible. While those in LA and
similar environs might chuckle at what we call hot and humid, we still adapt
to the conditions. Perhaps some other club members aren't as aggressive
about leaning as they should be and it hasn't been a problem until this long
muggy stretch.
--
Roger Long

The only other things I can think of are dirty fuel, leaking valves, or
perhaps the wrong type of spark plugs was installed.

"Roger Long" om wrote in message
.. .
I think the most significant point is that the engine got steadily better
over an hour of subsequent cruise flight. Four hours later, it's back to
normal. If there had been an underlying cause, it should have gotten worse.

Not true in general. For example, valve sticking will manifest itself when
the engine is first started which will clear up as things warm up. This doesn't
mean all is well with the engine however, eventually you may have a catastrophic
failure.

"Roger Long" om wrote in message ...
snip
I flew the plane the day before yesterday, 15 hours out of annual with new
plugs installed. ==========

The next morning, I started it up and taxied to the run up area. When I did
the mag check, the left was rough. The right was so rough that the airframe
was shaking visibly. There was no question about flying the plane.

When the plugs were removed, all were found fouled and one was completely
bridged. The engine ran acceptably on runup but was not as smooth as
before. It gradually returned to normal on a 4 hour flight. We're still
trying to figure out what happened. (Wild ass guesses appreciated)

OK, here's my WAG. You replaced all of the plugs and they all
started fouling. Did you replace the plugs with the same brand and
heat range? Most O-320 installations have a list of acceptable heat
ranges for the plugs. I've found over the years that there is usually
one specific heat range which will prevent fouling and work perfectly
in a specific installation. It's usually found by trial and error
and when you do find it, you stick with it.

John Galban=====N4BQ (PA28-180)

If only one magneto fails the check, and you have not been able to clear the
plug after a couple of tries, it helps to run up the engine for about five
minutes on the bad magneto.

Not that the previous poster implied this, but it is probably not a
good idea to do this five minute runup while going around the pattern,
don't ask me how I know. If it turns out to be a loose plug wire, it
doesn't fix itself, and others that the mechanic left only finger
tight may come off as well. Sometimes you can be TOO sure that you
have a fouled plug.

This IS why we do runups....but ya gotta listen to what the airplane
is trying to tell you.

JP Krievins

Roger,

You have people who are running the engine full rich all the time. It
was not built for 100LL fuel, so it needs to be leaned in cruise and
in descent and in climb above 5,000 density altitude. It doesn't hurt
to lean it when taxiing as well, but do so very aggressively, so that
if you forget to go to full rich before takeoff it will quit on you
rather than make a takeoff with the mixture other than at full rich.

Once you lean the airplane for cruise you need not change the mixture
on the descent. The checklist says to enrichen as necessary to avoid
roughness, so don't enrichen the mixture unless the engine runs rough.
When you run the prelanding checklist on downwind and pull the carb
heat, that's a pretty good time to go to full rich (if you want to) in
anticipation of a go around. Otherwise, leave it where it is until
you pull it the rest of the way out to shut the engine down as you
park. If you make a go around, push the mixture to rich, carb heat to
cold and apply full throttle. Some folks don't like the extra work
with the mixture at that time, so they go to full rich on downwind,
base or final to be ready for a go around. They lean again after
turning off of the runway to avoid plug fouling.

All the best,
Rick

"Roger Long" om wrote in message ...
The particular O-320 in our 172N is the sweetest running aircraft engine I
have ever known. We maintain it rigorously and proactively and lean
aggressively for all ground operations. I have not had to clear a mag once
in the 880 hours since it was installed.

I flew the plane the day before yesterday, 15 hours out of annual with new
plugs installed. I remember being impressed again with how smooth it was.
Two of our more experienced pilots flew it after me and the last confirmed
that it was smooth when shut down.

The next morning, I started it up and taxied to the run up area. When I did
the mag check, the left was rough. The right was so rough that the airframe
was shaking visibly. There was no question about flying the plane.

When the plugs were removed, all were found fouled and one was completely
bridged. The engine ran acceptably on runup but was not as smooth as
before. It gradually returned to normal on a 4 hour flight. We're still
trying to figure out what happened. (Wild ass guesses appreciated)

The lesson here is that fouling isn't always something that gradually gets
worse. It can build up quietly and then show itself suddenly when the
electricity goes through the crud instead of the mixture. This could have
happened on a lunch time stop over, running perfectly at shutdown and then
sick after start up and a 5 minute taxi. A rough, weak engine will probably
get you down safely if it goes bad in flight but may put you in the trees on
a tight takeoff.

Don't skip that runup just because it was running fine 15 minutes ago!

Rick,

That's pretty much the way I fly the plane although I land at a partially
leaned setting I know from experience will give me full power without
stumbling if I have to go around and forget the mixture.

I've posted stuff on our club web site about leaning and preach it every
chance I get. Everyone I've talked to assures me that they are leaning.
Than, I talked to someone today who said, "I lean all the time but I never
pull it out more than an inch because I'm afraid of overheating it." Ah,
the picture is becoming a little clearer. That engine hardly feels it at an
inch.

This engine has an unusually good mixture distribution for an O-320. There
is very little roughness or stumbling before it quits and it's easy to stall
leaning to the max on the ground. I think this makes people gun shy about
seeing too much shiny metal ahead of the red knob.

I guess I've got to push the issue more. It's hard though to change long
time habits learned from revered CFI's who had 10 times the hours I have.
We had three hours shop time instead of three hours flight time yesterday.
Price of habit I guess.

--
Roger Long

I noticed that the runup is pretty bad on my new plane. I started
leaning during taxi and it helped a little, but not as much as i thought it
would. Then I tried leaning for the taxi back, and flew again the next
morning and leaned again on taxi, big improvement. The one other partner
told me always to use two pumps of primer, even if it was just running five
minutes ago. I think they are drowning the bottom plugs, and then running
rich on taxi. The plane requires about two inches of leaning on the mixture
control to get it clean. I have yet to check the RPM rise before lean cutoff
to see if the idle mixture itself is too rich. We adjusted the mixture on
the IO-360 (on the bi-plane) and wow, what a difference in all around
performance! This one is an O-360 though.

Oh yeah, the roughness is about the same on either mag but goes totally
away when on both.
Suggestions?

Wayne


"Roger Long" om wrote in
message .. .
I think the most significant point is that the engine got steadily better
over an hour of subsequent cruise flight. Four hours later, it's back to
normal. If there had been an underlying cause, it should have gotten
worse.

The weather here in the east has been terrible. While those in LA and
similar environs might chuckle at what we call hot and humid, we still
adapt
to the conditions. Perhaps some other club members aren't as aggressive
about leaning as they should be and it hasn't been a problem until this
long
muggy stretch.
--
Roger Long

The only other things I can think of are dirty fuel, leaking valves, or
perhaps the wrong type of spark plugs was installed.

One thing to remember is that for most operations, even at sea level, full
rich is too rich. Consider this: The full rich mixture setting must not be
too lean for any condition likely to be encountered, as the pilot obviously
has no way to further enrich the mixture. But at -30 C at sea level (a temp
sometimes seen in winter in many parts of the country) and barometric
pressure of 29.92", density altitude is MINUS 5900 feet! If full rich
mixture is not too lean for those conditions it sure as heck will be too
rich at sea level in standard conditions. I'm not advocating leaning for
takeoff at or close to sea level, as a rich mixture may be required for
adequate cooling, but I always start to lean fairly early in the climb.

-Elliott Drucker