The O-320 in our C-172 was professionally recently professionally overhauled
(at 2300 hours and 32 years). This included an STC to 160 HP, new
millennium cylinders, new crank (a story, there..), new cam, new carb, a
repitched prop, and a powerflow exhaust.

We're past break-in now, and the plane flies great, climbs awesome. The
recurring problem is that the carb is set way too rich. After a few visits
to the A&P (each time involving a lot of carb twiddling), it is getting
better each time. It is rich enough that I find that carb heat in the
pattern induced pretty good roughness and often backfiring. In fact, I find
that I need to keep it leaned even on the ground or in the pattern (our
field is only 200' elevation). On the ground, the idle is at 650 with the
mixture in, put pull it out about an inch or 2 and it gets a lot smoother
and a little bit faster.

The mechanic feels that it will take us a few iterations to get it set up
right, since there is so much new stuff here. He feels that the exhaust may
be a big part of this.

One other clue, the EGT now reads a lot lower than it used too. They are
apparently going to re-calibrate the thermocouple so it will at least
register on the gage.

Anyone have any experience like this?

Thanks,

dave

Dave Gribble wrote:
> The O-320 in our C-172 was professionally recently professionally overhauled
> (at 2300 hours and 32 years). This included an STC to 160 HP, new
> millennium cylinders, new crank (a story, there..), new cam, new carb, a
> repitched prop, and a powerflow exhaust.
>
> We're past break-in now, and the plane flies great, climbs awesome. The
> recurring problem is that the carb is set way too rich. After a few visits
> to the A&P (each time involving a lot of carb twiddling), it is getting
> better each time. It is rich enough that I find that carb heat in the
> pattern induced pretty good roughness and often backfiring.

You're right, that's not good.

> In fact, I find
> that I need to keep it leaned even on the ground or in the pattern (our
> field is only 200' elevation).

Leaning on the ground is just normal good operating practice.

> On the ground, the idle is at 650 with the
> mixture in, put pull it out about an inch or 2 and it gets a lot smoother
> and a little bit faster.

That sounds like normal operation. Full-rich should be richer than an optimum
mixture. If full-rich gave you an ideal mixture, you wouldn't be able to
enrichen past that to give you the extra cooling you need during climb operation.

>
> The mechanic feels that it will take us a few iterations to get it set up
> right, since there is so much new stuff here. He feels that the exhaust may
> be a big part of this.
>
> One other clue, the EGT now reads a lot lower than it used too. They are
> apparently going to re-calibrate the thermocouple so it will at least
> register on the gage.
>
> Anyone have any experience like this?

Remove SHIRT to reply directly.

Dave

-----That sounds like normal operation. Full-rich should be richer than an
optimum
-----mixture. If full-rich gave you an ideal mixture, you wouldn't be able
to
-----enrichen past that to give you the extra cooling you need during climb
operation.

Correct me if I'm wrong, but I think the "idle mixture" does not have
anything to do with high power settings.
All the "ports" open up with full throttle and gives it more fuel than
needed to keep it cool.
John Prince


"Dave Butler" > wrote in message
...
> Dave Gribble wrote:
> > The O-320 in our C-172 was professionally recently professionally
overhauled
> > (at 2300 hours and 32 years). This included an STC to 160 HP, new
> > millennium cylinders, new crank (a story, there..), new cam, new carb, a
> > repitched prop, and a powerflow exhaust.
> >
> > We're past break-in now, and the plane flies great, climbs awesome. The
> > recurring problem is that the carb is set way too rich. After a few
visits
> > to the A&P (each time involving a lot of carb twiddling), it is getting
> > better each time. It is rich enough that I find that carb heat in the
> > pattern induced pretty good roughness and often backfiring.
>
> You're right, that's not good.
>
> > In fact, I find
> > that I need to keep it leaned even on the ground or in the pattern (our
> > field is only 200' elevation).
>
> Leaning on the ground is just normal good operating practice.
>
> > On the ground, the idle is at 650 with the
> > mixture in, put pull it out about an inch or 2 and it gets a lot
smoother
> > and a little bit faster.
>
> That sounds like normal operation. Full-rich should be richer than an
optimum
> mixture. If full-rich gave you an ideal mixture, you wouldn't be able to
> enrichen past that to give you the extra cooling you need during climb
operation.
>
> >
> > The mechanic feels that it will take us a few iterations to get it set
up
> > right, since there is so much new stuff here. He feels that the exhaust
may
> > be a big part of this.
> >
> > One other clue, the EGT now reads a lot lower than it used too. They
are
> > apparently going to re-calibrate the thermocouple so it will at least
> > register on the gage.
> >
> > Anyone have any experience like this?
>
> Remove SHIRT to reply directly.
>
> Dave
>

John P wrote:
> -----That sounds like normal operation. Full-rich should be richer than an
> optimum
> -----mixture. If full-rich gave you an ideal mixture, you wouldn't be able
> to
> -----enrichen past that to give you the extra cooling you need during climb
> operation.
>
> Correct me if I'm wrong, but I think the "idle mixture" does not have
> anything to do with high power settings.
> All the "ports" open up with full throttle and gives it more fuel than
> needed to keep it cool.
> John Prince

Don't know, but that sounds plausible. Thanks for the correction. The same
mixture knob controls the mixture at both idle and full power, though.

Dave
Remove SHIRT to reply directly.

On Wed, 31 Mar 2004 23:28:37 -0500, "Dave Gribble"
> wrote:

>Anyone have any experience like this?

No experience with the O320, but with the Lycoming O360's, the procedure to
set proper mixture should be doable on the ground.

1. Start engine and warm up until CHT's and oil temps are normal.
2. Mag check -- if normal, proceed.
3. Set throttle set screw so idle is per airframe mfg recommendation.
4. Slowly move mixture control towards idle cut-off and observe rpm. An
increase of more than 50 RPM indicates the mixture is too rich. (An
immediate decrease, if not preceded by a momentary increase, indicates the
mixture is too lean).
5. When performing step 4, don't let the engine cut out.
6. Adjust the mixture control in the proper direction; run up to 2000 RPM
to "clear" the engine, and repeat the above as necessary. If the mixture
change has changed the idling RPM, that should be re-adjusted as needed.
7. If the setting does not remain stable, check the idle linkage for
looseness.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Deakin's had an AvWeb column sometime in the last three months on
setting the proper ground mixture.

Dave Gribble wrote:
>
> The O-320 in our C-172 was professionally recently professionally overhauled
> (at 2300 hours and 32 years). This included an STC to 160 HP, new
> millennium cylinders, new crank (a story, there..), new cam, new carb, a
> repitched prop, and a powerflow exhaust.
>
> We're past break-in now, and the plane flies great, climbs awesome. The
> recurring problem is that the carb is set way too rich. After a few visits
> to the A&P (each time involving a lot of carb twiddling), it is getting
> better each time. It is rich enough that I find that carb heat in the
> pattern induced pretty good roughness and often backfiring. In fact, I find
> that I need to keep it leaned even on the ground or in the pattern (our
> field is only 200' elevation). On the ground, the idle is at 650 with the
> mixture in, put pull it out about an inch or 2 and it gets a lot smoother
> and a little bit faster.
>
> The mechanic feels that it will take us a few iterations to get it set up
> right, since there is so much new stuff here. He feels that the exhaust may
> be a big part of this.
>
> One other clue, the EGT now reads a lot lower than it used too. They are
> apparently going to re-calibrate the thermocouple so it will at least
> register on the gage.
>
> Anyone have any experience like this?
>
> Thanks,
>
> dave

Dave Butler > wrote:
: Don't know, but that sounds plausible. Thanks for the correction. The same
: mixture knob controls the mixture at both idle and full power, though.

Yes, but think of the mixture knob as an "absolute" metering adjustment, not a
"relative" adjustment. In other words, setting the mixture doesn't set the air/fuel
ratio... basically just the maximum amount of fuel that can flow. So, to lean on the
ground with the mixture you have to pretty much pull it all the way out. As Deakin
says, if you lean on the ground, doing anything shy of almost cutting it off does no
good, and possibly harm if you forget to enrichen rull before takeoff.

-Cory
--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

wrote:
> Dave Butler > wrote:
> : Don't know, but that sounds plausible. Thanks for the correction. The same
> : mixture knob controls the mixture at both idle and full power, though.
>
> Yes, but think of the mixture knob as an "absolute" metering adjustment, not a
> "relative" adjustment. In other words, setting the mixture doesn't set the air/fuel
> ratio... basically just the maximum amount of fuel that can flow. So, to lean on the
> ground with the mixture you have to pretty much pull it all the way out. As Deakin
> says, if you lean on the ground, doing anything shy of almost cutting it off does no
> good, and possibly harm if you forget to enrichen rull before takeoff.

If that's the case, that the mixture control simply meters the absolute amount
of fuel that's allowed to flow (don't know, taking your word for it), then
that's further support for the statement that the mixture control needs to be
able to allow a richer-than-optimum amount of fuel at idle.

Or, put another way, if the mixture is not over-rich at idle with the control at
the full-rich position, you won't be able to enrichen the mixture sufficiently
during climb to provide the necessary cooling. That's the statement I started
with. I think we are agreeing.

Dave
Remove SHIRT to reply directly.

Dave Gribble wrote:
>
> This included an STC to 160 HP, new
> millennium cylinders, new crank (a story, there..), new cam, new carb, a
> repitched prop, and a powerflow exhaust.

I installed tuned headers on a van once. It ran rich after that, due to the reduced
back pressure. I had to rejet the carb to fix it. That leads me to suspect your new
exhaust, but you've put a new carb on there too. Can you put the old one back on for
a flight?

George Patterson
Treason is ne'er successful, Sir; what then be the reason? Why, if treason
be successful, Sir, then none dare call it treason.

This is the exact procedure we've used to set the mixture. The idle is 650
rpm and the increase when you move to cut-off is only about 25 rpm.

The problem is that it still seems too rich, especially in the pattern with
carb heat on.

"Ron Rosenfeld" > wrote in message
...
> On Wed, 31 Mar 2004 23:28:37 -0500, "Dave Gribble"
> > wrote:
>
> >Anyone have any experience like this?
>
> No experience with the O320, but with the Lycoming O360's, the procedure
to
> set proper mixture should be doable on the ground.
>
> 1. Start engine and warm up until CHT's and oil temps are normal.
> 2. Mag check -- if normal, proceed.
> 3. Set throttle set screw so idle is per airframe mfg recommendation.
> 4. Slowly move mixture control towards idle cut-off and observe rpm. An
> increase of more than 50 RPM indicates the mixture is too rich. (An
> immediate decrease, if not preceded by a momentary increase, indicates the
> mixture is too lean).
> 5. When performing step 4, don't let the engine cut out.
> 6. Adjust the mixture control in the proper direction; run up to 2000 RPM
> to "clear" the engine, and repeat the above as necessary. If the mixture
> change has changed the idling RPM, that should be re-adjusted as needed.
> 7. If the setting does not remain stable, check the idle linkage for
> looseness.
>
>
> Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Thu, 1 Apr 2004 18:42:17 -0500, "Dave Gribble"
> wrote:

>This is the exact procedure we've used to set the mixture. The idle is 650
>rpm and the increase when you move to cut-off is only about 25 rpm.
>
>The problem is that it still seems too rich, especially in the pattern with
>carb heat on.

Then there is something else going on besides the mixture adjustment. Have
you checked with the folk that hold the STC for the engine and exhaust
mods?


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On 1-Apr-2004, wrote:

> Yes, but think of the mixture knob as an "absolute" metering
> adjustment, not a "relative" adjustment. In other words, setting the
> mixture doesn't set
> the air/fuel ratio... basically just the maximum amount of fuel that
> can flow.


I'm afraid this doesn't make any sense. The mixture control DOES control
air/fuel mixture -- hence the name. If it simply controlled fuel flow the
mixture would change radically every time you touched the throttle. In
fact, what is controlled by the red knob (mixture) is the ratio of air
volume to fuel volume. As density altitude increases (i.e. air density
decreases), a greater volume of air is needed to mix with a given volume of
fuel for optimal combustion, so one has to lean the mixture (increase air
volume/fuel volume ratio).

-Elliott Drucker

On 1-Apr-2004, Ron Rosenfeld > wrote:

> No experience with the O320, but with the Lycoming O360's, the procedure
> to
> set proper mixture should be doable on the ground.
>
> 1. Start engine and warm up until CHT's and oil temps are normal.
> 2. Mag check -- if normal, proceed.
> 3. Set throttle set screw so idle is per airframe mfg recommendation.
> 4. Slowly move mixture control towards idle cut-off and observe rpm. An
> increase of more than 50 RPM indicates the mixture is too rich. (An
> immediate decrease, if not preceded by a momentary increase, indicates the
> mixture is too lean).
> 5. When performing step 4, don't let the engine cut out.
> 6. Adjust the mixture control in the proper direction; run up to 2000 RPM
> to "clear" the engine, and repeat the above as necessary. If the mixture
> change has changed the idling RPM, that should be re-adjusted as needed.
> 7. If the setting does not remain stable, check the idle linkage for
> looseness.


If the goal here is to set the mixture control linkage so that "proper"
mixture occurs with the control at the full lean position, then this had
better be done at or near sea level density altitude. How is the procedure
done on the ground in Denver?

--
-Elliott Drucker

No, that will be next. I think there are actually 2 STCs, one for the
exhaust (Powerflow) and one for the 160 HP conversion (Superior Air Parts, I
think). I wonder if the problem is in combining these 2 STCs?

"Ron Rosenfeld" > wrote in message
...
> On Thu, 1 Apr 2004 18:42:17 -0500, "Dave Gribble"
> > wrote:
>
> >This is the exact procedure we've used to set the mixture. The idle is
650
> >rpm and the increase when you move to cut-off is only about 25 rpm.
> >
> >The problem is that it still seems too rich, especially in the pattern
with
> >carb heat on.
>
> Then there is something else going on besides the mixture adjustment.
Have
> you checked with the folk that hold the STC for the engine and exhaust
> mods?
>
>
> Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Fri, 02 Apr 2004 01:52:36 GMT, wrote:

>If the goal here is to set the mixture control linkage so that "proper"
>mixture occurs with the control at the full lean position, then this had
>better be done at or near sea level density altitude. How is the procedure
>done on the ground in Denver?

I don't know. All the manual says is that an "allowance" should be made
for setting idle for weather conditions and field altitude. It doesn't
state how to compute that allowance.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Thu, 1 Apr 2004 22:15:16 -0500, "Dave Gribble"
> wrote:

>No, that will be next. I think there are actually 2 STCs, one for the
>exhaust (Powerflow) and one for the 160 HP conversion (Superior Air Parts, I
>think). I wonder if the problem is in combining these 2 STCs?

I suppose it could be. But the 160HP converted engine should be listed as
an allowable engine for the Powerflo system. Or vice versa.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

wrote:
: I'm afraid this doesn't make any sense. The mixture control DOES control
: air/fuel mixture -- hence the name. If it simply controlled fuel flow the
: mixture would change radically every time you touched the throttle. In
: fact, what is controlled by the red knob (mixture) is the ratio of air
: volume to fuel volume. As density altitude increases (i.e. air density
: decreases), a greater volume of air is needed to mix with a given volume of
: fuel for optimal combustion, so one has to lean the mixture (increase air
: volume/fuel volume ratio).

OK... I'll concede that I oversimplified a bit....:) The air/fuel ratio (as
in mass-air/fuel, like 12.5:1, 14:1, etc) does change when the throttle is moved.
Even without changing altitude, if the mixture is set to peak EGT, for example,
increasing MP by an inch will lean the engine (unless it's starting to enter the
full-throttle fuel enrichment regime). It's probably somewhere between the two
extremes (absolute fuel vs. relative fuel).

-Cory
--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

On Fri, 2 Apr 2004 13:59:49 +0000 (UTC),
wrote:

> OK... I'll concede that I oversimplified a bit....:) The air/fuel ratio (as
>in mass-air/fuel, like 12.5:1, 14:1, etc) does change when the throttle is moved.
>Even without changing altitude, if the mixture is set to peak EGT, for example,
>increasing MP by an inch will lean the engine (unless it's starting to enter the
>full-throttle fuel enrichment regime). It's probably somewhere between the two
>extremes (absolute fuel vs. relative fuel).
>

Actually, you hit it pretty close. You do need to remember that aside
from the mixture control (and probably a crude economizer circuit), we
are talking about a one-barrel gasoline tractor carburetor.

On a MS carb, the "mixture control" is a chunk of brass on the end of
a two inch piece of flex cable (similiar to a tach cable). The mixture
knob rotates an arm clamped onto the top of the cable, rotating the
piece of brass on the bottom.

This piece of brass is stuck inside a brass cup on the bottom of the
fuel bowl with a "window" in it. At ICO, the window is "shut", at full
rich, the window is wide open.

The fuel gets sucked by the venturi out of the bottom of the bowl,
through the window, and out the main metering nozzle on the carb into
the induction airstream. The combination of the venturi type and the
main metering nozzle determines how much suction (relative to the
ambient pressure in the fuel bowl and the float/fuel level) is
generated "through" the valve.

Throttle plate position/engine rpm will indeed vary airflow through
the venturi, and consequently will affect the amount of suction. But
as you've indicated, the mixture control is a manually variable (with
the mixture knob) mechanical restriction to this suction. There is no
mechanical interconnect between the mixture valve and the throttle.

TC