Engine stumble, Thoughts?

Hi All!

OK, since the collective talent in this group greatly exceeds
mine and those around me...

Today the engine in our 151 (O 320-150hp)Warrior stumbled
during power reduction, and we were able to repeat the symptom.....

Here are details

OAT -5 C, 2350 rpm, kinda rough air, pulled back throttle to
decend after passing traffic..

I think we hit a bump which caused the throttle to be
retarded quicker than normal, engine stumbles for 1 second and
resumed operation at the new (lower ) throttle setting.

We were able to repeat this several times, carb heat on/off,
fuel pump on/off, leaned rich of peak/ full rich, left tank/right
tank, rt mag/left mag/both. - as long as we waited a couple of minutes
to try again. No evidence of carb icing, dew point was +10C

Moving the throttle immediately back to cruise, then sudden
retard, no problem. The stumble could be repeated by waiting 2 -3
minutes .

Recent maintenance, oil & filter change 1.5 hrs ago.

The stumble was like the mixture was momentairly too lean,
similar to the lean test during runup...

At 3500 ft, lean setting was at 60% of mix lever travel to
full rich..

Any thoughts from you engine talents out there?

Cheers!

Dave

float level and carb vent.


--
James H. Macklin
ATP,CFI,A&P

"Dave" wrote in message
...
| Hi All!
|
| OK, since the collective talent in this group greatly
exceeds
| mine and those around me...
|
| Today the engine in our 151 (O 320-150hp)Warrior stumbled
| during power reduction, and we were able to repeat the
symptom.....
|
| Here are details
|
| OAT -5 C, 2350 rpm, kinda rough air, pulled back
throttle to
| decend after passing traffic..
|
| I think we hit a bump which caused the throttle to be
| retarded quicker than normal, engine stumbles for 1
second and
| resumed operation at the new (lower ) throttle setting.
|
| We were able to repeat this several times, carb heat
on/off,
| fuel pump on/off, leaned rich of peak/ full rich, left
tank/right
| tank, rt mag/left mag/both. - as long as we waited a
couple of minutes
| to try again. No evidence of carb icing, dew point was
+10C
|
| Moving the throttle immediately back to cruise, then
sudden
| retard, no problem. The stumble could be repeated by
waiting 2 -3
| minutes .
|
| Recent maintenance, oil & filter change 1.5 hrs ago.
|
| The stumble was like the mixture was momentairly too
lean,
| similar to the lean test during runup...
|
| At 3500 ft, lean setting was at 60% of mix lever travel to
| full rich..
|
| Any thoughts from you engine talents out there?
|
| Cheers!
|
| Dave

Sorry all!

DP was MINUS 10C!
My bad w/the signs....

Dave

On Sun, 04 Dec 2005 20:20:24 -0400, Dave
wrote:

Hi All!

OK, since the collective talent in this group greatly exceeds
mine and those around me...

Today the engine in our 151 (O 320-150hp)Warrior stumbled
during power reduction, and we were able to repeat the symptom.....

Here are details

OAT -5 C, 2350 rpm, kinda rough air, pulled back throttle to
decend after passing traffic..

I think we hit a bump which caused the throttle to be
retarded quicker than normal, engine stumbles for 1 second and
resumed operation at the new (lower ) throttle setting.

We were able to repeat this several times, carb heat on/off,
fuel pump on/off, leaned rich of peak/ full rich, left tank/right
tank, rt mag/left mag/both. - as long as we waited a couple of minutes
to try again. No evidence of carb icing, dew point was +10C

Moving the throttle immediately back to cruise, then sudden
retard, no problem. The stumble could be repeated by waiting 2 -3
minutes .

Recent maintenance, oil & filter change 1.5 hrs ago.

The stumble was like the mixture was momentairly too lean,
similar to the lean test during runup...

At 3500 ft, lean setting was at 60% of mix lever travel to
full rich..

Any thoughts from you engine talents out there?

Cheers!

Dave

Hmmmm... OK..

Could you elaborate?

I think I see where you are thinking, deceleration if the
aircraft speed or engine rpm?

Suddenly closing the throttle would cause the mix to go lean?
- if the carb vent were plugged?

Thinking...

D


if On Sun, 4 Dec 2005 19:52:28 -0600, "Jim Macklin"
wrote:

float level and carb vent.

The fuel system must be vented, air has to get into the
tanks and the carb float bowl. Your fuel tanks are probably
OK since they do feed fuel and the tank has not collapsed.
Fuel flows out the carb jet due to differential pressure
between the venturi and the float bowl in a ratio determined
by the float setting which controls the fuel level in the
bowl.
Retarding the throttle drops the MAP (your Piper does not
have a MP gauge) and raises the pressure (decreases the
venturi effect) at the jet. This can cause a stumble,
particularly if the fuel level is not at the proper level
due to a float out of adjustment or a problem with venting.
When you retard the throttle rapidly, you can "detune"
engines with dynamic counter-balanced crankshafts (not on
your Piper model) and cause other effects on the prop and
other parts, such as alternator belts. But advancing the
throttle too fast can cause a hesitation if the carb does
not have an accelerator pump circuit to increase the fuel
flow for acceleration before the air-flow stabilizes.

If I was flying your airplane, I'd do a visual inspection of
the carb looking for fuel stains that might indicate a fuel
or air leak. At the next 100 hr/annual, I'd consider
dropping the float bowl and checking the float.


--
James H. Macklin
ATP,CFI,A&P



"Dave" wrote in message
...
| Hmmmm... OK..
|
| Could you elaborate?
|
| I think I see where you are thinking, deceleration if the
| aircraft speed or engine rpm?
|
| Suddenly closing the throttle would cause the mix to go
lean?
| - if the carb vent were plugged?
|
| Thinking...
|
| D
|
|
| if On Sun, 4 Dec 2005 19:52:28 -0600, "Jim Macklin"
| wrote:
|
| float level and carb vent.
|

I'd also have to say that in turbulence, floats are prone to bounce around a
bit.

Jim




"Morgans" wrote in message
...

"Jim Macklin" wrote in message
news:B3Nkf.11633$QW2.10651@dukeread08...
float level and carb vent.

I would have to go with float level, also.
--
Jim in NC

I think the Wrights were onto something. Drip gasoline onto the hot exhaust
manifold shroud and suck it into the engine before it catches fire. To hell
with this carburetor stuff.

{;-)


Jim

They do that under pressure now and call it fuel injection.


"RST Engineering" wrote in message
...
|I think the Wrights were onto something. Drip gasoline
onto the hot exhaust
| manifold shroud and suck it into the engine before it
catches fire. To hell
| with this carburetor stuff.
|
| {;-)
|
|
| Jim
|
|

"Jim Macklin" wrote in message
news:B3Nkf.11633$QW2.10651@dukeread08...
float level and carb vent.

I would have to go with float level, also.
--
Jim in NC

You can check the float level without disassembling the bowl by
screwing a jury rigged fitting into the 1/8" pipe plug bowl drain port.
Connect a short length of small clear flex hose to the fitting, hold
it up, & see where the fuel levels out to. I don't think you will find
it wrong though.

The comment on the accelerator pump maybe being goofy is interesting.

The float bowl vent is to the air intake side of the carburetor, but I
don't yet see how that could cause your symptoms.

Hmmmm......