Lycoming dieseling?

Hey all. Ran into something a bit odd yesterday. I was doing
some taxiing around the airport, and had to get out and check something.
To facilitate starting in just a minute, and since I was the only one
around, I shut it down with the mags, rather than the mixture cutoff. To
my surprise, right as it was shaking to a stop, it fired every once in
awhile... with the mags OFF! Mag drop, timing, etc was all checked not
too long ago, and in good shape. Compression is excellent, pops through
nicely on all four when pulled through by hand, and makes full static
runup.

Anyone heard of this? I guess I could potentially see that at
this ridiculously low RPM, there's almost 10x the time on the compression
stroke. Perhaphs this is enough time to make it light off once in awhile.
Also, I checked the MP as it was doing this, and since it's so low of RPM,
even at idle it's making 25". This lends credence to the "preignition"
theory.

Also, just for grins, I tried it on a friend's Cherokee 150 (O-320
150hp Lycoming 7:1 CR... mine's an O-360 8.5:1 180hp). I was somewhat
relieved to see his do this to.

Any thoughts?

-Cory

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

Where were the throttle and mixture set when you switched the mags OFF?

Re-read his post. He left the mixture on full to make it easier to start.

George Patterson
The optimist feels that we live in the best of all possible worlds. The
pessimist is afraid that he's correct.
James Branch Cavel

G.R. Patterson III wrote:

: john smith wrote:
:
: Where were the throttle and mixture set when you switched the mags OFF?

: Re-read his post. He left the mixture on full to make it easier to start.

Exactly... the point is that it didn't shut completely down with
the mags off. Throttle idled, and mixture leaned for ground taxi (very
lean).

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

wrote:
Exactly... the point is that it didn't shut completely down with
the mags off. Throttle idled, and mixture leaned for ground taxi (very
lean).

Would you have expected it to?

Cheers,
Sydney

Sydney Hoeltzli wrote:
: wrote:
: Exactly... the point is that it didn't shut completely down with
: the mags off. Throttle idled, and mixture leaned for ground taxi (very
: lean).

: Would you have expected it to?

Uh... YES. If the combustion process is doing what it's supposed
to, then without spark, there's no combustion. Although it may be
harmless at this RPM, it's still preignition (since the actual ignition
never happens). The question is whether this preignition is "normal" for
this type of engine in this condition, or whether it requires attention to
prevent destruction.

-Cory

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

Sydney Hoeltzli wrote:

Would you have expected it to?

Of course. When I turn the ignition off in my truck, it quits running. I
would expect no less in my aircraft. That's what IC engines are supposed
to do.

George Patterson
The optimist feels that we live in the best of all possible worlds. The
pessimist is afraid that he's correct.
James Branch Cavel

Same thing happened to me once, though it was caused by accidentally pulling
out the key in the process of a dead-cut check in a Piper Tomahawk.
Couldn'dfigure out why the engine still fires and coughs for a while when
there is supposed to be no spark. Then again I know jack about engines. Any
ideas?

--Jeremy

wrote in message
...
Hey all. Ran into something a bit odd yesterday. I was doing
some taxiing around the airport, and had to get out and check something.
To facilitate starting in just a minute, and since I was the only one
around, I shut it down with the mags, rather than the mixture cutoff. To
my surprise, right as it was shaking to a stop, it fired every once in
awhile... with the mags OFF! Mag drop, timing, etc was all checked not
too long ago, and in good shape. Compression is excellent, pops through
nicely on all four when pulled through by hand, and makes full static
runup.

Anyone heard of this? I guess I could potentially see that at
this ridiculously low RPM, there's almost 10x the time on the compression
stroke. Perhaphs this is enough time to make it light off once in awhile.
Also, I checked the MP as it was doing this, and since it's so low of RPM,
even at idle it's making 25". This lends credence to the "preignition"
theory.

Also, just for grins, I tried it on a friend's Cherokee 150 (O-320
150hp Lycoming 7:1 CR... mine's an O-360 8.5:1 180hp). I was somewhat
relieved to see his do this to.

Any thoughts?

-Cory

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

"NZLamb" wrote:

Couldn't figure out why the engine still fires and
coughs for a while when there is supposed to be no spark.

Perhaps a hot spot of carbon is lighting off the
combustible mixture? It's still pretty darned hot
in the cylinder - it was 1500+F only seconds ago.

--
ATP www.pittspecials.com