Engine overrun on shutdown question

[Saab Dastard]

Can anyone explain why a Warrior PA28 161 engine should overrun on shutdown?

2 days ago, following 75 minutes of circuits, I shutdown normally, throttle to idle, mixture to fully lean and the engine almost stopped, then picked up again. This repeated for (what seemed like) a couple of minutes at which point I switched the mags to OFF, but the engine kept running. Finally I turned the fuel selector to OFF, and the engine eventually stopped.

I have never known this aircraft to overrun before and I am curious as to why it should have done so.

It was very cold (hovering around freezing) and the last circuit ended with a glide approach with carb heat on, setting carb heat off before landing. After vacating, I stopped and set 1200 RPM while carrying out the after landing checks, then taxied back to the apron and carried out the normal shutdown process.

The only departure from the norm was having to wait a few minutes for other aircraft to vacate the parking area before parking, so the engine was sitting at 1200 RPM for 3-4 minutes longer than usual.

<img src="confused.gif" border="0"> <img src="confused.gif" border="0"> <img src="confused.gif" border="0">

Any thoughts?

SD

[Kermit 180]

If you mean that the engine cuts then coughs back to life and repeats this many times, then I experienced this phenonemon a few years ago. It was due to a corroded part of the idle cut off system in the carburettor.

It can also be caused by a mixture control not extending all the way out (or down in the PA28 Warrior's case). An overrich mixture can lead to a momentary surge of power after the mixture is put to the ICO position (hot temperatures caused by long periods on the ground after flight without sufficient airflow through the engine cowls).

May I also take the opportunity to point you to an engine troubleshooting website: <a href="http://www.aeas.com/proceed.htm" target="_blank">http://www.aeas.com/proceed.htm</a>

Kermie <img src="smile.gif" border="0">

[Saab Dastard]

Kermit,

Thanks for the information - I can suggest something to the CFI now!

Cheers

SD

[Polar_stereographic]

SD,

I've always thought that this was due to part of the idle fuels system bypassing the mixture control, so if it ran rich at idle, these is a chance that suficient fuel can get through the system even when in ICO.

Solution, and it's always worked for me, is to go to ICO from 1200 RPM rather than from idle.

PS

[DOC.400]

Hey Saab -you're flying!!

Only time it happened to me was when the mixture control cable BROKE!

Took 2.30mins to stop after shutting the fuel off.

DOC

[bingoboy]

OK Doc I'll rise to the bait. Didn't you have any mag switches ? <img src="tongue.gif" border="0"> <img src="rolleyes.gif" border="0">

[A and C]

Two things that it could be first that the mixture arm on the carb is not reaching the ICO stop , all this needs is the cable adjusting.

Or if this is not the problem then its a worn valve in the mixture/ICO port in the carb if this is the case then its time to take a good look at the other moving parts as it may be time for the carb to be overhauled.

[PA38]

If you switched the mag's off and it didn't stop, then it's more than a mixture problem...

[A and C]

PA38 as you say more that one problem but you have to start troubleshooting some place and if as i think the carb needs changing this could be the root of the problem because if it is running over rich then hot carbon deposits left in the cylinders by incompleat combustion MAY well be igniting the charge when the mags are switched off.

So a carb change and a good run at the correct mixture setting may well solve the problem.

[ 03 January 2002: Message edited by: A and C ]

[ 04 January 2002: Message edited by: A and C ]</p>

[Saab Dastard]

Thanks for the replies guys

Doc - yup, if you call 75 minutes of circuits (14 landings) flying.

Having said that, 13 of the 14 were bloody good, greased it on the C/L every time (one was 2 touches and one go <img src="wink.gif" border="0"> )

SD

[Captain Numpty]

I fly 28's and have never experienced or heard of this so it's interesting to know should it ever happen. However, I was wondering if there is any chance it could attributed to say a "Hot Spot" on a cyclinder head or timing problem? Afterall, Motor cars are known to do this. I had an old Cavalier that used to continue running even after I had locked it and walked away LOL!!!! God it was spooky.

Anyway, interesting comments made and noted in the other postings.......thanks Chaps & safe landings!

C.N.

[Captain Numpty]

Sorry A & C......just clocked your comments......my sentiments exactly.

C.N. <img src="rolleyes.gif" border="0">

[gasax]

You guys should try flying behind a Gipsy Major. The manual states that in hot conditions the engine should be shutdown by - idling at more than 1000 rpm for over 2 minutes. rpm should then be reduced to a minimum, the magnetos earthed and the throttle smartly fully opened.

And it works. If the engine is hot and the weather too the Gipsy has a very nasty habit of 'running on', due to the rich idle mixture and hot spots in the combusiton chamber. The idle period helps to cool things, the minimum speed mag cut gives the best chance of a controlled shutdown and the open throttle ensures the maximum amount of cold air getting into the combustion chamber.

Having said all that it is pretty unusual to get these symptoms with an O-320 - but not impossible.

Cheers