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