Do you mean after a few hours of flying, before shutting down, or after a few hundred hours of flying at any point?
When I was running in my new cylinders and pistons I put my hand behind the exhaust and I could feel all the metal flecks being thrown out. Died down after an hour or two.
I mean after about half an hour of flight.
I could feel all the metal flecks being thrown out.
That’s unbelievable 
Is the half-hour thing not just down to proper warm-up, oil circulation, clearances, deposits on the plugs being burned off etc..? My only thought would be that if so, does it really take a whole half hour or would 5-10 minutes be enough?
I guess the peppering could have been carbon deposits on the cylinder heads being thrown out, but the cylinders had a heavy cross-hatch honing pattern so my assumption was that it was the edges being knocked off that.
kwlf wrote:
Is the half-hour thing not just down to proper warm-up, oil circulation, clearances, deposits on the plugs being burned off etc..?
My thoughts as well. Lots of people don’t lean on the ground. This could cause “rough” running for, I don’t know, 5-10 minutes or more. Low idling also causes the same problem. The idle RPM on a Rotax 912 UL is about 1600-1700. The manual say always run it on on min 2000 RPM on the ground due to risk of fouling the plugs. Rotax has automatic mixture control.
Have you ever noticed that engines seem to sound and feel at their most perfect, just before they go bang?🙂
Having been involved on the edges of motor racing and rallying over the years, its often what the drivers and mechanics tell you.
I’ve not noticed that one, but then never had one go bang!
I do remember reading about what happened when the London & North Eastern Railway pushed the steam locomotive Mallard to some 126mph, a world record for steam traction that still stands. Someone writing about it said that for the record attempt they chose an engine that was six months old. Not six weeks old when it would be tight, and not six years old when it would be worn. Six months, or ‘just run in’ was thought to be optimum.
Have you ever noticed that engines seem to sound and feel at their most perfect, just before they go bang?
That was well known for the 1970s motorbike 2-stroke engines, if the oil tank ran out 
