Curiouser and curiouser....
Regular readers know my separator/evacuator installation is home-built, and the exhaust tap is not located on a tailpipe, but rather on the #4 headpipe, about 18" down from the cylinder flange. It was installed approximately 20 months and 140 hours ago. I've been keeping a regular eye on it; last look inside the exhaust tap was when Vic and I were comparing notes after S&F.
I have about 18 hours of flying later this month, so yesterday the cowl was off for a quick general inspection. Removed the reed valve of course. This is the deposit found in the tube. Not a lot, but it's there, a small annular ring of buildup:
What is really interesting is the location. It's about halfway along the tube length,
not near the weld to the exhaust header. There's nothing closer to the valve end, nor any deposit closer to the exhaust header end. It's a ring. There is either a specific temperature at that point, or it has something to do with wave mechanics.
The material itself is interesting. First, it's not the color of burnt oil, but quite gray, like lead sludge. It also has a soft and sticky component. You can see how it adheres to the scraper. One onlooker commented that it seemed like gray anti-seize mixed with soft grit. It could be smeared between the thumb and index finger.
This is an engineering/design problem, and when we finally figure out the mechanics we'll probably laugh about it. In the meantime, pull your valve and take a look in the tube.