I have a copper colored soot clinging to the underbelly of my aircraft. It is about the color of rust water. It begins a few inches back from the exhaust and widens as it reaches all the way to the stabilator.
This Rotax 912iS has exclusively consumed 91 octane unleaded mogas (well, rarely Swift UL94 has been used but MOSTLY 91 mogas). The copper soot accumulated from the very beginning of engine usage and still accumulates at the same rate after 335 hours. Another Van's RV-12iS flyer I have met has the same copper soot on the underside. This stuff is difficult to clean off. Any thoughts on what could produce a soot of this color?
