Rotax exhaust white grayish soot?

Sounds like lead residue from the 100LL. 

Disregard. Created a new topic since soot is different color.

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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.

Rotax 912iS that has exclusively consumed 91 octane unleaded mogas. The copper soot accumulated from the very beginning of usage and still accumulates at the same rate after 335 hours. Any thoughts on what could produce a soot of this color?

Oil vapors from the oil tank vent tube can cause this. Same color and I see it all the time. I use a product called "Oil Eater". It is water based and biodegradable and won't harm any type of aircraft skin and I wipes any oily or greasy residue off right off really easy.. It doesn't have all the harsh solvents in it like some fluids have.

Roger Lee wrote:

Oil vapors from the oil tank vent tube can cause this. Same color and I see it all the time. I use a product called "Oil Eater". It is water based and biodegradable and won't harm any type of aircraft skin and I wipes any oily or greasy residue off right off really easy.. It doesn't have all the harsh solvents in it like some fluids have.

For the grayish white soot or the rust/copper soot?

Copper colored. Greyish is usually exhaust. I see the grey all the time around the exhaust knuckles where they meet the muffler from leaks. If it was a big leak then that can be blown out to the bottom of the plane. I'd start by looking at my exhaust / muffler knuckles for leaks since it's a common occurrence.