Black residue from muffler

Richard, I had the same problem.  Was running Ethanol Free Mogas.....switched to Swift fuel and the problem disappeared. 

Hope you like that M24. I have the M16 Plus. Check your spark plugs. I have 100 hours on my 915 now and when I pulled them for the 100 hour service the electrode residue on all were white (exactly how they looked at 25 hours) and I'm not producing any noticeable soot from the exhaust. Having said that, my machine is red and my tail boom and tail planes do get dirty from exhaust gases, and that's normal. I have over a hundred hours on white M16s and about 10 hours in a white M24, and yes they get dirty. If you are getting soot on your wheel pant I'd speculate you are getting that during startup. My engine runs pretty rough for the first 5 seconds on first start of the day. I assume the ECU is trying to figure out the mixture settings based on whatever atmospheric conditions it discovers on that first start. That's probably where your dirt is coming from. I'd guess the wheel pant would not get hit by exhaust gas in flight. 

I burn pump gas with ethanol. I get Swift fuel or unleaded when I'm out on cross country flights when I can. I probably burn 100LL less than 10 percent of the time, and when I do, I use Decalin in the fuel to capture the lead. Before the community freaks out over this, I consulted two old and presumably smart Rotax guys in the Rotax booth at Oshkosh and I've taken the Rotax service class at Lockwood Aviation. They all recommend using a lead scavenger in your fuel when burning 100LL. Lead is bad. (Ok, the guys at Oshkosh were not that old. I should have said they looked experienced.)

For all reading this, I highly recommend taking the service and perhaps deeper maintenance classes at Lockwood. I learned a lot. They have a two-day service class which is generic to the 9-series engines with a follow-on single-day line maintenance class for the iS series engines. I took the service class and will likely go back for the iS class in the future. 

Great stuff!  Thanks Michael!