When we first got our boat it did the same thing (slight exhaust looking residue on the back). The guy before us didn't use the boat hardly at all. After we ran it several week ends it stopped. I don't know if we ran the carbon out of it but it seems to run better now and we no longer get that black residue.
Hope this helps.
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Troy
2008 Centurion Avalanche
1994 Four Winns Horizon 180 SE 5.0L OMC Cobra (Sold)
2002 Ford F150 Harley Davidson Edition (
http://www.nhtoc.com) 378 hp & 465 tq at rear wheels
2008 Honda Accord EX-L w/Nav