+1 to aussietrev on this one.
To be smokey, it has to be running rich. The 'no change with carby substitution' test tells us that the problem lies elsewhere, so the muffler is the next suspect on the list.
It may not just be carbon either - I picked up a couple of Echo SRM200DB brushcutters for crazy cheap recently, and one had the muffler [no spark arrester mesh on this model] plugged solid with mud wasps' handiwork.
It was filled completely; they had even got through the small holes in the baffles, and filled the cylinder exhaust port as well! Luckily, the port was closed by the piston, so they didn't get into the cylinder itself.
Had the machine running about 15 mins after I finished cleaning the crud out of the muffler!
