Looks a little rich and too much oil... 40:1 is a bit much as evidence by the large amounts of black deposits. In short order you'll see some problems with the rings sealing. You are uing a good quality oil so thin it down to 50:1 and check the plug again after a few runs...<cite>
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http://www.troybuiltmodels.com/ns/manuals/zdz_all.pdf