Looking for some help on where to start repairs on my Stihl HS80. I can't get the engine to turn over. Using the normal start procedure, I pull and pull but it won't start. I noticed black soot and wetness from the exhaust under trimmer. I think some black smoke too. It's overdue for routine maintenance, so I'm going to order the filters & plug to replace. Any ideas what might be the root cause or where to start looking?