Plug sounds OK, they shouldn't be oily.

Odds are good the spark arrestor is clogged up. Needs to be cleaned. These are screens in the muffler, usually they are behind the outlet deflector. Some Homelites have them inside the muffler requiring disassembly of the muffler to get to them. If cleaning the screen doesn't correct, remove the muffler and look inside the exhaust port to see if it has carbon build up blocking the port.

Here's a tip. If the screen is carboned up there is almost always dark gooey stuff around the deflector. You won't normally see this if the port is clogged.