I'm working on a Husqvarna 128DJX for my dad that's smoking while under load. At idle, there is no visible smoke. At half throttle, you see white smoke, and at full throttle, you really see it. On top of that, there's what looks like black oil all over trimmer on the left side where the exhaust is.
I have changed the spark plug, replaced the air filter, cleaned the spark arrestor, dumped the fuel and put in fresh, and replaced the carburetor. I took everything apart and visually inspected the outside of the motor where the head gasket is, but did not see any leaks nor hear any air escaping.
The oil to gas ratio on this model is 50:1 and he's been buying the pre-mixed cans that are ethanol free for years.
I am out of ideas; what could be causing the white smoke and oil-like deposits on the trimmer?
EDIT: Link to parts diagram