Lean mixture on the pilot circuit.
It's possible that you have some worn things in your engine, which might cause it.If you had leaky valve seats, that can cause spitting back thru the intake. And leaky valve guides can cause oily spark plug.And an exhaust leak at the head joint might cause a lean mixture condition and spitting, so check your header pipe fit into the head for any leaks or visible carbon showing around the header pipe joint to the head.
thanks--will check as best I can. The cylinder head rebuild by the shadetree jackass I paid to do the initial work sure isn't precluding any of that.How would you check for a leaky valve seat or guides?I've been regularly looking for soot at the exhaust header join, though, and not found any.