Before tearing anything apart try adjusting the valves.

https://www.youtube.com/watch?v=U65dTaEn62Q

Just like that.

I had a similar problem and after reading your thread I thought it might be my seals. Pulled the valve cover and figured I'd see smoke coming from the valve seals when it was running. No smoke. Decided to readjust my valves since the cover was off. I had a misfire at idle on the #6 cylinder. It was because one of the valves was not adjusted right. fixed the problem and now I have a smooth idle again plus no more black smoke.