Hi, i,m a new member to the TBN. I bought a 73 massey mf135 with a perkins diesel and used it a few times with a rotary mower. It worked fine for a while then i noticed the exhaust started getting black and it started losing power. I shut it off and had a hard time getting it started again but when i did i noticed it was spitting air back through the air cleaner. Someone told me it was a stuck valve so i took it apart and took the valve cover off and all the valves are moving fine. any suggestions from anyone? Do i need to pull the head?