I have a '75 280 with a problem. A popping sound(like Compression) in the #4 cylinder when the plugs are out. The compression in all valves are 160. Also how do I look for damage in the spring rocker arm & pivot. Let me know if you have any suggestions. Stacey

since no one has responded , I will. It is not clear at all what you are asking . Plus the information given , what do you mean by , '' a poping sound like compression '' ? '' compression on all valves is 160'' ? Are you indicating that you have checked the compession of the cylinders and they are all at 160psi ? What I think you are saying is that , with the plugs removed and the engine turned over you are hearing a poping noise form the # 4 . If that is it , look to the valve springs . You may have a broken spring , I don't own a '75 L-28 but I believe they have double valve springs , an inner and a outer . If a broken spring is the case , the repair can be made without pulling the head . However it requires some special procedure to keep the valve from falling into the cylinder when the keeper is removed , but it can be done. All the best , Gary