You've got a blown valve an a possible had gasket leak. All of it caused by detonation or incorrect timing, or WRONG HEAT RANGE PLUGS. Either way, start with a compression test, identifying the motor and proper plugs for it. Any knocking noises? Any rattling? especially at idle when you first start the engine, before oil has a chance to circulate? What engine? Cyls? Horses? Volume?
Hmm, Champion Plugs. Had the same problem over the weekend. I installed them and I know I put them in tight. Two of mine looked like the center one. All of them were loose on Monday when I removed them. I haven't done a compression test yet, but the engine seems to be running fine now. I dumped the champions and went to AC MR43T's. The champions were in for about 3 hours of running time. Maybe a little more but not much. That is what kept me from the skip day party. I won't be using champions ever again.
Yep, you were right about the compression test. Cylinders 3 and 5, zero. All the rest are at 150. Thought I was going to be okay as I started on the starboard side, 2, 4, 6, 8. They all read fine. Then did 7, again, ok. Then the bad news started and went downhill from there. So, we are all done for the summer. Only a few good weekends left anyway and I am not gonna bust my butt trying to hurry and get it done and back in. I will have her pulled next week probably. Then I will do the work myself. She did 38MPH on 6 cylinders. Can't wait to see what she can do with 8 healthy ones. There is always next year.