I have this os max 35 that has gotten this cracking sound when it's running. It causes the a vibration in the plane and a loss of power. I took it out of the plane and took the back plate and head off . I didn't see anything that look bad. The head bolts were not as tight as they could have been. So I put it back together and ran it on a test stand. That didn't change anything. When it ran good I got 12.5k rpm . Now I only can get 10.5k rpm. It idles good and starts ok. Tried different glow plug and prop. To me it sounds like the fuel is not igniteing at the right time. Nothing I have tried has helped. I'm lost . Has anyone seen a problem like this or have any ideas for a solution.