I suspect that the the cyl head screws are loose. That engine has a 4-bolt head with phillips screws -- they often loosen -- it is a chronic problem. I usually carry a supply of socket-head screws in my flight box for student's engines. I can't remember the correct size right off the top of my head.

Make sure that the head is seated flat on the block (not cocked), then tighten the screws incrementally in a criss-cross pattern. It should be OK, but you will have to re-tune the air-bleed screw.

It would be a good thing to also check the back-plate screws & the carb hold-down screws.