For what it's worth, it only takes a pin hole leak in one of the flanges next to the head to burn the valves in that cylinder. Been there, done that on a VW flat four. If you have a lot of welds they all need to be checked more for air leaks leading to a lean condition as well as maybe a gas leak to the outside. Assuming a partial vacuum in the log, the air leak won't hurt high up, but right near the head that will lead to a very lean condition at that port.

Don Shillady
Retired Scientist/teen rodder