For what it's worth, I just did a valve job on a 79 Volare with lean burn. It had 2 cylinders with no compression. Took it apart and two exhaust valves looked like they had a cutting torch taken to them. When will a super lean condition cause driveability problems? Depends on alot of things! I think alot of the cracked head problems are due to running the engines on the ragged edge of driveability
