could be lean. My 230 had extremely worn valve guides and very low spring tension. #6 was leaking past the exhaust valve when running, leakdown test did not reveal it as the valve had time to "seat". I had hardened seats (1970 car, suspect 68 engine) but the valves still needed to be shimmed tighter, as if they had pounded into the seats. I also had two worn out cam lobes. You may have similar issues causing a difference in combustion
