Some stock BBC's have weak valvesprings, my 71 402 did the same thing: 402s are good engines.
Im told some 72s had hardened valve seats & some did not: it could have a valve issue.
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Two 1975 455 Grandvilles &
'79 455 Trans Am
‘69 Camaro SS 396/375 (owned since ‘88)
‘67 Corvette 427/435
‘23 Lexus LS500 awd
‘20 Audi R8 & ‘25 Toyota RAV4 (wife’s cars)
‘95 Ford F250 4wd 7.3 p-stroke
& countless Jeeps & off road vehicles & motorcycles 
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