The exhaust valve not opening enough will cause a backfire thru the carb, as the cylinder IS getting gas and firing, but the burnt gas can't get out of the cylinder, so it will back up into the intake when that valve opens.
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Mitch
1970 Chevelle SS
1966 Chevelle SS
1967 Camaro ss/rs
1938 Business coupe, street rod
2000 FXSTS, original owner, 13k miles
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